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Buriedbedrockvalleysandglacialandsubglacial
meltwatererosioninSouthernOntario,Canada
ArticleinCanadianJournalofEarthSciences·May2011
DOI:10.1139/E10-104
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CunhaiGao
OntarioGeologicalSurvey
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Buried bedrock valleys and glacial and subglacial
meltwater erosion in southern Ontario, Canada
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Cunhai Gao
Abstract: Morphometric features from a recently compiled bedrock topography map by the Ontario Geological Survey suggest a glacial erosion origin for the buried large bedrock valleys and troughs in southern Ontario. The bedrock valleys at
Milverton, Wingham and Mount Forest are tunnel valleys, resulting from subglacial meltwater erosion beneath the Huron
ice lobe, probably during or shortly after the Late-Wisconsinan glacial maximum. Diagnostic features for this interpretation
include abrupt valley beginning and termination, uneven longitudinal valley profiles and up-slope gradients. The Dundas
bedrock valley is the western extension of the Lake Ontario Basin. No comparable bedrock valleys were found to connect it
to the Milverton valley for a joint drainage system as previously suggested. The Laurentian bedrock trough is the southeastward extension of the Georgian Bay Basin, both developed along shale bedrock between the Precambrian shield highlands
and the Niagara Escarpment, resulting from long-term mechanical weathering associated with Pleistocene glacial erosion.
This bedrock low has a floor that exceeds 50 km in width and is 26 m and more below the current water level of Georgian
Bay. It could drain Georgian Bay should the drift cover be removed. There is no evidence to suggest that a preglacial river
channel, if it existed, is still preserved in the floor of the Laurentian trough as previously suggested. The framework for an
intensely glacially sculpted bedrock surface differs from the previous view for simple modification of a preglacial landscape
and is, therefore, important in regional subsurface geological mapping and groundwater studies.
Résumé : Les caractéristiques morphométriques illustrées sur une carte topographique du socle rocheux faite par la Commission géologique de l’Ontario suggèrent que les grandes vallées et fosses enfouies du sud de l’Ontario aient une origine
d’érosion glaciaire. Les vallées rocheuses à Milverton, Wingham et Mount Forest sont des vallées de tunnels et proviennent
de l’érosion par l’eau de fonte sous les glaciers, sous le lobe glaciaire Huron, probablement durant ou peu de temps après le
maximum glaciaire du Wisconsin tardif. Les caractéristiques de diagnostique pour cette interprétation comprennent des débuts et des fins de vallée abruptes, des profils longitudinaux et des gradients de pente amont irréguliers. La vallée rocheuse
Dundas constitue l’extension vers l’ouest du bassin du lac Ontario. Aucune autre vallée rocheuse n’a été découverte la reliant à la vallée Milverton pour constituer un système de drainage conjoint, tel que suggéré antérieurement. La fosse laurentienne dans le socle est le prolongement vers le sud-est du bassin de la baie Georgienne, les deux s’étant développés le long
du socle de shale entre les hautes terres du bouclier précambrien et l’escarpement du Niagara par la météorisation mécanique à long terme associée à l’érosion glaciaire au Pléistocène. Ce creux du socle rocheux a un plancher qui a une largeur
de plus de 50 km et il est à 26 m ou plus sous le niveau d’eau actuel de la baie Georgienne. Si le couvert glacio-sédimentaire devait être retiré, ce creux pourrait drainer la baie Georgienne. Il n’existe aucune preuve suggérant qu’un chenal de rivière préglaciaire, s’il avait existé, soit encore préservé sur le plancher de la fosse laurentienne, tel que déjà suggéré. Le
cadre pour une surface de socle intensément sculpté par les glaciers diffère de l’ancienne vue d’une simple modification
d’un paysage préglaciaire et il est donc important dans la cartographie géologique régionale subsurface et les études de
l’eau souterraine.
[Traduit par la Rédaction]
Introduction
Buried bedrock valleys have been reported in southern Ontario since the late 19th century (Spencer 1881, 1890; Karrow 1973; Flint and Lolcama 1986; Eyles et al. 1993, 1997).
Early studies are based on hand-contoured maps, and details
on the bedrock valleys are generally lacking as to the geometry, longitudinal profile, and their spatial relationships with
other bedrock valleys in the vicinity. This is probably part of
Received 30 June 2010. Accepted 2 December 2010. Published
at www.nrcresearchpress.com/cjes on 4 May 2011.
Paper handled by Associate Editor Timothy Fisher.
C. Gao. Sedimentary Geoscience Section, Ontario Geological
Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5, Canada.
E-mail for correspondence: [email protected].
Can. J. Earth Sci. 48: 801–818 (2011)
the reason for the various interpretations proposed for their
origins, including relict preglacial channels, glacial scours,
tectonically controlled Tertiary river valleys or subglacial
meltwater valleys or channels (Spencer 1907; Straw 1968;
Karrow 1973; Brennand and Shaw 1994; Eyles et al. 1997;
Kor and Cowell 1998).
Previous compilations for bedrock topography are lacking
quality controls on water-well records, the major source for
the depth-to-bedrock information. The current water-well database archived at the Ontario Ministry of Environment in
Toronto has more than half a million records for southern
Ontario. These records are notoriously inconsistent in quality,
containing georeferencing errors and incorrect geological descriptions largely owing to the reporting procedure, inaccurate locations sketched and the lack of detailed material
information because of the commonly used wash-bored drill-
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802
ing method, and, lastly, the fact that most water-well drillers
are not trained professional geologists (Russell et al. 1998).
The database has been systematically filtered for georeferencing errors in recent subsurface mapping (Kenny et al. 1997;
Logan et al. 2005); however, the geological content of the records has rarely been critiqued.
Recently, the Ontario Geological Survey developed protocols and a methodology to generate digital regional bedrock
surface maps (Gao et al. 2006, 2007). Using this methodology, rigid quality control measures were employed to track
and eliminate problematic data during the compilation. The
resultant map has enabled better delineation of the bedrock
topography, in particular, the regional extent of significant
buried bedrock valleys or depressions. This paper introduces
briefly the methodology, describes in detail major bedrock
valleys in southern Ontario, and discusses the possible causal
mechanisms. In the discussion that follows, some large bedrock depressions with a size of 20 km and greater in width
are referred to as troughs.
