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 thisproblem, 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 branchextending 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 Bridgea 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).

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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). The follows. terraces are generalized in Fig. 2. They will be consi- If it is maintained that the coarsegravel is the deredhere only so far as they are relevant to the product of gorge excavation, then terrace 2 in Fig. 3 Spean gorge. must be visualized as being continued up-valleyby the The higher terraces of the Spean valleywere as- bedrock floor of the gorge of that time. By the time sociated with an intermittently falling lake level dam- terrace 3 had been formed the rock floor of the gorge med by ice at the Spean gorge. Great quantities of would have been some 10 m lower, and by the time sanddeposited in the lake now form these higher the present floodplain(5, Fig. 3) was produced the terraces, many ofwhich are conspicuous, partly be- Spean wouldhave cut down through a further 10- cause of their morphology and partly because they are 15 m of bedrock, all this over a distance of several well drained andthus very suitable for cultivation. The kilometres. Hence, the lower suite of terraces should terraces show that the lake was infilled with sediment consist of coarse gravel derived fromthe excavation of from E to W and was ultimately extinguished by thk the gorge. Yet itconsists almost entirely of sand. Thus, sedimentary infill. These higher terraces (now dissec- the Spean gorge must have been in existence before ted) occu in the horizontally-shaded area upstream the terraces were formed. Since the terrace sequence from moraine E in Fig. 2. began to be formed when the icemargin stood at

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FIG 2. Some landforms of the Spean Bridge-Gairlochy area; for reference letterssee text.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/2/215/4885948/gsjgs.136.2.0215.pdf by guest on 03 October 2021 FIG. 3. Terraces along the lower Spean and in the Gairlochy area (slightly generalized). The profile of the Spean continued by that of the Lochy is shown. Levelled terrace fragments are shown by continuous linesand interpolated terraces by broken lines. A and B are the two lowest terraces associated with glacier damming of the Spean. Terraces 1-5 are discussed in the text. The Spean gorgeis represented by vertical shading.

moraine E, when the gorge was still partly ice- channel by Peacock (1970) on his large-scale map of covered, it follows that the gorge was either cut sub- thisspecific locality. Atone place (NGR NN glacially or is an ancient feature reoccupied by the 17937770) there is a large section in the ice-contact Spean; moreover, it could be of dual origin. fluvioglacial deposits that mantle the northern side of The coarseness of the debris in the higher suite of the gorge. This section shows excellent bedding inthe terraces at the gorgeexit is compatible with this sands and gravels and, since it extends down to the conclusion.When this suite was being formed, the base of the gorge, proves that the deposits are here in lower part of the gorge would still have been occupied situ and have not merely slipped down into the gorge. by glacier ice and the coarse debris would be derived Thus, the gorge was already in existence to the full from erosion of drift (till, kame and kettle deposits'), extent of its present depthwhen glacier ice last wasted steep slopes in which now border the gorge. Much of away from this locality. Hence, like the Spean gorge, the sand of the lower suite of terraces wouldhave the Lundy channel was either cut subglacially or is an been derived from erosion of the extensive sand de- ancient feature. posits in the Spean Bridge to Roybridge area. It is possible to proceed a step further. The area in which the Lundy channel occurs is extensively mantled in glacial till. Therefore, if the channel is an ancient The LdYgo%e feature one wouldexpect it to havebeen at least The Lundygorge is amajor meltwaterchannel partly infilled with till. Yet the ice-contact fluvioglacial leading into the Lundy valley. It is 1km long, 100 m deposits mentioned above show that this was not so wide, and up to 30 m deep and is geologically control- when the gorge became ice-free. Thus, if the channel led. Part of thesouthern side of the channel is a is an ancient feature it appears that it was cleaned out precipitous cliff in metamorphosed limestone and the by meltwaters flowing subglacially immediately before opposite side is mantled in fluvioglacial deposits as- the fluvioglacial deposits were laid down. Alternatively, sociated with a considerable area of kames (see bot- it was cut subglacially in its entirety at this time. tom left, Fig. 2). In Fig. 2 the Lundy channel is continued eastwards The channel was interpreted byWilson (1900) as by a broken line. This line corresponds with a broad, the overflow of an ice-dammed lake held up in the shallow,eastward-directed meltwater channel shown Spean valley, and he stated that certain river terraces on Peacock's (1970) mapand on the official geological in the Speanvalley are at the same altitude as the map. The eastward flowwas partly accompanied by channel intake. Accurate levelling of all the Spean deposition within the channel, as shown by the low, terraces shows that this correlation is incorrect. Al- eastward-sloping terraces mapped by Peacock. These though Wilson was involved inthe original mappingof delicate features clearly could not have survived the the area by the then Geological Survey, the meltwater westward meltwater flow that cut or re-excavated the channel is not shown on the officialgeological map Lundy gorge: thus they post-date this westward flow. eventuallypublished in 1975(1:50,000, sheet 62s, For the eastward flow to haveoccurred, the gorge drift edition); neither is it markedas a meltwater itself must have been blocked: this can only have been

