Permafrost, Phillips, Springman & Arenson (eds)
© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7
Postglacial talus-derived rock glaciers in the Gaspé Peninsula, Québec (Canada)
B. Hétu
Université du Québec, Rimouski, Canada
J.T. Gray, P. Gangloff & B. Archambault
Université de Montréal, Montréal, Canada
ABSTRACT: Many talus-derived rock glaciers have been observed in the Gaspé Peninsula and three of these
were studied in detail. Stratigraphy and 14C dates have established that two rock glaciers in a coastal valley
(15–70 m a.s.l.) were active during the Younger Dryas and Early Holocene. They attest to the presence of permafrost bodies near sea-level after 9 ka BP. A third talus-derived rock glacier, situated inland in the Cascapédia
Valley, with a steep front at 250 m a.s.l., and a massive ice lens exposed during recent excavation, has readvanced
over an organic soil dated at 2190 BP (DIC-1898), suggesting a Neoglacial active phase.
1 INTRODUCTION
Fossil rock glaciers have been observed at several locations in the NE Appalachian highlands and on the
southern rim of the Canadian Shield in eastern North
America, notably in New Hampshire (Thompson
1999), in the Gaspé Peninsula (Archambault 1991,
Baron-Lafrenière 1983, Gray & Brown 1982, Gray &
Hétu 1981, Hétu & Gray 2000a), in Charlevoix (Govare
1995) and in Newfoundland (Grant 1994). With the
exception of the rock glaciers in the coastal valleys of
Gaspé (Archambault 1991), discussed in this paper, the
age of these features remains unknown, being vaguely
attributed to an early postglacial period situated from
14–10 ka BP according to the study region.
At Mont-Saint-Pierre, rock glaciers fed by debris
from talus slopes have advanced over proglacial deltas
and shell-bearing marine terraces, the latter 14C dated
(Archambault 1991). The main objective of this paper
is to present new data on the chronology of protalus rock
glaciers situated at relatively low altitudes in Gaspésie.
This study will shed new light on the postglacial history
of permafrost in eastern Canada.
Figure 1. Location of rock glacier study sites. The MontSaint-Pierre Valley contains 8 rock glaciers. Thick lines:
isochrones for deglaciation, after Richard et al. 1997.
Goldthwait Sea invaded the coastal valleys in the
vicinity of Mont-Saint-Pierre up to an altitude of 55 m
(Hétu & Gray 2000b).
3 METHODOLOGY
Eight fossil rock glaciers have been observed near the
coast in Mont-Saint-Pierre Valley and two have been
excavated in order to study their stratigraphic context.
Evidence from rock glacier still possessing buried
ground ice in the Cascapédia Valley (fig. 1) in the
interior of the Gaspé Peninsula will also be discussed.
2 STUDY AREA
The study area is situated in the northern and central part
of the Gaspé Peninsula (Québec, Canada), on the south
shore of the Saint Lawrence Estuary (fig. 1). The present
mean annual temperature is 3.5°C (July: 17.4°C,
January: 10.0°C) at sea-level, and ca 4°C at the
summit of Mt. Jacques-Cartier (1268 m) (Gagnon 1970).
This region was covered by a regional ice-cap during the Last Glacial Maximum, and deglaciation
occurred diachronously, from 13.3 ka BP on coastal
headlands, to 10 ka BP inland (Hétu & Gray 2000b,
Richard et al. 1997). As deglaciation proceeded, the
4 STRATIGRAPHIC FRAMEWORK OF
THREE ROCK GLACIERS
4.1
The Coulombe rock glacier
The Coulombe fossil rock glacier is located near the base
of the west flank of the valley of Mont-Saint-Pierre,
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glacier front itself. The exposure is composed essentially of beds of normally sorted angular needle shaped
clasts, alternating with well-sorted beds of angular
gravels. The stratigraphic transition from basal beds dipping at a low angle to steeply dipping beds is attributed
to frontal advance of the rock glacier overridding debris
previously accumulated as talus at the base of the front.
Exposure B, also in the lower part of the frontal slope,
shows a similar burial by rock glacier advance of gently
inclined basal talus debris (units 5–7: dip of 12°),
beneath steeply dipping debris (unit 8: 25°). Units 1–4,
below the colluvial debris form the uppermost part of
the proglacial delta sequence.