Geological setting
Southern Ontario is underlain by a Precambrian basement
containing Proterozoic gneissic rocks and an overlying Paleozoic cover rock (Fig. 1A; Ontario Geological Survey 1991;
Johnson et al. 1992). In the basement across southwestern
Ontario lie northeast-trending tectonic highs, referred to as
the Findlay and Algonquin Archs, which separate the Michigan intracratonic basin to the northwest from the Appalachian foreland basin to the southeast (Fig. 1B). The
Paleozoic bedrock thickens toward the basin centers, and, in
southern Ontario, it comprises Cambrian to Devonian-Mississippian carbonate and clastic sedimentary rocks, reaching a
maximum thickness of 1.5 km (Johnson et al. 1992). In this
region, the widespread dolostone of the Middle Silurian
Amabel and Guelph Formations is erosion-resistant, whereas
rocks containing shale and evaporites, such as the Upper Ordovician Queenston and Upper Silurian Salina formations,
are susceptible to erosion. The Niagara Escarpment, a prominent regional landmark, is a result of erosion of soft shale
bedrock below a resistant cap rock of dolostone. The Onondaga Escarpment is another bedrock high on the Niagara
Peninsula. However, in contrast to its prominent relief in the
New York State, it has a subtle surface expression as discontinuous low ridges along the northern shore of Lake Erie.
The study area was overridden by Pleistocene ice sheets
from the north. During the retreat of the ice sheet in the
Late Wisconsinan, discrete ice lobes developed in the Great
Lakes Basins, which expanded and retreated independently
or semi-independently (Barnett 1992). The Nissouri ice advance deposited a variably textured clay to stony till named
the Catfish Creek till during the glacial maximum at about
20 000 years before present (BP); ice re-advance occurred
during the Port Bruce and Port Huron stadials at about 15 000
and 13 000 BP, respectively. The ice advances and subsequent retreats in this region have generated widespread till
plains and a series of moraines, including the interlobate
Waterloo and Oak Ridges moraines rich in sand and gravel
deposits (Ontario Geological Survey 2003).
Can. J. Earth Sci., Vol. 48, 2011
Methods
Depth-to-bedrock information was extracted from waterwell, petroleum, and geotechnical drill records, as well as
from published geological maps. Detailed descriptions of the
methods have already been released, and the following is a
summary of the quality control procedures adopted in this
compilation. Readers can refer to Gao et al. (2006, 2007) for
details.
The water-well database that contains over half a million
records is the largest source for depth-to-bedrock information.
The water-well records were systematically filtered to remove
georeferencing errors, including unreliable locations, ground
surface elevations inconsistent with digital elevation model
(DEM) values, and wells located within lake boundaries. Applying these restrictions provided an initial database with
more than 350 000 water-well records for southern Ontario.
The drift–bedrock contact was then assigned through an automation process (Gao et al. 2006, 2007). However, water-well
records containing ambiguous or questionable entries for bedrock, such as basalt, conglomerate, greywacke, slate, sandstone, and soapstone that do not occur or have limited
occurrence in southern Ontario, were inspected and the
drift–bedrock contact was manually assigned.
The borehole records with assigned drift–bedrock contact
were further filtered and those with inverted stratigraphy, e.
g., a granite (Precambrian) overlying a limestone (Paleozoic)
or with duplicate locations, but having different depths to
bedrock, were removed. Lastly, water-well records with excess depth to bedrock (>8 m) in the known thin-drift areas
(<1 m) mapped by Ontario Geological Survey (2003) were
inspected and many were removed because of the incorrectly
assigned drift–bedrock contact resulting from misused terminology and misinterpreted geologic material. After these filtering processes, a database with about 250 000 drill records
was obtained to determine the bedrock elevation surface using the ESRI® ArcGIS® ordinary kriging routine (Gao et al.
2007). Over 16 000 water-well records not reaching bedrock
but deeper than the interpolated bedrock surface were subsequently used to “push down” or adjust the surface.
The initially interpolated bedrock surface contains many
excessively high peaks and deep holes. More than 1200 drill
records causing such anomalous areas were carefully inspected and compared with the boreholes within a radius of
1 km. Many had erroneously assigned drift–bedrock contact
resulting primarily from incorrect use of the geologic terminology and misinterpretation of boulders as bedrock in the
borehole records. For instance, “stones,” although normally
used to indicate a drift deposit, has been used by drillers to
indicate bedrock, thereby causing unrealistic holes or highs
in the generated bedrock surface. After removal of the problematic data, the database was updated and kriged to refine
the bedrock surface; the drift thickness map was generated
subsequently by subtracting the bedrock surface elevations
from the ground surface elevations (Gao et al. 2006, 2007).
Bedrock valleys and troughs
Many bedrock valleys and
surficial deposits in southern
rock low referred to here as
Lake Erie (Figs. 2, 3, 4, 5).
depressions exist beneath thick
Ontario, including a large bedthe Long Point trench beneath
Although some of the bedrock
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Fig. 1. (A) Bedrock geology of southern Ontario (modified from Ontario Geological Survey 1991). (B) Generalized basement contours
(metres above sea level) and location of structural basins (modified from Johnson et al. 1992). Inset map shows location of southern Ontario.
Fm., Formation; Gp., Group.
valleys have been reported before (e.g., Karrow 1973; Eyles
et al. 1997), the current work provides much needed details
on their planform, floor topography, and longitudinal profiles, as well as their spatial relationships with the adjacent
bedrock lows.
Apart from the Laurentian and Ipperwash bedrock troughs,
named following Spencer (1890) and Karrow (1973), broad
bedrock lows also include the Walkerton and Brantford–
Welland troughs, both occurring along the Salina Formation
and walled by the Onondaga Escarpment, with a tilted floor
in transverse profile corresponding to the dip of the regional
Paleozoic bedrock (Figs. 3A, 4, 5). In the Walkerton trough,
a thalweg exists along the buried Onondaga Escarpment
(Fig. 4), which corresponds more or less to Karrow’s (1973)
Walkerton valley. Small re-entrants are developed on the
scarp (Figs. 3A, 4). The Brantford–Welland trough occurs
east of Woodstock and has an elongate, rectangular plan
view (Figs. 3A, 4, 5). Although the current data does not allow mapping into the New York State, this bedrock low
likely extends farther east because of the continuity of a similar bedrock setting.