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accomplished by glacier ice. Hence it may be inferred Gilbert 1972) as well as erosive ability, and the vol- that, after the gorge was cut or re-excavated subgla- ume of water in the Speanlake was apparently greater cially, the ice collapsed into the gorge and ice-contact than the volume released during any ice-dammed lake fluvioglacial deposits (mentioned above)were laid flood studied in recent years, the possibility that the down within it. Furthermore, the terraces demonstrate outflowing waters of the 260 m lake followed a wholly deposition within a channel that was already in exis- englacial route has to be rejected. If drainage was tence. Inother words, thebroken line of Fig.2 initiated englacially and the water then cut down to representspart of the originalwestward-directed the base of the ice, the resultant geomorphic evidence Lundy channel that has been partly obscured by later wouldbe indistinguishable from that produced by river deposits. drainage that was subglacial from its initiation. Subglacial drainage of lakes held up by temperate glaciers is today by far the most common method of Lake drainage in Glen Spean drainage (e.g. Liestal 1956; Marcus 1960; Gilbert 1971; Mathews 1973; Nye 1976). Following from the Drainage of the 260m lake preceding discussionthis appears to be the only We may now turn to the problemof disposing ofthe method available for theSpean lake. It has been 5 km3 of water that constituted the Spean ice-dammed argued above that both the Spean and Lundy gorges lake. One possible method by which the 260m lake were already in existence before deglaciation of the level might have been lowered is by the water flowing localitiesin which they occur. Furthermore, it was over the ice dam. This is common for lakes held up by inferred that the Lundy gorge was cut or cleaned out sub-polar glaciers (Maag 1969), butrare for temperate by subglacialmeltwaters immediately before it was glaciers, although instances are known. An example deglaciated. It is also interesting that at one point by has been referred to by Liest~l(1956). He stated that the Spean gorge there are three giant pot-holes, the in August 1897,Demmevatn inNorway began to deepest extending down 15 m (Peacock 1970), which inundate the ice dam and a channel was soon formed point to powerful water flow. Thus it is concluded that in the ice, ‘graduallydeepening andenlarging to a vast the Spean and Lundy gorges are routes by which the crevasse’. Referring to the water flowing through the water escaped from the Spean ice-dammed lake. crevasse he wrote ‘obviously it cut through to the very It is possible that the gorgeswere cut largely or bottom’. entirely by the escaping lake waters. However, it is Another possibility is the initiation of lake drainage also possible that the gorges are the cumulative result in a submarginal position. Drainage of this type was of this and similar,much earlier, periods of lake witnessed by Stone (1963a)at LakeGeorge in drainage. It is becomingincreasingly apparent from Alaska. The outflowing water caused the ice to col- the study of ocean-floor sediments that Scotland has lapse, resulting in a gorge walled by rock on one side been subjected to many periods of partial glaciation, and by ice on the other. The gorge, which was exca- so that a lake may have been held up in Glen Spean vated in 4 days, was 8 km long, 30-90m wide, and on a number of occasions extending back into the 90 m deep. Quaternary. However,such considerations donot Demmevatn wassmall in comparisonwith the affect the sequence of events being considered here. Spean lake and Lake George had only a third of the The Spean gorge is considered to have functioned volume of thelatter. If the Speanlake drained in firstas the lake outlet for tworeasons. Firstly, the either of these two ways, vast areas of hillslope would Lundy channel begins nearly 4 km W of the ice dam have been swept clean of drift and considerable ero- (Fig. 1). Secondly, the Lundy channel appears to be sion of bedrock by the torrent should have occurred. closely related to end moraines A to E (see below). In view of thecomponent of ice-surface gradient There are nowmany reports on the drainage of towards the NE (Sissons, in press b), it is more likely modern ice-dammed lakes from countries such as that such evidence would be found on the hillslopes N Alaska, Canada, Iceland,Norway, and Argentina. of Spean Bridge, yet neither here nor on the southern Somedischarges have been witnessed and many slopes of Glen Spean is there such evidence. Hence,it glaciers have been visited soon after lake discharge. is concluded that the Spean lake drained in neither of The drainage of such lakes isnormally catastrophic these ways. and the floods that result are usually referred to by the Another possible method of lake drainage is by an Icelandic term ‘jokulhlaup’. By analogywith these englacial tunnel or tunnels. The hypothesis that such modem events, it is suggested that one may envisage an escape route remained entirely in the ice and thus an enormous floodflowing along the route of the left no evidence of its former presence would appear Spean gorge and thencealong the Great Glen toreach to find no support in the literature on modern ice- the sea atthe present site of Inverness. If this dammed lakes. Since the outflowing waters of self- jokulhlaup resulted in lake drainage down to the level draining lakes, even at typical temperatures of 0.5 to of the outlet, which is much the most common occurr- 2-0°C, have a great melting capacity (e.g. LiestBl1956; ence at the present day, then 5 km3 of water escaped.