Exposure C, at the northern extremity of the rock
glacier, at ca 25 m altitude shows the lateral transition
from rock glacier deposits to those associated with the
fluvioglacial delta (fig. 2). Steeply inclined strata in
unit 1 at the base of the sequence represent delta foreset beds. They consist principally of smoothly rounded
clasts, but with angular shale particles from the rock
glacier as a secondary component. Units 2 and 3 show
a progressively increasing proportion of angular shale
and sandstone clasts dispersed in a matrix of coarse,
sorted, stratified and well rounded sands. Clearly, the
sandy matrix is associated with continued transport by
glacial meltwater, which also eroded, and incorporated
into its bed, angular clasts from the rock glacier front.
This important observation indicates that rock glacier
advance was initiated prior to the end of proglacial
delta construction. Further advance of the rock glacier
is demonstrated by unit 4, entirely composed of angular fragments of locally derived shales and sandstones.
This unit can be followed directly into the steeply sloping rock glacier front, 2 m upslope.
The stratigraphic relationships between proglacial
delta and colluvial rock glacier deposits, in these
exposures, allow a chronological framework to be established for the active phase of the rock glacier. The northern part of the rock glacier front was in contact with a
delta still in its final phase of construction, but the
southern part must have advanced later over the same
delta surface, after the latter had been abandoned due
to glacio-isostatically induced emergence. A shell-bearing marine terrace at about the same altitude (25–30 m)
on the opposite flank of the valley yielded the following 14C ages: 10 330 100 BP (DIC-1647) and
10 160 120 BP (Beta-32027) (cf. Hétu & Gray
2000b). These ages indicate that the rock glacier was
still actively advancing at the end of the Younger
Dryas phase.
3 km inland. Its front, oriented ENE, forms a pronounced lobe overlying a proglacial delta surface situated at 25–35 m above present sea-level, and gives way
upslope to a talus slope. The delta was built for the most
part during an early phase of the Younger Dryas event
(Hétu & Gray 2000b). The morphometry of this rock
glacier permit its classification as a talus-derived rock
glacier (Humlum 1996). It forms a long concave segment extending outwards from the base of the talus
slope, and is much wider along the slope (730 m) than
long downslope (170 m). It is relatively thin, given its
spatial dimensions. Its frontal thickness, 20–25 m at the
centre, diminishes at its northern extremity to 15 m and
at its southern extremity to 15–18 m. A minimum volume of 2 005 500 m3 has been calculated for this rock
glacier at the base of a slope of ca 650 m length. The
volume of debris in the rock glacier, allowing for the
calculation of porosity, represents an average removal
by erosional processes of 4 m from this slope.
The top surface of the rock glacier which has a general slope of 3°– 6° towards the valley-floor is characterised by low transverse undulations, which may be the
surface expression of thrust planes associated with
internal mass movement. Although a closed depression
15 m in diameter and 2–3 m deep in the northern part is
the only indicator of the former presence of buried massive ground ice, it should be stated that the surface morphology has undoubtedly been modified by slope
processes which have continued to operate well after the
rock glacier itself became inactive. A debris flow cone
has spread out over the entire rock glacier surface on its
southern side, fed from two long gullies on the slope
above. A 14C age of 1090 50 BP (DIC-1644) for an
organic horizon buried beneath 1.25 m of debris in the
central part of the rock glacier indicates the occurrence
of a similar event in another gully. Periodic transfers of
debris by such events are likely to have filled small
depressions associated with melting ground ice.
Three exposures were excavated by mechanical
shovel between the toe of the rock glacier and the
underlying proglacial delta (fig. 2). The rock glacier
debris is easily distinguished from the underlying
deltaic sediments on the basis of clast form and lithological provenance. The rock glacier clasts are angular
and of very local origin (shales, siltstones and sandstones), whereas the delta clasts are well rounded and
of varied petrography (granites, syenites an metabasalts
brought by glaciers and meltwater from distant sources
make up 2.9% of the total number).
The deltaic sediments represented as stratigraphic
unit 1 in Exposure A, are overlain by units 2–13 within
the rock glacier front (fig. 2). The latter are composed
of two groups distinguished by a contrast in dip angle.
Units 2–6 are inclined towards the valley floor at
15°–18°. The overlying units (7–13) are inclined in the
same direction at 34°, an identical angle to the rock
4.2
The Mercier rock glacier
This fossil rock glacier, also a talus-derived rock glacier, is situated on the eastern side of the valley. It
390
Figure 2. Stratigraphy of the frontal zone of Coulombe rock glacier. Exposures A, B and C are localised in photo D.