Narrow but deep bedrock lows or gorges include the Milverton, Wingham, and Mount Forest valleys to the west and
northwest of Kitchener (Figs. 3A, 4, 5). Smaller such bedrock valleys are also present, including the Elora and Rockwood valleys at Elora and Guelph (Figs. 3A, 4), named
following Greenhouse and Karrow (1994). Previously, the
Milverton and Wingham valleys are referred to together as
the “Wingham Valley” (Karrow 1973). In this study, this
name only refers to the valley between Wingham and Ethel
because of a bedrock high at Ethel bisecting the valleys
(Fig. 3A). The bedrock valleys are lacking branching features, contradicting Eyles et al.’s (1997) work that outlines
well-developed dendritic or arborescent bedrock valleys. This
difference cannot be attributed to map scales because the current geographical information system (GIS) map can be
viewed at substantially larger scales than the earlier maps
used by Eyles et al. (1997). It remains unclear why their
compilation differs so strikingly from the current work. It is
speculated here that those small tributaries on their maps are
probably not the data-based delineations contoured by computers but derived largely from the authors’ interpretations.
On the Niagara Escarpment, many re-entrant valleys exist.
Among them, the Dundas valley is one of the largest. Other
large ones include the Owen Sound, Beaver, Meaford, and
Colpoy Bay bedrock valleys (Figs. 3A, 4). Straw (1968) has
provided details on the geometry and dimension of these reentrants. Although some of them terminate at the foot of the
escarpment, others incise into the floor of the Laurentian
trough (Figs. 3A, 4). On the Bruce Peninsula, the large reentrants continue beneath the lake, connecting to the linear
depressions in the floor of Georgian Bay (Figs. 3, 4).
On the Niagara Peninsula, the Erigan bedrock valley consists of broad bedrock lows in the middle and re-entrants at
St. Johns and Lowbanks, namely the 12 Mile Creek and
Lowbanks re-entrants on the Niagara and Onondaga escarpments, respectively (Fig. 6). North of the Niagara Escarpment
to Lake Ontario, no prominent bedrock valleys are found,
which differs from Flint and Lolcama’s (1986) compilation
that outlines several well-defined, deep bedrock valleys (see
Fig. 2B). Irregular, linear bedrock lows occur between
Wainfleet and Fraser (Fig. 6), probably corresponding to the
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Fig. 2. Bedrock valleys proposed by (A) Spencer (1907), (B) Flint and Lolcama (1986), (C) Karrow (1973), and (D) by Eyles et al. (1997).
Crystal Beach channel, as previously mapped by Flint and
Lolcama (1986) (see Fig. 2B). However, this valley is shallow and lacking well-defined shoulders.
The St. Davids valley is confirmed, but its northern part
near the Whirlpool is not well defined, probably owning to
the sparse boreholes available there (Fig. 6). Organic material
recovered from the valley fill has been radiocarbon-dated at
22 800 to 24 800 BP, indicating a Late-Wisconsinan affinity
for the fill (Hobson and Terasmae 1969). It is believed that
this valley was created during the last interglacial Sangamonian Stage and filled during the Late to Middle Wisconsinan
(Feenstra 1981). Later, it was truncated by the present Niagara River at the Whirlpool.
Long Point trench
Defined by numerous offshore petroleum-well records, this
bedrock low extends for over 150 km beneath Lake Erie and
pinches out to the west in a direction at 10° to 25° oblique to
the basin long axis (Figs. 3A, 4). Its deepest part is located
off Long Point, where the trench stands at 20 m above sea
level (asl) and is 12 km wide. The trench appears to crosscut
the Ipperwash trough (Figs. 3A, 4). No petroleum-well records are available across the international border on the
USA side, and the eastward extension of this trench remains
unknown. However, seismic surveys in this area indicate its
probable extension further to the east-northeast (Morgan
1964). The trench aligns with the deep, east–west-trending
trough in the floor of the eastern Lake Erie Basin.
Till and postglacial lacustrine sediments probably fill the
trench. Off Long Point, under the lake, a large moraine ridge,
the Norfolk Moraine (Coakley et al. 1973), occurs as an arcuate, transverse ridge aligning nearly perpendicular to the
trench (Fig. 7). The trench has an irregular longitudinal profile (Fig. 8A). It probably connects to a large bedrock valley
in the onshore area at Bothwell (Figs. 3A, 4). However, in
the offshore area west of the Median across Port Stanley,
only a limited number of petroleum-well records are available, rendering delineation of bedrock features there difficult.
As such, the relationship between these two bedrock depressions remains to be confirmed.
Laurentian trough
The depth-to-bedrock information is relatively sparse in the
Laurentian bedrock trough largely owing to the presence of
thick drift of the Oak Ridges Moraine (Fig. 3B). There is a
need in the future to refine the floor topography. Nonetheless,
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Fig. 3. (A) Bedrock topography of southern Ontario and Lake Erie. Bathymetric data for the Great Lakes came from the National Oceanic
and Atmospheric Administration (1999). asl, above sea level. (B) Drift thickness with hill-shade relief of the present ground surface. Boxed
areas are enlarged in Figs. 6, 9A, and 9B, as well as in Figs. 10A and 10B.
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Fig. 4. Highlighted bedrock valleys and troughs in southern Ontario. Contour interval = 25 m. Coded bedrock depressions are mentioned or
discussed in the text. Refer to Fig. 1 for bedrock geology.
the current compilation shows a bedrock trough 50 to
70 km wide with a floor mostly below the 150 m contour.
The trough stretches from Georgian Bay to Lake Ontario in
the Ordovician shale and limestone, bordered to the east by
the gentle Precambrian highlands and to the west by the
steep Niagara Escarpment (Figs. 3A, 4, 5A).
