Jt geot. Soc. Lond. Vol. 136, 1979, pp. 215-224, 3 figs. printed in Northern Ireland. Catastrophic lake drainage in Glen Speanand the Great Glen, Scotland J. B. Sissons SUMMARY: The 260 m ice-dammed lake in glens Spean and Roy finally attaineda volume of 5 km3. It is suggested that it was initially drained subglacially and catastrophically through the Spean gorge and Loch Ness. Maximal flow of water may have been about 22,500m3s-'. In Glen Spean the ice. dam,7 km long and up to 200 m high, collapsed. Near Fort Augustus a vast depdsit of sand and gravel was laid down in relation to a Loch Ness suddenly raised in level. Subsequently,the level of theSpean-Roy lake varied constantly as it wasintermittently emptied by other sudden floodsof lesser volume, some of which followed the Lundy gorge and one of which produced a now abandoned waterfall site near Loch Lochy. Some of these events are related to the formation of end moraines and fluvial terraces in the area around Spean Bridge and Gairlochy. The outline sequence of eventsin Glen Roy and shoreline. This view is not accepted, because in Glen vicinity during the last glaciation was established by Roy the highest parallel road extends to the southern- Jamieson (1863), but neither he nor anyone else has most position at which it could have existed without ever consideredin detailthe problem of how the the associated lake levelbeing lowered, while the ice-dammed lakes were finally drained. In this paper middle road on the western side of Glen Roy termi- the author offersa solution to this problem, chiefly nates exactly in line with a cross-valley moraine that withreference to the Spean ice-dammedlake. The records the response of the ice dam to the dropin lake area covered by the lake just before it was drained is level when this road was abandoned. shown in Fig. 1. At that time, it was 35 km long with a major branch extending up Glen Roy. Its area was End moraines and related featurea 73 km2. The volume of the lake was derived from the Ordnance Survey 1: 10,000 map using a rectangular W of Spean Bridge a series of ridgescrosses the grid of points spaced at distances equivalent to 200 m.. valleys of the River Spean and the Allt Achadh na The value obtained wasalmost exactly 5 km3, i.e. Dalach. The ridges lettered A to E in Fig. 2 are lines almost twice the volume of Loch Lomond, the second of elongated mounds 10-40 m broad and 1-6 m high largest Scottish loch by volume, and two-thirds that of composedmainly of glacialtill and, occasionally, Loch Ness, the largest modern loch. The problem is partly of sand and gravel or lacustrine silt. Peacock therefore what happened to this enormous volume of (1970) noted 2localities where the ridgesoverlie water. laminated silt. Livingston (1906)interpreted theridges as end moraines, but erroneously attributed them to ice moving westwards. On the other hand, Peacock, Landforms in the Spean referring to these and other ridges, proposed several Bridge-Gairlochy area explanations, concluding that 'it is doubtful whether any of the morainicforms were generated in the The western ends of the 260 m parallel road immediate vicinity of an ice edge' (1970, p. 189). That ridges A to E were formed by glacier ice is The lakes of Glen Roy and vicinity were held up by demonstrated by their being composed mainly of gla- ice of the Loch Lomond Advance, whose limit inGlen cial till. The clarity of the features and, in places, their of Roy is 5 km NE Roybridge. During subsequent ice steep sides, suggest that they did not accumulate in a retreat, the Roy lake eventually dropped to the level supraglacial or englacialposition: hence they were of the lowestparallel road in Glen Roy,which is produced either beneath the ice or at its margin. A 260 m. Only the 260m road is present in Glen Spean, subglacial interpretation appears unsatisfactory for it and it records the edge of a lake that gradually ex- appears to require the former existence of basal cre- tended as the ice retreated. The ice dam (Fig. 1:) vasses 5.5 km long (for ridges B to E) and is difficult correspondsapproximately with the western end of to reconcile with the lake deposits beneath the ridges the parallel road on the northern and southern flanks at two localities. The remaining interpretation, namely of Glen Spean. It could be argued that the 260 m lake that the ridges are end moraines, encounters none of extended further westward but did not leavea thesedifficulties and accordswith the form,length, 0016-7649/79/0300-0215$02.00 and trend of the features. That the ice came from the @ 1979 The Geological Society W is shownby erratics and striae(Wilson 1900). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/2/215/4885948/gsjgs.136.2.0215.pdf by guest on 03 October 2021 216 J. B. Sissons Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/2/215/4885948/gsjgs.136.2.0215.pdf by guest on 03 October 2021 Catastrophiclakedrainage, Glen Span & GreatGlen 217 Local bends in the ridges, which cause them occasion- The highest terrace of all slopes westward down the ally to resemble eskers, can be attributed to the active Spean valley to terminate 3 km E of Spean Bridge at ice margin being affectedby dead ice blocks outside it. an altitude of 112-115 m. The intake of channel G W of ridge E on the southern side of the Spean (Fig. 2), beneath peat, is at 11 1.5 m. The kame-and- there is a large area of sand and gravel disposed as kettle topography S of the lowerSpean includes a kames, kettles, and a kame plateau. These forms show large kamewith deltaic bedding (H, Fig.2), now that the icein this locality became stagnant after almostdestroyed by man, the extensiveflat top of moraine E was formed. Thetrend of some of the which (300 m broad) was at 113 m. Ice-contact slopes features, especially F (Fig. 2), points to deposition in largely surround the kame. Thus the kame was depo- crevasses parallel with the ice margin represented by sited in a lacustrine environment amidst decaying ice, moraine E. In thevalley of the Allt Achadh na Dalach the lake extending to the terrace termination 3 km E there are 3 well-marked ridges lyingW of and trending of Spean Bridge and overflowing through channel G. parallelwith moraine E. Severalsections (e.g. Na- Since moraine E, requiring active icefor its formation, tional GridReferences (NGR) NN 18228047, must have been produced before the stagnation in its 17858019,17887952) show that these ridges are immediatevicinity represented by kame H and as- largelycomposed of bedded sand andgravel, thus sociated kames and kettles, and since the kames (as indicating fluvial deposition. Locallythe ridges are low shown by their ice-contactslopes) were deposited androunded, but more often they are sharp and amidst decaying ice,it follows that moraine E approx- steep-sided, the most conspicuous feature attaining a imates the position of the ice margin when the 113 m height of 30 m. This form indicates the presence of lake came into existence (Sissons, in press a). glacierice onboth sides of the ridgeswhen they While the 113 m lake level and subsequent lower accumulated: this and their parallelism withmoraine E lake levels existed, the lake waters could not escape point to deposition in crevasses parallel with the ice through the Allt Achadh naDalach valley, for this was margin. blocked by glacier ice. It may therefore seem that the failure of the Spean to follow this valley is a simple example of glacialdiversion of drainage, which re- TbeSpeangogeinrelPtiontotheSpeanrrnd its terraces sulted in the excavation of the Spean gorge. However, the explanation is more complex. Near Spean Bridge a valley 1-2 km wide occupied Following the period of extensive sand deposition in by a tiny misfit stream, the Allt Achadh na Dalach, the Spean valley upstream from moraine E, the ice leads WSW from the Spean valley. As can be seen dam ceased to function and the Spean began to cut from the trend of the 150 and 100m contours (Fig. 2), down into the sand accumulation. Someof the terraces the Allt Achadh na Dalach valley is a continuation of recording this incision lead into the gorge, where they the Spean valley and was apparently formerly followed are lost.Towards Gairlochy, terraces reappear and by the River Spean. Now, however, at the entrance to become conspicuous. Here they may be divided into a the valley, the Spean turns abruptly to enter a gorge higher suite (1 and 2, Fig. 3; widely-spaced shading, that has a maximal depth of about 25-30 m. Thus the Fig. 2) and a lower suite (3-5, Fig. 3; closely-spaced Spean has cut through its former valley side, the gorge shading, Fig. 2). The higher suite consists mainly of in which it flows following a shallow col. gravel, often coarse, which has come out of the gorge, Along the courseof the Spean, and thencealong the this being shown by the gradients of the terraces. It Lochy valley and also alongside Loch Lochy,there is a might therefore seem that this coarse debris can be sequence of fluvial terraces. These have been accu- readily explained as the product of gorge excavation. rately levelled at closely-spaced intervals and are dis- However, this interpretation canbe eliminated, as cussed in detail elsewhere (Sissons, in press a).