LAKES IN RELATION TO GEOLOGICAL FEATURES 483 
high plateau and debouch on the Great Glen, or on the intermediate 
plateau and coastal plain of the Beauly Firth. It is worthy of note 
that each of these lakes occupies the same relative position in each 
valley, where the trunk glacier had received its main accessions of ice 
from the tributary glens and before it proceeded to fan out. 
Another group of valley rock-basins occurs in Easter Ross. In 
the central portion of that county, the River Bran and the Black 
Water, tributaries of the Conon River, drain a low plateau occupied 
by granulitic Moine schist and augen gneiss, which is bounded on the 
east by more elevated ground extending from Sgurr a' Mhuilinn 
north-eastward to Ben Wyvis, composed of muscovite-biotite gneiss. 
During the period of confluent glaciers, the central low plateau acted 
as a reservoir of ice, part of which passed outwards through the main 
gaps in which Loch Luichart and Loch Garve are situated. Another 
portion was deflected northward by Ben Wyvis, and moved down the 
valley of the Glass, in which lies Loch Glass. 
CORRIE ROCK-BASINS 
Corrie rock-basins are of minor importance, for they are invariably 
small and shallow and confined to mountainous regions. They occupy 
the floors of cirques or corries, with rocky barriers in front and with 
prominent clifls of rock behind them. Round the lip of each tarn 
there is clear evidence of differential erosion by ice under extreme 
pressure due to the downward movement of the mass. Sometimes 
the rocky barriers are concealed by moraines deposited by the 
corrie glaciers. 
On the northern declivity of the Ben More range in Assynt, 
Sutherlandshire, there are excellent examples of corrie rock-basins. 
For instance. Loch a' Choire Dearg and Loch a' Choire Ghuirm are 
situated at a height of about 1750 feet on the north shoulder of 
Glas Bheinn (2341 feet), and lie on a glaciated floor of Lewisian 
gneiss, while the walls of the cirque are composed of Cambrian 
quartzite. Several additional examples occur along the base of the 
escarpment of Cambrian quartzite extending eastwards to Ben More, 
the finest being the tarn at the head of Coire Mhadaidh Bheag at a 
height of 2500 feet. The glaciated floor of this rock-basin is com- 
posed of Torridonian and Lewisian rocks partly encircled bv cliff's of 
Torridon Sandstone and Cambrian quartzite. 
Loch Toll an Lochainn is one of the best examples of this type of 
basin in the North-West Highlands. It occurs at a height of 1700 
feet in the An Teallach range, Ross-shire. The lake is floored by 
well-glaciated Torridon Sandstone, and is surrounded on three sides 
by cliff's of massive grit belonging to the same formation. Again, in 
the hollow between Sgurr na Lapaich (3775 feet) and Riabhachan 