Linear depressions exist in the floor of the Laurentian bedrock trough. By connecting some of the deeper ones, a thalweg was defined, which rises toward the mid-point along the
trough (Fig. 4). Although it is similar in alignment and location to those mapped previously (White and Karrow 1971;
Eyles et al. 1993), this thalweg remains to be confirmed because of the limited data available. It has been interpreted as
a relict channel of the preglacial Laurentian River draining
the upper Great Lakes Basins (see Fig. 2A; Spencer 1890,
1907; White and Karrow 1971; Eyles et al. 1993; Holysh et
al. 2004).
Among the re-entrants that extend into the Laurentian
trough, the one at Caledon East (Figs. 3A, 4) contains about
30 m of glaciofluvial sand and gravels resting on the valley
floor at the base of the Niagara Escarpment, forming the regionally significant aquifers (Davies and Holysh 2005; Russell et al. 2006). It extends to the southwest, trending
similarly but not overlapping with the present-day Grand
River, and it is probably linked to the buried, southwesttrending Rockwood bedrock valley (Figs. 3A, 4). Drilling
along the base of the Niagara Escarpment in a poorly defined
branch of the bedrock valley at Georgetown (Figs. 3A, 4, 9B)
also indicates thick sand and gravels below Late-Wisconsinan
till deposits (Meyer and Eyles 2007).
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Fig. 5. Cross sections A–A', B–B’, C–C', D–D', and E–E'. Thick line indicates bedrock surface and thin dotted line the ground surface. Refer
to Fig. 4 for locations. asl, above sea level.
Mount Forest, Wingham, and Milverton valleys
These bedrock lows are rectilinear to slightly curved
gorges with a width ranging from 2 to 4 km and depth of 40
to 70 m, trending to the southeast across the Algonquin Arch
(Fig. 9). Their longitudinal profiles are irregular with multiple thresholds and enclosed depressions, and some valley
segments begin and terminate rather sharply (Figs. 8B, 8C,
9). The boreholes in the valleys are not all distributed along
the thalwegs, and some areas even do not have data (Fig. 10).
Nonetheless, the overall geometry of these valleys is welldefined owing to the large number of drill records available
in the vicinity that clearly define the valley shoulders
(Fig. 10).
The Mount Forest valley extends from Mount Forest to
Drayton for 35 km in the Salina Formation, and, despite an
extremely undulating longitudinal profile, it has a gradient to
the southeast (Fig. 8C). This valley not only differs from the
broad Walkerton trough in planform but also cuts into the latter (Figs. 3A, 9A). The Wingham valley stretches from
Wingham to Ethel for about 30 km, with an uneven floor
dipping to the northwest; it is truncated at Wingham by the
northward-aligned Hutton Heights valley (Figs. 8B, 9A).
Some of the bedrock highs in this and other bedrock valleys
may be artificial because of the lack of data in these areas
(Fig. 10A). After removal of these highs, the valleys exhibit
a much subdued floor topography (Figs. 8B, 8C).
The Milverton valley extends for 46 km with an overall
gradient of 0.3 m/km to the southeast. At Milverton, a closed
depression or basin stands as the deepest part of the valley,
reaching to 268 m asl or 70 m depth below the valley
shoulder (Figs. 8B, 9). A bedrock high about 6 km northwest
of Milverton divides the valley into the western and eastern
segments (Fig. 9A). However, the lack of data in this area
(Fig. 10A) means that this bedrock high needs to be confirmed by drilling or ground geophysical survey in the future.
The valley crosscuts several bedrock units including, from
west to east, the Detroit River Group and the Bois Blanc,
Bass Island, and Salina formations (Fig. 9). East of Wellesley, it diverges into a broad lowland underlain by the Salina
and Guelph formations.
In Eyles et al.’s (1997) compilation, both the eastern and
western segments of the Milverton valley are shown on the
computer-contoured bedrock surface map. However, they interpreted the western segment atop the Algonquin Arch as a
series of solution holes in limestone bedrock. Although this
interpretation is consistent with their proposition that the
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Fig. 6. Erigan valley on the Niagara Peninsula, contoured at 5 m interval. Lower case letters indicate bedrock units. Refer to Fig. 1 for bedrock geology.
buried valleys were tectonically controlled river channels,
draining the flanks of the Arch, the current work shows that
the western segment is an integral part of the Milverton valley that cuts across this tectonic high (Figs. 4, 9A).
The Milverton valley is thought to extend eastward to connect the Dundas valley, forming a joint preglacial river (Karrow 1973; Eyles et al. 1997). However, no comparable
bedrock gorges were found between Copetown and Wellesley
for such drainage connection (Fig. 9). The absence of large
bedrock valleys in this area cannot be ascribed to the lack of
data. Although limited data is available between Kitchener
and Paris, sufficient data is seen elsewhere along the proposed river pathway. This is particularly true for the area between Wellesley and Kitchener and between Paris and
Copetown, where dense data points are available but no bedrock gorges are delineated (Fig. 10B).
Alternatively, the shallow bedrock lows found in a recent
geophysical study are suggested to be the connection between the Milverton and Dundas valleys (Zwiers et al. 2008,
2009; Bajc et al. 2009). It is thought that bedrock gorges
were created on both Niagara and Onondaga escarpments
and connected by shallow valleys between the escarpments
in a way similar to the present Niagara River (A.F. Bajc, personal communication, 2010). This proposition invokes the
presence of a high-relief Onondaga Escarpment to form bedrock gorges with a size comparable to the Milverton valley.
However, as evidenced by the current work as well as previous compilations, the Onondaga Escarpment is subtle even
along its most prominent segments on the Niagara Peninsula,
and there is no prominent bedrock cliff at the outlet of the
Milverton valley at Wellesley (Figs. 3A, 9). As such, it is unlikely that the Milverton valley resulted from river-headward
erosion on bedrock scarps. Indirect evidence comes from the
Mount Forest bedrock valley, which is developed in a single
bedrock formation without any salient slope break or bedrock
escarpment at its outlet at Drayton (Fig. 9A). It should also
be borne in mind that the present Niagara River has not generated on the Onondaga Escarpment any bedrock valleys with
a size comparable to the bedrock gorge below the Niagara
Falls.
