LAKES IN RELATION TO GEOLOGICAL FEATURES 475 
concentric ridges of gravel and morainic material indicate their lower 
limits. Within the valleys the hill-slopes are terraced with lateral 
moraines, and the floors are strewn with mounds and ridges, often of 
horseshoe shape, marking stages in the farther retreat of the ice. 
That old wind-gaps between adjoining ridges were used as overflow 
channels is shown by the occurrence of gravels at these levels where 
only dry hollows now exist, and by the occurrence of rock notches across 
comparatively steep slopes. These phenomena, which are of comiTion 
occurrence in certain districts of the Highlands, point to temporary 
drainage deflected by the ice, which continued long enough to enable 
the streams to entrench themselves. Sometimes lakes of considerable 
extent were impounded by ice-barriers, as in the case of the Parallel 
Roads of Glen Roy, where each terrace marks the temporary margin 
of a lake the height of which is determined by the level of the lowest 
col free from ice. Another characteristic feature of this period of 
retreat of the glaciers is the deposition of a series of fluvio-glacial 
gravels due to the escape of melt-water, which led to the reassortment 
of the morainic material, sometimes round masses of ice isolated from 
the retreating glaciers. 
The last phase of the later glaciation was characterised by the 
occurrence of small glaciers in the high corries, which sometimes gave 
rise to small rock-basins, terminal moraines, and groups of mounds. 
In the North-West Highlands these local glaciers survived to a late 
period in the geological history of the counti-y, as they rest on the 
deposits of the 50-ft. beach at the head of Loch Torridon. 
The Distribution and Probable Origin of Scottish Lakes 
The numerous lakes in Scotland, ranging in size from small tarns 
on the high plateaux and pools on the drift plains to large sheets of 
water in the valleys, may be arranged in the following groups : — 
i. Lochans lying in hollows in, or surrounded by, peat. 
ii. Lakes due to tlie action of the wind : (1) by the interruption of drainage 
in the case of sand-dunes, as, for instance. Loch Strathbeg near 
Fraserburgh, Loch Wester in Caithness, and numerous lakes on the 
west side of South Uist ; (2) by the removal of disintegrated rock, as, 
for example, on high granite plateaux. 
iii. Lakes due to river action : (1) those formed on flat cols by cones of debris, 
of which Loch na Bi, near Tyndrum, is an instance ; (2) crescent-shaped 
or " oxbow " lakes resulting from the isolation of stream-meanders on 
flood-plains. 
iv. Lakes due to wave action on the seashore, where sheets of water are 
enclosed by gravel bars (Loch Sine, on the west side of Loch Eireboll). 
V. Lakes caused by chemical action on limestone plateaux (Loch Borralaidh 
and Loch Croisaphull near Durness, Loch Maol a' Choire or the Gillaroo 
Loch near Inchnadamff). 