Photo E is a detailed view of the rock glacier sediments (19 cm long notebook for scale).
transition to a shell-bearing marine terrace with
(Mytilus edulis, Macoma balthica, Balanus crenatus
Species), which furnished a date of 9150 95 BP
(DIC-1281). Several trenches excavated for garbage
disposal revealed the stratigraphic contact between
the base of the rock glacier and the underlying fluvial
sediments. Significantly, no trace of a buried soil was
observed at this contact.
These trench exposures also displayed the internal
structure of the rock glacier front. One, cut into a low
terrace abutting against the rock glacier front at its
southernmost extremity, shows several matrix dominated diamicton layers, between 0.25 and 1.75 m thick,
extends outwards from the base of a 640 m long talus
slope, whose upper half is subject to frequent geomorphic disturbance, but whose lower half is entirely
wooded. Contrary to the Coulombe rock glacier the
Mercier rock glacier is very narrow (30–130 m) in
comparison to its length (750 m). It also has a much
higher (40–50 m) and steeper (35°–40°) front. The top
surface slopes downwards at 10°–15°, with several
undulations, towards the front. No ground ice collapse
pits, or thrust-related furrows and ridges were observed on this surface.
This rock glacier advanced over an alluvial plain,
situated at 20–25 m a.s.l. The latter shows a gradual
391
and debris flows from the latter could easily have traversed its surface, several times, to progressively build
a colluvial debris fan (fig. 3). The dated organic layers
suggest that these geomorphic processes occurred
mainly between 6300 and 3700 BP. No debris flows
have since reached the base of the rock glacier.
The chrono-stratigraphic evidence permits the active
phase of the Mercier rock glacier to be established
between the deposition of the underlying alluvial plain,
at 9200 BP and the construction by debris flows of the
diamictons, the earliest of which began to mask its front,
towards 6300 BP.
intercalated with nine organic horizons two of which
contained terrestrial shells (fig. 3). The clasts in these
diamictons are composed of angular fragments of the
local shale, siltstone and sandstone bedrock and have
an obviously local origin, as opposed to glacial
diamictons in the region. The matrix is composed of
poorly sorted silty sands.
The organic layers, from 2–4 cm thick have 2.7%–
12.1% carbon content (mean 6.8%; n 7). Five
dated layers gave ages between 6300 100 BP and
3690 70 BP (fig. 3). Two of the nine layers exposed
contained millimetric sized molluscs of four terrestrial
species. For organic layer 2 Helicodiscus parallelus
(Say), Helicodiscus singleyanus (Pilsbry) and Discus
cronkhitei catskillensis (Pilsbry) were noted, and for
organic layer 7, the same species and also Pisidium
casertanum (Poli). The three species belonging to the
genus Helicodiscus and Discus are closely associated
with the rotting tree trunks and decaying leaf litter of a
damp forest floor. Pisidium casertanum is a freshwater
mollusc associated with stagnant ponds and marshes.
Thus, these organic layers clearly represent buried
Ah soil horizons. They show that the diamictons were
laid down during recurring disturbance of a damp and
poorly drained forest floor environment. Archambault
(1991) has interpreted them as solifluction lobes related
to the thawing of permafrost in the rock glacier core.
However such an explanation is contradicted locally
by the palaeo-ecological evidence, adduced from the
terrestrial molluscs above, and also regionally by palynological data, which attests the presence of a closed
forest cover on the northern Gaspé coast for at least the
last 7000 years (Marcoux & Richard 1995).
Recent observations suggest an alternative interpretation for these diamictons. On the 18th of June, 1998
debris flows triggered on the talus slope by a torrential
rainstorm extended across a 100 m long section of the
Coulombe rock glacier, spreading debris onto the delta
surface, below. The Mercier rock glacier at its southern
extremity is only 30 m from the base of a steep slope,
4.3
The Cascapédia rock glacier
This talus-derived rock glacier (fig. 4) located in the
interior of the Gaspé Peninsula, alongside highway
299, ca 50 km north of the Baie des Chaleurs (48°27N;
66°12W), has been described in some detail by Laurin
(1981). As in the case of the examples discussed above,
it forms a protalus lobe, extending several hundred
metres down to a base elevation of 250 m a.s.l. Its steep
front exceeds 30 m in height, over a width of ca 100 m.
Its top surface is characterised by small circular depressions, possibly formed by ground ice thaw, and by transverse ridges associated with multiple flow phases. The
rock glacier is separated from the talus slope above by
a 7–8 m deep depression (fig. 4). The debris both on
the talus slope and the rock glacier surface is composed of large angular blocks (a axis: 10–120 cm) of the
locally outcropping sandstones of the York Formation.