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Again, by analogy with the present, the flood would moraines in questionare cross-valleymoraines and probably have lasted about a week but most of the there is again the associationwith a former ice- dischargewould have occurred in one or two days. dammed lake (Sissons 1977). Thisstrongly suggests Clague & Mathews (1973) founda relationship be- that the Spean endmoraines resulted from the special tween maximum flood discharge and available water glaciological conditions related to a glacier terminating storage for 10 modem ice-dammed lakes. If the equa- in a lake. When the lake fell from the 260 m level the tion they derive isvalid then peak flow during the chaotic collapse of the ice-wall would have precluded Spean jokulhlaup (assuming complete lake drainage) the formation of an end moraine. Subsequently, as the would have been about 22,500m3s-’. It is not dii5cult ice continued to decay during a period of oscillating to envisage such a vast dischargeof water contributing lake level (see below), the ice-wall would have become to the excavation of the Spean gorge. lower and thus more favourable for the formation and By analogywith present glaciers ending in deep preservation of end moraines. It is suggested that their water, the Spean ice terminus before lake drainage abruptappearance (moraine A) correlates with the maybe envisaged as vertical,possibly overhanging, replacement of the Spean gorge outlet of the lake by and perhaps floating. Modern ice-dammed lakes often the Lundy outlet, for this is the only specificevent that have large numbers of icebergs floating in them (e.g. appears to have occurred in the period involved. Since Thorarinsson 1939; Nichols & Miller 1952) and the the entrance to the Lundy channel is 70-80 m above Spean lake, with an ice-wall 7 km long and maximal the entrance to the Spean gorge, a consequence of its water depth of 200m, isunlikely to have been an coming into operation would have been that the ice- exception. When sudden lake drainage occurred, the dammed lake could no longer drain completely. The ice-wall, at least 200m high by the present course of chances of collapse of the now much diminished ice- the Spean, would have collapsed. At first this would wall would therefore have been greatly reduced and have provided a great number of icebergs that would end-moraineformation would thus havebeen sud- have become grounded as the lake continued to drain, denly favoured. but during the final stages of drainage and after its completion ice-blocks would have accumulated a 7 as The period constantly vprying lake level km longbelt across the Spean valleymarking the of collapsed ice-wall. Marcus(1960), describing the scene The precedingdiscussion has not dealt with lake after the drainage of ice-dammed Tulsequah Lake in levelsbetween the time the260 m lake levelwas British Columbia, a lake far shallower than the Spean abandoned and the time the Lundy outlet ceased to lake, referred to the ice barrier’s floating tongue col- function.During this time the icemargin retreated lapsing and breaking into thousands of pieces ranging 2.5-5 km (to moraine E) and an ice barrier with its in size from particles to masses of ice hundreds of feet upper edge at an altitude of at least 260m was re- long. Referring to the floor of the lake he mentioned placed by an ice terminus that must have been far less ‘a countlessnumber of bergs’,‘bergs scattered in steep, for large parts of the ice-marginal zone then profusion’and individual bergs up to 125ft [38m] became stagnant (shown by the kames around H (Fig. high. Other descriptions of drained lake floors littered 2), in the Allt Achadh na Dalach valley and by the withicebergs are given by Thorarinsson (1939), Lundy channel). Using the minimal altitude of 260 m Liest~l(1956), Aitkenhead (1960),Lindsay (1966), and an estimateof 1 in 50 for the ice surface gradient and Moravek (1973). One may therefore imagine such at the beginning of this period, by the time the ice a scene in the Spean valley. margin was at moraine E (altitude 60-160 m) the ice surface here had been lowered by 150-300m. The removal of such a thickness of ice must have taken a LakedrpinagethrooghtheLundy~ considerable time. The problem to be considered is The 5 end moraines A to E all end near or trend what happened to the lake during this period. towards the eastem half of the Lundy channel (includ- Catastrophic drainage almost always results in the ing the portion represented by the broken line in Fig. lowering of modem ice-dammed lakes to the level of 2). This suggests that they have some connection with the outlet (e.g. Thorarinsson 1939; Liestd 1956; Mar- the channel. Thesudden appearance of clear end cus 1960;Stone 1963a,b; Lindsay 1966; Mathews moraines in this part of the Spean valley