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Fig. 7. Moraines and eskers in southern Ontario, formed during the Late Wisconsinan (Modified from Ontario Geological Survey 2003).
Dundas valley
This is a partly filled, deep bedrock gorge that crosscuts
the Niagara Escarpment in Hamilton and extends into Lake
Ontario (Figs. 3A, 4). Because of the thick overburden material in the valley, water wells often terminate in the valley infill, leading to the limited depth-to-bedrock information
available. This is probably the reason why the valley has an
extremely uneven floor shown in this compilation. Future
drilling may help better delineate the floor topography.
The valley floor is at 50 m asl in Hamilton but plunges
into a small but very deep basin with a base at 30 m asl at
Copetown (Fig. 9B). The valley widens and rises sharply by
more than 120 m prior to its closure about 3 km west of Copetown (Fig. 9B). Further to the west, no comparable bedrock gorges exist. The recent drilling and subsurface
mapping in this area has reached the same conclusion (Bajc
et al. 2009; Zwiers et al. 2009). Instead, a broad, shallow
bedrock low, referred to here as the Innerkip valley, occurs
in the direction of the Dundas valley. It cuts into the Onondaga Escarpment forming a small but deep re-entrant at Innerkip (Fig. 9B). Prior to its termination, the Innerkip valley
broadens in a way similar to the terminus of the Dundas valley at Copetown (Fig. 9B). Irregularly shaped, poorly defined
shallow bedrock lows also occur around Brantford (Fig. 9B).
However, the data in this area is relatively scanty, and the
morphometry of these bedrock lows needs to be refined in
the future.
Origin of bedrock valleys and troughs
Long Point trench
The broad plan view morphology and thick fill of glacigenic material suggest that this bedrock feature can best be
explained in the context of glacial erosion (Figs. 3, 4). Fast
moving ice was probably responsible for the formation of
this bedrock low. Instead of entering the Ipperwash trough,
the proposed westward-moving ice stream appears to crosscut
it (Figs. 3A, 4). The reason for this may be that the Huron
ice lobe moved in a direction against the Erie lobe, which
could potentially have jammed the trough. The other reason
is probably related to the alignment of the Ipperwash trough
that orients at a high angle to the direction of the proposed
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Fig. 8. Longitudinal valley floor profiles. Upper line is the actual measurement, and the lower one the profile after the removal of the bedrock
highs where no borehole records exist. (A) Long Point trench. (B) Wingham and Milverton valleys. Note the former is cut by the Hutton
Heights valley. (C) Mount Forest valley. (D) Erigan valley. a.s.l., above sea level.
ice stream. The Norfolk Moraine off Long Point marks the
stillstand position of the retreating ice lobe after the Port
Bruce ice advance (Coakley et al. 1973; Barnett 1992). The
Long Point trench has an alignment oblique to the basin
axis, suggesting no direct link between this bedrock low and
the proposed preglacial river that drained eastward along the
long axis of the Lake Erie Basin (see Fig. 2A; Spencer 1890,
1907).
Laurentian trough
The morphology and alignment suggest that the Laurentian
trough is the southeastward extension of the Georgian Bay
Basin (Figs. 3A, 4). The fact that both are located along soft
shale bedrock between the Precambrian Shield highlands and
the Niagara Escarpment suggests bedrock control on their development. It is likely that the Laurentian trough resulted
from long-term mechanical weathering associated with Pleistocene glacial erosion.
The Georgian Bay Basin, which stands at 80 m asl and
lower in the centre, ascends to the southeast along a ramp to
120 m asl at Wasaga Beach to connect the Laurentian trough,
suggesting increased erosion toward basin centre (Fig. 3A).
Like the northeastern part of the Huron Lake Basin, the
Georgian Bay Basin has a floor dissected with numerous linear depressions or valleys (Fig. 3A). These valleys were
probably scoured by subglacial meltwater during the Late
Wisconsinan (Kor and Cowell 1998). The linear bedrock
lows in the floor of the Laurentian trough can probably be
ascribed to a similar origin. Presently, thick drift material of
Late-Wisconsinan age occurs in the Laurentian trough
(Fig. 3B; Barnett 1992). Conceivably, drift material of preWisconsinan ice advances could have existed there and acted
as a surface armour, preventing the trough from excessive
glacial excavation. The convergence in planform and rise in
floor elevation toward the centre of the trough suggests that,
apart from the Georgian Bay ice lobe, the northward-moving
Ontario lobe also played a role in shaping this bedrock low,
consistent with the occurrence in the trough of the interlobate
Oak Ridges Moraine resulting from the coalescence of these
ice lobes during the Late Wisconsinan (Figs. 3B, 7; Barnett
1992).
Spencer (1890, 1907) first recognized the Laurentian
trough as part of the Georgian Bay Basin. However, the overall geometry and floor depth of the trough remained unknown at that time. Based on the drill records available, he
noted a bedrock surface deeper than the water level of GeorPublished by NRC Research Press
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Fig. 9. Bedrock valleys and the surrounding bedrock topographic features contoured at 10 m intervals. (A) Milverton, Wingham, and Mount
Forest valleys. (B) Dundas and Innerkip valleys. asl, above sea level. Boreholes OGS 03-5, OGS 04-04, DV-06, and DV-08 are from Bajc and
Hunter (2006), Bajc et al. (2009), and Zwiers et al. (2009); UW34-78 and 83-81 from Greenhouse and Karrow (1994); and G2, SL3, and M2
from Meyer and Eyles (2007). Refer to Fig. 4 for the coded bedrock valleys. Lower case letters are bedrock formations (refer to Fig. 1 for
bedrock geology).
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Fig. 10. Drift thickness and location of drill records. The bedrock topography is outlined by 10 m contours. The hill-shade relief represents
the present ground surface. (A) Milverton, Wingham, and Mount Forest valleys and surrounding area. (B) Dundas and Innerkip valleys and
surrounding area.
