IRISH GEOGRAPHY
(BULLETIN OF THE GEOGRAPHICAL
SOCIETY OF IRELAND)
A GLACIALLY BREACHED WATERSHED IN
DONEGAL
BY G. H. DURY
Glacial breaching of watersheds involves the crossing of
high ground by ice which, eroding the rock over which it
passes, modifies or destroys sections of the water-parting.
According to the severity of erosion within the breaches, the
watersheds of post-glacial times may depart little or much
from the pre-glacial lines. Effects of glacial breaching on the
pattern of streams range from negligible alteration to profound
rearrangement. Where glacial breaches are identified it may
be possible to reconstruct the former system of watersheds,
and thus to assess the influence upon the drainage-net of
glacial and pro-glacial erosion.
Glacial breaching has long been recognised on the European
mainland. Its effects upon the axial divide of the Scandinavian peninsula were noted bv Suess (1) as early as 1888.
Penck, Bruckner and H. Soldi in the early 1900s (2, 3, and
4) laid down principles which were well summarised and
illustrated by W. M. Davis in 1912 (5); Penck applied the
terms diffluence and transfluence to the two aspects of icemovement across watersheds. Tarr (6) in 1908 insisted on the
occurrence of through valleys in the Scottish Highlands, and
both Louis (7) and J. SOlch (8) in the 1930s recognised glacial
breaches there. Paradoxically enough, the influence of the
teaching of normal erosion by W. M. Davis and his followers
seems to have been so strong that for a long time interpretations of the drainage of the Scottish Highlands took little
account of the influence of ice on the pattern of streams.
Many anomalous elements of drainage were thought to be
explicable by river capture. In recent years, however, D. h.
Linton has re-examined a number of supposed captures
(9,10,11,12,), and has shown that glacial breaching of watersheds rather than diversion of competing rivers in the subaerial
171
CORRIE
CULLY
SIDE OF TROUGH
HIGH PLATEAUX
6 PEAKS
CONTOUR. F
BENCH . U N
PRESENT DIVIDE
FORMER DIVIDE
LOCATION-viTEXl
*•...•"
DERHYVEAGH
MOUNTAINS
FIG.
1.
Based on the Ordnance Survey by permission of the Minister for Finance.
cycle is their true cause. The present writer has described a
glacial breach in the main divide of the N. W. Highlands
through which a river now passes (13). Glacial breaching
is known to have entirely disrupted the major watershed
which, in the Snowdon district, bounded the preglacial
basin drained by the headstreams of the Dee (14, 15). It is
clear that the present pattern of relief and drainage in formerly
glaciated highlands can scarcely be understood unless the
effect of glacial breaching is assessed. The object of this paper
is to describe some of the glacial breaches which have already
been identified in Donegal, and to attempt a reconstruction of
part of a pre-glacial watershed.
The district to be discussed includes the Errigal—Muckish
ridge and its immediate surroundings (Fig. 1). Faults running
roughly parallel to the long axis of the ridge bring thick bands
of quartzite and of mica-schist against one another. Summits
exceed 2000 feet in height, and the ridge rises abruptly both
from the lowland on the N.W. and from the structure-guided
valley on the S.E. Height and distinctiveness alike are probably accounted for by the resistance to erosion of the quartzite ; in the study of glacial breaches it is often found that
breached divides are located on particularly resistant rocks,
a circumstance which suggests that pre-glacial drainage had
become well adjusted to structure. Peat conceals part of the
floor of the trough drained by the Calabber (10 in Fig. 1), but
at the lower end parallel bands of basic igneous rock intruded
into mica-schist are well exposed. It is obvious that this assemblage of rocks is far less resistant than the rocks of the
Errigal-Muckish ridge.
The ridge, more than seven miles long, is known to have
been crossed by northward-moving ice. Charlesworth's
detailed study of 1924 (16) shows conclusively where the
main ice-shed lay, and in which directions the basal ice moved.
There is no need to emphasise the great number of fieldobservations on which his account is based, but it may be well
to recall that in establishing the line of the main ice-shed and
in tracing directions of ice-movement he made use of the
combined evidence of striae, roches moutonnees, and erratics.