suggests the 1973): hence it is reasonable to infer that this applied occurrence of some specific event that favoured their to the drainage of the 260m lake. Somesubglacial formation. Sucha change is unlikely to have been tunnels then stay open for a considerable time, even climatic, since the deposits associated with the Loch until the followingsummer (e.g. Aitkenhead 1960), LomondAdvance in Scotlandnormally form such but normally the tunnel is soon closed and refilling of clear systems of end moraines only at and very close to the lake begins. The latter seemsmuch the more the limit of the advance. The only exceptions knownat probable in the Spean case, for the tunnel would have presentare the Glen Spean area and Glen Doe had to pass beneath ice at least 300m thick and (tributary to Glen Moriston): in the latter area the end closure of the tunnel by plastic deformation is to be

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/2/215/4885948/gsjgs.136.2.0215.pdf by guest on 03 October 2021 222 I. B. Sissons expected. Furthermore, the replacement of the Spean since the entrance to the Lundy channel is well above lake outlet by the Lundy outlet shows that the tunnel the lowest part of the lake floor. Thus, although there did not remain permanently open.It is therefore infer- is noreason to assume that the lake filled to its red that the lake came into existence again. maximalpossible level eachor anytime, it seems However, there appear to be no shorelines related likely that the jokulhlaups would have tended to di- to this reconstituted lake and there are no horizontal minish successively in volume, as often happens with or gently-sloping river terraces along the rivers Roy or modem glacier floods. Span that can be related to such a lake. The absence of such terraces is particularly significant for, after the lid joLnlhlnup period now under discussion, glacier damming resulted The in the formation of the extensive terraces of sand in As long as the Lundy channel functioned asthe lake the Roybridge to Spean Bridge area mentioned previ- outlet the altitude of the lake surface could not fall ously. These terraces are horizontal or gently sloping, below about 130 m. The Span terraces and associated some of them being almost continuous for a kilometre evidence show that, after the channel was abandoned, or more, withwidths commonly between 100and an ice-dammed lake at 113 m existed in the Spean 200m. These extensive features point to very rapid valley. This lake hadcertainly come into being im- fluvial deposition, for the damming ice had a stagnant mediately after moraine E had been formed. It was marginal zone and 8 terraces were produced while its suggestedabove that end moraines A to E were margin (to the extent that it had a definite margin) produced while the Lundy channel was functioning. retreated little more than 1km (Sissons, in press a). It On this basis it might be further suggested that end is therefore concluded that the level of the lake varied moraines ceased to form when the channel was aban- constantly after the initial jokulhlaup that lowered it doned, a result of the change in glacier balance as- from the 260m level. sociated with the drop in lake level. Alternatively, it The rate of refilling of the lake would have been might be suggested that the final end moraine records mainly conditioned by the precipitation in its catch- the last forward movement of the ice margin in re- ment. Precipitation during the Loch Lomond Stadial sponse to the fall in lake level. On both interpreta- in the English Lake District has beenestimated as tions, however, moraine E approximates the position rather less than present (Manley 1959) and in the SE of the ice margin when the Lundy channel was aban- Grampians has been calculated as similar to present doned, after which large parts of the ice-marginal zone (Sissons & Sutherland 1976). Definedstrictly, the became stagnant. LochLomond Stadial had ended at the time under When the Lundy channel functioned, the waters of consideration, so that it might be argued on this basis theSpean lake ultimatelydischarged into Loch that precipitation amounts may well have been rather Linnhe. However, channel G and all the Spean ter- similar to present amounts. The volume of present races(excepting only the verylast ones) wereall precipitation for the area that would have drained to associatedwith drainagethrough LochNess to the thelake was determinedfrom the officialmap of Beauly Firth. One might therefore anticipate that British rainfall at 1: 625,000 scale by superimposing abandonment of the Lundy channel markedthe begin- on it a grid of squares with side equivalent to 1 km. ning of this changed direction offlow. There is one The value derived was 1.6 km3 a year. Although such interesting feature that supports this view. a