Forest clothes ca 70% of both surfaces.
Brief excavations of the steep rock glacier front in
1979 by the Ministère des Transports du Québec for
road materials exposed a 5–10 m thick debris rich ice
lens. The presence of ground ice elsewhere in the rock
glacier is confirmed by cold air ventilation from the
interstices between boulders at the base of a 3 m deep
depression on the top surface (Gray & Brown 1982).
Figure 3. Stratigraphy of old debris flow at the Mercier rock glacier front.
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valley. They continued their advance after 9200 yr BP,
possibly until the end of the early Holocene cold phase
between 8650 and 7250 yr BP, evidenced by the palynological evidence of Marcoux & Richard (1995).
They are now fossil features. A younger generation of
rock glaciers formed at higher elevations in the Gaspé
interior as glacial retreat progressively liberated the
steep slopes of valleys and cirques. One of these, the
Cascapédia rock glacier has present day ice masses in its
core, and displays evidence of Late Holocene activity.
The presence of the coastal rock glaciers, 1000 m
below the lower limit of regional permafrost (Gray &
Brown 1979, Gray & Brown 1982), suggests a mean
annual air temperature (MAAT) during the Late
Glacial-Holocene transition at least 6.5°C cooler than
that of 3.5°C prevailing today on the coast (assuming
an adiabatic lapse rate of 0.65°C par 100 m). The
resultant value of 3.0°C or colder fits well with the
observation by Humlum (1998) that « very few examples of active rock glaciers occur for a MAAT higher
than about 2°C, and the majority of active rock glaciers occur at sites with MAAT below 6°C… »
(p. 390). However, a word of caution should be injected
here, as local terrain factors such as steep shadowed
slopes, and the insulating effects afforded to internal
ice masses by cold dense air in the overlying debris
cavities (the Balch ventilation effect; Gray & Brown
1979), may have permitted the continued existence of
buried ice and active advance of rock glaciers well
below the regional permafrost limit.
The presence of permafrost on the marine terraces
and deltas which emerged during the Younger Dryas is
well established, on the basis of numerous ice wedge
casts both in southern Québec (Govare & Gangloff
1989, Hétu 1994) and in the Maritime Provinces
(Anderson & Macpherson 1994), but it was not previously possible to determine how long this permafrost
lingered. The data presented here indicates that, at
least sporadic permafrost was present near sea-level
as late as 9200 BP. The data from the Cascapédia rock
glacier indicates the continued presence through the
entire Holocene of such permafrost bodies in the Gaspé
Peninsula, at an elevation as low as 250 m a.s.l., as
well as probable rock glacier advance, within the last
2000 yrs BP.
Figure 4. Upper: Long profile of Cascapédia rock glacier.
Lower: Stratigraphy of pit at the base of the rock glacier
front (arrow).
A pit excavated to a depth of 5 m at the base of the
rock glacier front revealed a clast supported angular
debris layer of 0.75 m, consisting exclusively of the
locally derived sandstone lithology, overlying a glacial diamicton composed of a mixture of rounded and
angular clasts of local sandstone and foreign crystalline
origin, set in a sandy matrix (fig. 4). The upper 60 cm
of this diamicton appears to be characterised by a
buried podzol. A 2–10 cm thick zone of finely disseminated organic material dated at 2190 60 BP (DIC1898) marks the transition from the angular clast layer
to the diamicton. The upper 50 cm of the latter displayed the orange B horizon of this soil. This date and
the presence of present-day ice bodies in the interior
of the rock glacier suggest that the rock glacier
remained active until late in the Holocene.
5 DISCUSSION AND CONCLUSION
ACKNOWLEDGMENTS
The exposures described here from three rock glaciers, as well as summary observations on several
other rock glaciers in the coastal valleys and in the
interior of Gaspé, indicate the existence of two generations of such features. The oldest fossil Rock glaciers
in the coastal valleys began to form during, and probably also prior to the Younger Dryas phase, as glaciers
gradually liberated the base of steep talus slopes of the
Diane Coll, Louise Dion and Richard Laurin assisted
in the field-work. The figures were drawn by Suzanne
Gagnon. This research was made possible through
Natural Sciences and Engineering Research Council of
Canada grants. The authors also express their thanks
to Dr O. Humlum who reviewed the text and made
several useful suggestions.
393
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