He remarked (op. tit., p. 209) that ice from the south passed
over the whole length of the Errigal-Muckish ridge, the lower
layers being constrained by the hills and the lines of flow
guided by the deep portals. But although he noted that ice
entered and eroded the heads of certain valleys (see, for
example, pp. 206-208), and carefully reviewed the evidence
of the diversion of drainage at and near the ice-fronts, he
does not seem to have discussed in detail the effect on water173
174
sheds of glacial breaching. These remarks are by no means
intended as adversely critical; Charlesworth's treatment of
drainage in relation to glaciation was far in advance of that
of the geologists who, in the Memoir on North-West and
Central Donegal (17), had stated in 1891 ' The glaciation of
the Ice Age did not alter the direction of the streams, but only
modified slightly the channels, in some places eroding hollows
which afterwards became lakes ; so that, on the disappearance
of the ice and snow, the streams resumed their former courses.'
Nevertheless, it can be maintained that there are still glacial
effects in Donegal which await study and description.
At glacial maximum, then, ice moved east, north and west
from the Derryveagh and Glendowan Mountains. As indicated on Charlesworth's general map, the basal ice swung
obliquely across the lower valley of the Owencarrow towards
Sheephaven, and across the valley of the Gweebarra towards
the Rosses. The Errigal-Muckish ridge was overwhelmed.
Northeast of Muckish (2197 ft.) a long spur projects to
Crockatee (962 ft.). Part of the crest of this spur—none of
which is shown in Fig.l—appears to belong to an erosional
platform at about 800 ft. above sea-level; remnants of the
same platform have been identified elsewhere in Donegal.
Although at glacial maximum the spur was thickly covered by
ice, the crestal watershed here seems to have been little
modified, probably because the ice here was little constricted.
In Muckish Gap, on the other hand, northward-moving basal
ice was confined in a pass between Muckish itself and Crocknalaragagh (1554 ft., 3 in Fig. 1), and considerable erosion took
place. The whole southwestern side of Muckish was severely
shorn (Fig. 2b). The sill of the pass appears to have been
ground down, erosion being greatest on the northwestern
side, that is,. at the outlet end, so that the watershed was
displaced southeastwards, i.e. in the direction opposite to
that of ice-movement. Thus it came about that the present
watershed lies at the very entrance to the gap from the
southeast, where a fine roche moutonnie (1 in Fig. 1 ; Fig. 2c)
occurs with its scoured side facing the broad valley of the
Calabber and its plucked side overlooking the deep trench of
the inner gap.
Displacement of a watershed in the direction opposite to
that of ice-movement is to be expected wherever ice has
crossed pre-glacia] divides, whether as the diffluent ice spilling
laterally from a valley glacier across a minor watershed, or
as the deeper transfluent ice of a sheet that rises high enough
to cross a major watershed. Where, as in Muckish Gap, the
watershed has been merely displaced and not destroyed, it may
175
well be possible to suggest a level for the sill of the pre-glacial
col. The present watershed lies above 800 feet on solid rock at
the end of the pass, and it may be assumed that the pre-glacial
sill rose higher. An upper limit is perhaps set by the hanging
headstream of the Duvowen River on the southwestern side,
which commences a sharp fall at about 1000 ft. This stream
may formerly have drained to the south, not to the north. If
so, the watershed in the gap has been displaced nearly half a
mile and the sill lowered by some 150-200 ft.
Immediately north of the present sill glacial erosion of the
floor of the pass was severe. The transfluent ice seems to have
spilled into a corrie on the northwestern flank of the ridge.
Similar observations of the entry of ice into a corrie across
the headwall have been made elsewhere in Donegal, and a
few of these will be noticed presently. It can be suggested that,
when a corrie is so invaded by transfluent ice, the headwall
continues to retreat, for it is subject to the erosive processes
which attack rock-steps in glacial troughs. Indeed, it may
suitably be pointed out here that Lewis (18) maintains that
the erosion of corrie-headwalls and that of rock-steps in glacial
troughs is probably similar, because in each case the ice is
liable to rotational slipping and the steep face is repeatedly
plucked. When a corrie is buried by transfluent ice the headwall may be severely attacked, provided that the ice can
escape freely through the new outlet; perhaps the deep
trenching in the inner part of Muckish Gap is to be accounted
for in this way. In the waning stages of glaciation, however, a
corrie glacier seems to have re-established itself, depositing the
large steep-sided mounds of debris which are prominent
features today ((2) in Fig. 1).
The second gap to be described lies between Crocknalaragagh and Aghla Beg. It is now occupied by Lough Aluirg.