volume would have been increased by melting of At J (Fig. 2), a rock ridge has on its NW side a glacierice in thecatchment and wouldhave been feature that resembles an abandoned quarry. That it is diminished by evaporation, it is clearly very different not such is shown by thick peaty deposits on its floor from the 5 km3 of the 260 m lake. Hence it may be (3.5 m deep at one point) and by massive blocks of concluded that, if the 260 m lake refilled, it could not rock heavily encrusted with lichens at the base of its have done so in a year and that it probably took 3 back wall. The latter is very steep and about 30 m high years. and curves round into the side walls,which decline Duringthe period under consideration, as noted rapidlyin height north-westwards. The rockwalls already, the icemargin retreated 2.5-5 km and, at enclose a flat peaty floor 100m broad, which merges moraine E, thinned by 150-300 m. This amountof ice into a well-marked channel, 80-100 m wide and cut in wastage cannot have taken place in 3 years and might drift, that leads north-westwards. At the top of the have lasted over a period ten times as long. Hence it backwall a series of ridges and hollowsalong the seems that there were several jokulhlaups along the strike of the bedrock is suggestive of water erosion. Spean gorge and, later, along the Lundy gorge. As the This feature is clearlyan abandonedwaterfall site. The ice wasted while the Spean gorge was functioning as former waterfall displaysno connection with anyother the outlet, the maximal possible lake level would have meltwater feature in its vicinity and its large size is become progressively lower. When the Lundy channel locallyanomalous. Since the channelleading away came into operation the volume of water required to from the waterfall site is excellently preserved despite fill thelake wouldhave been still further reduced, being cut in drift, it is unreasonable to argue that this

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meltwater route is an ancient feature: formation late might reasonably be expected to have left a continu- in the deglaciation of the area is indicated. Interpreted ous open routeway from the vicinity of Gairlochy to as the route bywhich the lake that hadpreviously Inverness. Most of this distance is occupied by lochs been discharging intermittently through the Lundy Lochy,Oich, and Ness and therefore presents no gorge was lowered, the waterfall route ceases to be problems. Between LochNess and Inverness the River anomalous and it provides the final link in the sequ- Nessfollows a pronouncedbroad cut flanked by a ence of events already discussed. The incision of the steep erosional bluff along much of its eastern side. waterfall into a local hill top canbe attributed to Betweenlochs Lochy and Oich there is anopen superimposition from glacier iceof the river that cut it. routeway except for a few kames that can readily be The route of this final jokulhlaup is suggested in Fig.I attributed to fluvial deposition during subsequent ice and accords with the fact that the 113 m lake, which decay. However, commencing a few kilometres SW of came int~existence immediately thejokulhlaup Fort Augustus and extending to Loch Ness there is a ceased, overflowed in the same general direction (G, veryextensive spread of sand and gravel(Fig. 1). Fig. 2). Although now dissected by the River Oich and lesser streams, thisaccumulation appears to constitute a Sequence of events major obstacle to free flowof a torrent through the Great Glen. Thesequence of events suggested above can be The largest surviving portion of the deposit, measur- summarised as follows: ing 3 X 1km, forms a conspicuous terrace, although for 1. Ice-dammed lake at 260 m, recorded especially by 1-5km part of the southern portion is so extensively the shoreline at this altitude. kettled that hardly any of the terrace surface exists. In 2. Catastrophic drainage of this lake along the Spean the kettled area the maximal altitude is about 40m gorge, resulting in collapse of the ice dam. (estimate from 1: 10,000 map). Fort Augustus is partly 3. Period of oscillating lake level,with lesser built on another portion of the terrace, whose altitude jokulhlaups, first along the Spean gorge and later is 31-33 m (estimate), while another terrace fragment through the Lundy gorge. by Loch Ness declines to 31 m (accurately levelled). 4. Jokulhlaup along the now-abandonedwaterfall The deposit is anomalous in several respects. route. 