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gian Bay (176 m asl) and suggested the presence in this bedrock low of a preglacial Laurentian River channel that
drained the upper Great Lake Basins into the Lake Ontario
Basin (see Fig. 2A). In the subsequent studies, thalwegs, similar to that in Fig. 4, have been mapped in the Laurentian
trough and are believed to be the relict channel of this preglacial drainage often referred to as Laurentian Channel (White
and Karrow 1971; Eyles et al. 1993; Holysh et al. 2004).
However, as the current compilation shows, the trough has a
floor that exceeds 50 km in width and is 26 m and more below the current water level of Georgian Bay. The thalweg of
the trough is not the only area that could drain Georgian Bay
should the drift cover be removed. While it is possible that a
preglacial river, as Spencer (1890, 1907) speculated, flowed
in a pathway along this trough, any such valleys would have
been altered or removed during the subsequent Pleistocene
glaciations that shaped Georgian Bay and other Great Lake
Basins.
The “Laurentian Channel” is thought to have remained
open and drained Georgian Bay during the last interglacial
because of the occurrence of the Sangamonian deposits in
the Laurentian trough (Eyles and Williams 1992; Karrow et
al. 2001). But these sediments alone may not be used as evidence for the presence of such a drainage system. This is because they could simply be the deposits in local lakes or
rivers draining into Lake Ontario just like the present-day
Don River. Based on geophysical data, Eyles et al. (1985)
proposed the subsequent development of a large delta stretching from Barrie to Toronto in the channel during the early
Wisconsinan, implying that the channel was still open by
this time. However, except for the deltaic sediments in the
Scarborough Formation seen in the shore bluffs of Lake Ontario at Scarborough and the Rouge River valley in Toronto
to the north, there is no convincing sedimentological evidence to confirm this suggestion.
The Ipperwash trough can be regarded as a smaller version
of the Laurentian trough, resulting from glacial erosion of
Devonian shale. The ice lobes from the Lake Huron and
Erie basins moved in opposite directions and probably coalesced in this bedrock low. Consequently, the bedrock low
shows a convex-up longitudinal profile with a sill located
near the mid-point along the trough (Figs. 3A, 4). The Walkerton and Brantford–Welland bedrock troughs differ from
the Laurentian and Ipperwash troughs in morphology in that
they do not terminate in lake basins on both ends (Figs. 3A,
4). But both troughs are closely related to the bedrock and
align with the regional ice flows. It is likely that they resulted
from long-term weathering, in particular, the erosion by
Pleistocene glaciers.
Milverton, Wingham, and Mount Forest valleys
Assuming that, as Karrow (1973) suggested, the Milverton
and Wingham bedrock valleys had an eastward flow direction, then prominent upslope gradients occur (Fig. 8B). They
have irregular longitudinal profiles and contain segments that
start and terminate abruptly. Such features exclude the possibility for a normal fluvial origin. Instead, the valleys resemble tunnel valleys or channels carved by subglacial meltwater,
as described elsewhere (e.g., Sjogren et al. 2002; Jørgensen
and Sandersen 2006; Kristensen et al. 2007). Their alignment
with the pathway of Huron ice lobe of the Late Wisconsinan
813
is consistent with such an interpretation. Although the waterwell records provide limited details on the subsurface material, those in the Milverton valley indicate frequent occurrence of sand and gravels resting on the valley floor. Recent
boring at Wellesley confirms the occurrence of glaciofluvial
sand and gravels below Late-Wisconsinan till in the Milverton valley (Fig. 11; Bajc and Hunter 2006; Bajc et al. 2009;
Zwiers et al. 2009). Such a valley infill is consistent with a
tunnel-valley origin. It is worth noting that the term “tunnel
channel” is used to imply that water completely filled the valleys when they were formed (e.g., Clayton et al. 1999; Fisher
et al. 2005). To avoid this genetic connotation, the term “tunnel valley” was used in the discussion that follows.
The Milverton, Wingham, and Mount Forest bedrock valleys do not show prominent branching (Figs. 3A, 4, 9),
which is in contrast to an anabranching channel network
commonly proposed for tunnel valleys (Brennand and Shaw
1994; Praeg 2003). However, recent studies indicate that
anabranching is not ubiquitous, and the network may have resulted from onlapping or crosscutting of tunnel valleys
formed at various stages (Jørgensen and Sandersen 2006;
Kristensen et al. 2007). Valley crosscutting is seen at Wingham, where the Hutton Heights valley cuts through the
Wingham valley (Figs. 4, 8B, 9A).
Subglacial meltwater drained along the tunnel valleys, carrying subglacial debris to lakes or to outwash fans at ice
sheet margins. As such, the terminus of tunnel valleys commonly marks the marginal zone of the ice lobes (Mullins
and Hinchey 1989; Hooke and Jennings 2006). The Milverton valley ends at Wellesley, suggesting that the convergence
and breakup of the Laurentide ice sheet during the associated
ice advances was centered in this area, corresponding well to
the location of the interlobate Waterloo Moraine (see
Figs. 3B, 10). Between Drayton and Elora, a thick drift deposit occurs at the outlet of the Mount Forest bedrock valley
(Figs. 3B, 10), suggesting that its emplacement was probably
related to this tunnel valley. Future detailed boring in this
area may provide insight into this deposit and its relationship
with this bedrock valley.
Dundas valley and other re-entrants
The Dundas bedrock valley protrudes as the western extension of the Lake Ontario Basin (Figs. 3A, 4), likely resulting
from repeated cycles of glacial erosion during the Pleistocene. A series of arcuate-shaped moraine ridges occur around
the head of this bedrock gorge (Fig. 7). The valley infill contains primarily Late-Wisconsinan glacigenic deposits, as revealed by recent drilling in the valley (MacCormack et al.
2005; Zwiers et al. 2008, 2009; Bajc et al. 2009). Such a
landform and sedimentary setting indicates the latest glacial
overriding and erosion of this bedrock depression. A glacial
origin of the Dundas valley is consistent with the interpretations of other re-entrants on the Niagara Escarpment (Straw
1968; Kor and Cowell 1998).