The present watershed runs round the southern end of the
Lough, i.e. as in Muckish Gap, along the southeastern flank of
the ridge. Lough Aluirg spills nouthwards through a tiny
gorge between two corries (4, 5 in Fig. 1), the floors of which
are broken by mounds of moraine and by rock-knobs with
their plucked faces towards the north. On both sides of the
pass occur other rock-knobs similarly oriented, while the low
divide at the southern end of the lough consists of rock in place
with a plucked side facing into the pass. Here again it is quite
clear that northward-moving ice entered and traversed a preglacial col. The line of the pre-glacial watershed is possibly
represented by spurs which run down from either side towards
the outlet of the lough. If so, and if no great lowering has taken
place, the former sill probably reached a height of about
176
1000ft. above present sea-level. The composite valley-head
now represented by Lough Aluirg and the flanking corries
drained, on this view, to the south; the watershed is therefore
thought to have been displaced from one end of the pass to the
other, a distance of half-a-mile.
Between Aghla Beg (1860 ft., (6) in Fig. 1) and Aghla More
(1916 ft.) there are two rather shallow corries which contain
the tarns of Lough Nabrackbaddy (7) and Lough Feeane
(8). Although this part of the divide was also crossed by
ice there was no deep col ready for conversion to a glacial
breach. Seen from the south, the crestal watershed hereabouts
is manifestly intact. . Immediately southwest of Aghla More,
however, there seems to have been a marked col, which was
traversed, and very severely eroded, by a powerful stream of
basal ice. It was converted to the trough now occupied by
Altan Lough (Fig. 2a).
Although there is little to show exactly where this part of
the pre-glacial watershed ran, a few relevant observations can
be made. Perhaps the' most important of these is that the
whole side of Aghla More has been shorn. Now the shearing
of a mountainside in a glacial breach corresponds, but on a
larger scale, to the truncation of spurs in a glacial trough.
The shorn face, or the truncated end of a spur, is highest on the
line of the pre-glacial crest. For this reason it seems likely that
the watershed formerly ran fairly directly from Aghla More to
the high ground on the opposite side of the pass, as is suggested
by the thick pecked line in Fig. 1. It is true that spurs descending from either side tend to enclose the head of Altan Lough
( e.g. (9) in Fig. 1), and that these might be taken to represent the now-destroyed sill; on the other hand, they may
be no more than the remnants of unimportant lateral divides
between small feeders of the former main stream. At the
lowest, the former sill-level seems unlikely to have lain much
below 700 ft; at the highest, it can scarcely have exceeded the
height of the subdued summit of Beaghy ((12) in Fig. 1) which
is bounded by the 1000-ft. contour.
Hereabouts the low ground is much encumbered by coarse
rock-debris and by blanket bog. Visible rock-knobs are
consequently few at low levels, but the passage of ice into the
breach is well attested by a large plucked rock-face which,
nearly half a mile ESE of the head of Altan Lough, faces into
the breach. The actual watershed lies well outside the gap,
about a mile from the lough ; it consists of an imperceptible
divide on the very boggy floor of a glacier trough ((11) in Fig.
1). Streams which formerly discharged towards the NE along
a broad valley are now directed through the gap to feed the
177
Tullaghobegly River; indeed, one stream from, the flanks of
Dooish, in the Derryveagh Mountains, now crosses both the
line of the trough and that of the former watershed on the
farther side.
Southwest of the Altan breach ice passed across the flank of
Errigal, descending into the two corries marked (13) and(14)
in Fig. 1. The headwall of the latter, eroded in Errigal itself,
is clearly seen from the east to cut sharply across the smooth,
scoured hill-face on the opposite side of the crest. The corrie
seems, as it were, to have been punched out of the floor of a
col; the pre-glacial watershed here may therefore have run
north of the line marked in Fig. 1, especially if the sill of the
gap has been eroded by ice, as the scoured aspect of its floor
suggests.
The reason for the severer scour in the Altan breach than in
Muckish Gap or at Lough Aluirg is clear from Charlesworth's
map. Immediately to the south lies the central mass of the
Derryveagh Mountains, on the line of the main ice-shed. It
is clear that great masses of ice accumulated here even before
glacial maximum, for the NW face of the mountains is scalloped by large corries ; those at the head of the Poisoned Glen
discharged round the west of Errigal, but that now occupied
by Croloughan Lough ( (16) in Fig. 1) added its output to the
congested ice between the Derryveagh Mountains and the
Errigal-Muckish ridge.