1. Somelarge outwash spreads wereformed in 5. Lake at113 m, overflowing through channel C; associationwith Loch Lomond Advance glaciers on (Fig. 2) and beginning of sand infilling of the Spean the W coast of Scotland, butnone in the interior valley. approaches the dimensions of the Fort Augustus fea- 6. Lower lake levels and continuationof sand Wing. ture. 7. Failure of the ice dam, leading to drainage along 2. The principaloutwash spreads associatedwith the Spean gorge, dissection of the sand infill, and Loch Lomond Advance glaciers in Scotland typically terrace deposition in the Gairlochy area. begin at the advance limitor just within it, yet the Fort Augustus feature begins 4 km inside the limit; further- Drainage of the Glen Gloy ice- more, there is no outwash spread at the limit. dammedlake 3. The major W coast outwash spreads associated with Loch Lomond Advance glaciers are characterized The only shoreline of widespread occurrence in Glen by deep, well-defined kettle holes or begin at ice- Gloy is at 355 m and terminates nearthe mouth of the contact slopes that are often the sides of kettle holes. glen at the broken line shown in Fig. 1. There are no The FortAugustus feature has neither of these charac- meltwater channels or belts of water-swept rock on teristics. Its chaotically kettled area has no parallel in the slopes of lower Glen Gloy or its vicinity. However, features of comparable age. at the entrance to the glen the RiverGloy fails to 4. The outwash ends 15 m above the present level of follow its former valley that leads WSW, instead turn- Loch Ness. This cannot be explained by postulating a ing abruptly through 100" to enter a rock gorge that high sea level and consequent raising of the level of desit due N towardsLoch Lochy. For reasons LochNess, for the evidencefrom former marine similar to those discussed above, it is suggested that shorelinesin Scotland (especially the Main Late- catastrophic drainage of the Gloy lake along the gorge glacial Shoreline) strongly suggests a sea level at In- occurred on one or more occasions. Since the Gloy verness at thattime no higher than present sealevel. lake was 95 m higher than the 260 m Spean lake it is If it is postulated that the outwash was deposited by inferred that it was initially drained first. the torrent thatdrained the 260 m lake, these anomalies can be explained. Jokulhlaup deposits 1. The size of the deposit ceases to be remarkable. Liest~l(1956) stated that a Norwegian jokulhlaup The vastvolume of water that isbelieved to have carried enormous quantities of gravel and built up a flowed through the Great Glen in a very short time largegravel cone; this jokulhlaup had a volumeof

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only 0-025-0.030km’ in contrast with the 5 km3 avail- of lowering of the loch outlet since it was formed. On able in the 260 m lake. this basis the jokulhlaup raised the level of Loch Ness 2. The position of the outwash is reasonable in 8-5m (31-22.5 m). At the calculated peak discharge relation to the probable amountof glacier decay since rate mentioned earlier, and not allowing for outflow at the maximum of the advance. At the time the lake was the northern end of the loch, this would have taken drained the Treig glacier had thinned 200 m; if the only 6 hours. This figure shows that, despite the large same amount of thinning applied to the FortAugustus area of the loch (56-4 km’), raising of itslevel is glacier its terminus would have been approximately at feasible. The actual time taken would exceed 6 hours the prohal end of the outwash. because of outflow from the loch and because peak 3. The chaotic kettling of part of the outwash can be discharge is only momentary (although most of the attributedto the great quantities of iceblocks that discharge from modern ice-dammed lakes takes place would have been carried by the torrent. in a very short time). 4. The altitude of the Loch Ness end of the outwash It is therefore suggested that the extensive outwash can be attributed to temporary raising of the level of spread near Fort Augustus resulted from the first, Loch Ness by the floodwaters. The highestraised catastrophic draining of the Spean ice-dammed lake. shoreline of the loch near Fort Augustus was levelled Since there is only one outwash spread, it is suggested as 22.5 m (NGR NH 384084), which is 6.5 m above that subsequent jokulhlaups along the Spean gorge the level of the loch. This shoreline post-dates the were much less devastating: they could well have been outwash since it is cut into the latter. Near the north- annual events, as is common with ice-dammed lakes em end of the loch the highest shoreline was levelled today. as 18.8m (NGR NH 589335). These two features probably correlate with each other, the discrepancy in altitude representing isostatic tilting since they were ACKNOWLEDGEMENTS.The writer is very grateful to the Car- formed. The shoreline remnant near the northernend negie Trust for the Universities in Scotland for financing field is nearly 3 m above loch level and implies this amount work and to his wife for levelling assistance.

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