Based on the morphometric features and glacial deposits,
Straw (1968) proposed a glacial origin for the re-entrants on
the Niagara Escarpment, including the Dundas valley. Because of the slight variation in valley strike, he suggested
that these valleys were a result of multiple glacial scouring
and expansions during the Late Wisconsinan. Kor and Cowell (1998) noticed that the deepest parts of Georgian Bay
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814
often coincide with the thalweg of the re-entrants at the
shoreline on the Bruce Peninsula. This relationship is interpreted as a result of raised subglacial stream velocity and increased erosion from flow convergence in the re-entrants in
front of the escarpment. Based on these observations, they
interpreted the re-entrants as tunnel valleys scoured by catastrophic release of stored subglacial meltwater. The Caledon
East re-entrant contains thick glaciofluvial sand and gravels
along the valley base, consistent with a subglacial flood erosion origin (Davies and Holysh 2005; Russell et al. 2006). A
more gradual sedimentary process is suggested for a similar
fill in the bedrock valley at Georgetown (Fig. 9B; Meyer and
Eyles 2007). In that study, however, the bedrock valley and
those in the vicinity are poorly defined, and their relationship
with and possible controls on the sedimentation remain unknown.
The small basin in the front of the 12 Mile Creek re-entrant at the base of the Niagara Escarpment (Figs. 6, 8D) is
thought to be a plunge pool of the Erigan preglacial valley
draining the Lake Erie Basin into the Lake Ontario Basin
(Feenstra 1981; Flint and Lolcama 1986). As mentioned earlier, similar pool-like basins also occur elsewhere at the base
of the escarpment, resulting from flow convergence in a subglacial meltwater setting (Gilbert and Shaw 1994; Kor and
Cowell 1998). Subglacial meltwater erosion is suggested to
have played a key role in shaping the 12 Mile Creek re-entrant
and the surrounding landscape (Tinkler and Stenson 1992).
The re-entrant is bordered to the south by the Fonthill
Kame, a deltaic complex formed when the Ontario ice lobe
was fronted by proglacial Lake Warren to the south (Feenstra
1981). Various glacial lakes developed in the middle part of
the Erigan valley, where broad bedrock basins developed in
less resistant Salina Formation (Fig. 6; Feenstra 1981). During the last deglaciation, for example, glacial Lake Wainfleet
drained into the Lake Ontario Basin through several outlets
on the Niagara Escarpment (Pengelly et al. 1997). As such,
the Erigan valley may have resulted from a combination of
glacial, subglacial meltwater and lacustrine erosion. Even if
a preglacial river draining the Lake Erie Basin did exist, as
suggested by Spencer (1907), the original valley may have
been intensively, if not entirely, altered or modified.
Discussion
The present bedrock topography has some inheritance
from a preglacial landscape (Horberg and Anderson 1956;
Karrow 1973). However, as evidenced by the bedrock valleys
and troughs, intense glacial and subglacial meltwater erosion
occurred during the Pleistocene, leading to deep carving and
alteration of the bedrock surface. In a glacial environment,
the resultant landscape tends to be uneven and complex,
marked by numerous highs and lows; the linear bedrock
lows such as tunnel valleys commonly occur as discrete depressions without tributaries. Realization of this is important
in regional subsurface geological mapping and groundwater
studies.
Catastrophic outburst of trapped subglacial meltwater has
been suggested to be responsible for the formation of tunnel
valleys under the Laurentide and Scandinavian ice sheets
(Wright 1973; Piotrowski 1997; Clayton et al. 1999; Cutler
et al. 2002; Russell et al. 2003; Fisher et al. 2005; Hooke
Can. J. Earth Sci., Vol. 48, 2011
and Jennings 2006; Jørgensen and Sandersen 2006). It may
be speculated that meltwater ponding at ice base occurred in
parts of the Huron Lake Basin, and the catastrophic release
of the stored subglacial water created the Milverton, Wingham, and Mount Forest bedrock valleys. It is suggested that
deep permafrost at ice margins impedes the substrate drainage and freezes the glaciers to the ground, helping raise the
hydraulic head for catastrophic release of stored subglacial
water (Piotrowski 1997; Clayton et al. 1999; Cutler et al.
2002; Hooke and Jennings 2006). As indicated by ice-wedge
casts and polygons, permafrost did occur in southern Ontario
during the Late Wisconsinan (Morgan 1982; Gao 2005). The
presence of permafrost would, thus, have created favorable
conditions for tunnel valleys to form in this region. It is noteworthy that subglacial sediments can be squeezed by deformation into small tunnels at ice base owing to increased
pore pressure, and the regular basal meltwater then removes
the debris under steady state. Such a process can gradually
generate deep tunnel valleys (e.g., Boulton and Hindmarsh
1987). However, the fact that the tunnel valleys in the study
area are in the Paleozoic bedrock, which does not deform appreciably under glacial stress, negates this model.
In explanation of the present landscape, Shaw and Gilbert
(1990) and Sharpe et al. (2004) proposed the outburst of two
subglacial megafloods responsible for the development of
tunnel valleys and drumlin fields across much of the Great
Lakes region during the Late Wisconsinan. The Milverton,
Wingham, and Mount Forest bedrock valleys have an alignment oblique to, and a flow direction against, the proposed
flow lines toward the south-southwest and west, indicating
that they are not related to such flood events. Instead, these
bedrock valleys were likely carved by different, both temporally and spatially, subglacial meltwater floods. Similarly, the
tunnel valleys in the floor of Georgian Bay show various
flow directions (see Fig. 3), and they may record multiple releases of stored subglacial water, as suggested by Kor and
Cowell (1998).
Faults exist in the Paleozoic bedrock in southern Ontario
and the basement faulting, as reflected by the aeromagnetic
lineaments, can displace or have the potential to fracture the
Paleozoic cover rock (Boyce and Morris 2002). Areas with
fractured Paleozoic bedrock are susceptible to erosion. This
is probably true for the Laurentian, Ipperwash, and Walkerton troughs that are within aeromagnetic linear zones. As opposed to this situation, the Long Point trench and the
Milverton, Wingham, and Mount Forest valleys have an
alignment oblique to the aeromagnetic lineaments. Although
the Dundas valley may have been fractured along some large
aeromagnetic linear zones (Boyce and Morris 2002), other
large re-entrants, such as the Owen Sound, Meaford, and
Beaver valleys, do not align with any aeromagnetic lineaments or faults. As such, faults may have provided favorable
conditions for selective glacial erosion, but they are not the
controlling factors on the development of the linear bedrock
depressions in southern Ontario.