It can be concluded from what has been said that the effects
of glacial breaching on the watershed of the Errigal-Muckish
ridge are clearly displayed and simple to understand, and that
they become more marked as Errigal is approached from the
direction of Muckish. Two related problems, however, are still
unsolved. One is the origin of the Poisoned Glen, the other
is the relation of the Owenbeg to the Owenwee.
Although the composite valley adjoining the Derryveagh
Mountains on the N.W. side is less clear-cut than is the narrow,
fault-guided trench which contains Glen Lough, Lough Veagh
and the Gweebarra, it is nevertheless well-marked ard is
obviously aligned on the underlying structures. In these respects it resembles the Errigal-Muckish ridge. It has been
suggested above that pre-glacial drainage was well adjusted to
structure, and the anomalous course of a stream from the
Derryveagh Mountains across the line of the valley and through
the Altan breach has been laid to the account of glacial erosion.
But the Cronaniv Burn, draining the Poisoned Glen, crosses the
continuation of the same line above Dunlewv. The Poisoned
Glen is so large, and the branching corries at its head so
typical of a headwater basin modified by glacial erosion, that
178
it seems most likely to represent a pre-glacial valley.
The question then arises whether this valley was drained in
pre-glacial times along the line of Dunlewy Lough or whether
it had an outlet to the NE or to the SW. A former southwestern
exit is highly unlikely, for the land on this side rises well
above 1000 ft. On the NE, however, a sizeable bench ( (15) )
in Fig. 1) stretches from the valley of the Owenwee into that of
the Owenbeg. If this bench, lying mainly between 600 and 900
ft., represents an old valley-floor, through which the glacial
troughs and the post-glacial gullies have been cut, the preglacial watershed appears to have crossed it near the present
highest point, where solid rock in place is exposed. If so, the
divide between drainage to the NE and that to the SW ran
roughly east-west from Dooish to Errigal, and the Poisoned
Glen discharged then as now round the western side of the
latter. This conclusion, however, is but tentative. It may
need revision in the light of future work in the field.
BIBLIOGRAPHY
1. E. Suess. The Face of the Earth II (1888) ; translated by H. B. C.
Sollas. Oxford Clarendon Press 1906 ; 326-345.
2. A. Penck. Glacial Features in the Surface of the Alps. Journal of
Geology 13 (1905) 1-19.
3. A. Penck and E. Brückner. Die Alpen im Eiszeitalter. (1909).
Leipzig.
4. K. Sölch. Studien über Gebirgspässe. etc. Forschuagen zur
deutschen Landes und Volkskunde. 17 (1908).
119-273.
5. W.M.Davis. Die erklärende Beschreibung der Landformen (1912).
Leipzig, p. 425.
6. R. S. Tarr. Glacial Erosion in the Scottish Highlands. Scottish
Geographical Magazine. 24 (1908). 579-580.
7. H. Louis. Glazial rhorphologische Studien in den Gebirgen der
Britischen Inseln. Berliner Geographische
Arbeiten. Heft 6 (1934). 31-35.
8. J. Sölch. Geomorphologische Probleme des Schottischen Hochlands.
Mitteilungen der Geographischen Gesellschaftin
Wien Band 79 (1936) ; see especially 36-7. 44.
48-49.
9. D. L. Linton. Watershed Breaching by Ice in Scotland. Transactions Institute of British Geographers Publication 15 (1949) 1-6.
179
10. D. L. linton. Some Scottish River Captures Re-examined. Scottish
Geographical Magazine, 65 (1949), 123-133.
11. D. L. Linton. Some Scottish River Captures Re-examined, 11.
Ibid., 67 (1951), 31-44.
12 .D. L. Linton. Problems of Scottish Scenery. Ibid., 67 (1951), 65-85.
13. G. H. Dury. A Glacial Breach in the North Western Highlands.
Scottish Geographical Magazine, 69 (1953), 106-117
14. D. L. Linton. Midland Drainage. Advancement of Science, 7 (1951),
449-456.
15. G. H. Dury. Diversion of Drainage by Ice. Science News, 38 (1955);
see especially 51-54, 61-64.
16. J. K. Charlesworth. The Glacial Geology of North-West Donegal.
Proceedings, Royal Irish Academy, B, 36 (192124), 173-314.
17. Memoir of the Geological Survey of Ireland. Sheets... comprising North-west and Central Donegal (1891),
18. W. V. Lewis. Valley Steps and Glacial Valley Erosion. Transactions
Institute of British Geographers, Publication 13»
(1947), 19-44.
180