The age of the bedrock valleys and troughs is difficult to
determine owing to a poor understanding of the subsurface
drift stratigraphy and the lack of detailed borings in these
bedrock lows. The bedrock troughs likely have experienced a
long-term weathering process typified by multiple cycles of
glacial erosion during the Pleistocene. In the Laurentian
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Fig. 11. Borehole records in the lower part of the Milverton bedrock valley (Bajc and Hunter 2006; Bajc et al. 2009; Zwiers et al. 2009). Note
the presence of sand and gravels and the Catfish Creek till in the bedrock valley. m asl, metres above sea level.
trough, the York till of the penultimate Illinoian glaciation
has been recorded below the last interglacial deposits (Eyles
et al. 1985; Eyles and Williams 1992; Karrow et al. 2001).
The occurrence of this till suggests glacial overriding and associated erosion of this bedrock low at least during the Illinoian Stage.
The Milverton, Wingham, and Mount Forest bedrock valleys are buried features without surface expression. This suggests an age older than the surficial sediments of till plains,
moraines, and eskers emplaced by the retreating glaciers after
the Port Bruce ice advance at 15 000 BP. The eskers indicate
a regional hydraulic gradient to the east and east-southeast
beneath the Huron ice lobe. They crosscut the Mount Forest
and Milverton valleys (Fig. 7), suggesting the development of
a later hydraulic gradient that differed from the previous one
responsible for those bedrock valleys. The eskers, on the
other hand, align well with the Wingham and Hutton Heights
valleys (Fig. 7), suggesting a similar hydraulic gradient under
that part of the ice lobe or, alternatively, the re-use of these
valleys by subglacial meltwater or both.
The drilling in the lower part of the Milverton valley at
Wellesley shows glaciofluvial gravelly deposits and till correlated to the Catfish Creek till (Fig. 11; Bajc and Hunter
2006; Bajc et al. 2009; Zwiers et al. 2009). This till was deposited during the Nissouri ice advance (Bajc and Shirota
2007), and its presence suggests that this valley probably developed during or shortly after the Late-Wisconsinan glacial
maximum around 20 000 BP. There is also a possibility that
this valley predated the Late Wisconsinan, e.g., developed
during the Early Wisconsinan, but was re-used during the
Nissouri ice advance. To prove this, it requires a thorough
understanding of the relationship between the valley infill
and the drift in the immediately adjacent region. However,
the current data available does not allow any detailed evaluation of these sediments as to their sedimentary facies, age,
and lateral extent. Boreholes in the re-entrants along the base
of the Niagara Escarpment all indicate a valley fill with a
Late-Wisconsinan affinity and hence a minimum age for
these bedrock lows (Davies and Holysh 2005; MacCormack
et al. 2005; Russell et al. 2006; Meyer and Eyles 2007; Bajc
et al. 2009; Zwiers et al. 2009). Glacial deposits predating
the Catfish Creek till have been reported to occur in the
Elora and Rockwood bedrock valleys, suggesting an age
older than the Nissouri ice advance (Greenhouse and Karrow
1994). This means that the buried bedrock valleys in southern Ontario likely developed during various stages. Only can
future detailed boring in this region provide the needed insights that would help better understand the subsurface till
stratigraphy and provide age control on the buried bedrock
valleys.
Conclusions
The recent compilation of the bedrock topography by the
Ontario Geological Survey has enabled better delineation of
the regional extent of significant buried bedrock valleys in
southern Ontario. This is largely attributable to the rigid quality control measures adopted in this compilation to track and
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816
eliminate the problematic borehole records, in particular,
those in the water-well database that contains over half a million records. The morphometric features suggest that these
large linear bedrock depressions were carved through glacial
and subglacial meltwater erosion. The bedrock topography is
uneven and complex, containing numerous highs and lows.
This view differs from the previous views for simple modification of a preglacial landscape and features such as river
valleys.
The Laurentian bedrock trough is the southeastward extension of the Georgian Bay Basin, resulting from long-term
mechanical weathering associated with Pleistocene glacial
erosion. It has a valley floor that exceeds 50 km in width
and is 26 m and more below the current water level of Georgian Bay. This bedrock low could drain Georgian Bay should
the drift cover be removed. There is no evidence to suggest
that a preglacial river channel, if it existed, is still preserved
in the floor of this bedrock depression. The Milverton, Wingham, and Mount Forest bedrock valleys are discrete, rectilinear to slightly curved bedrock gorges without prominent
branching features. Their undulating longitudinal profiles
and upslope gradients exclude the possibility for a normal
fluvial origin. Instead, such features indicate a tunnel valley
origin related to subglacial meltwater erosion under the
Huron ice lobe. The till stratigraphy of the valley infill suggests the development of these bedrock valleys probably during or shortly after the Late-Wisconsinan glacial maximum.
Lastly, the Dundas and Milverton bedrock valleys are two
different systems and there are no comparable bedrock valleys existing between them for a joint drainage system as previously suggested. The Dundas valley is the westward
extension of the Lake Ontario Basin, resulting from glacial
to subglacial meltwater erosion. This is evidenced by the valley infill consisting of glacial and related deposits of the Wisconsinan.
Acknowledgments
The author wishes to thank Jiro Shirota, Steve van Haaften, and Frank Brunton for technical support in compilation
of the bedrock topography map for this study, and Peter Barnett for useful discussions. Andy Bajc, Ross Kelly, and Cam
Baker reviewed an early version of the manuscript and provided critical but helpful comments and suggestions. Thanks
are also extended to John Shaw, Phil Kor, and the Associate
Editor Timothy Fisher for their detailed reviews and comments that greatly improved the quality of this manuscript.
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