LAKES m RELATION TO GEOLOGICAL FEATURES 481 
the lower ground. The barrier at its outlet consists of massive gritty 
greywackes belonging to the Silurian system. 
Loch Dee is situated near the south-eastern gap, and partakes of 
the character of a plateau basin and of a valley rock- basin ; for there 
must have been a considerable escape of ice by the col at the head of 
the Black Laggan valley, though the drainage of that stream is 
northward towards the cauldron. The long narrow peat-moss 
traversed by the Cooran Lane to the north of Loch Dee probably 
conceals a silted-up valley rock-basin. 
Loch Trool, occupying the south-western gap, is a typical rock-basin 
excavated along the strike of the altered Silurian strata. The deepest 
sounding (55 feet) occurs near the head of the lake, where the valley is 
most constricted ; and the basin gradually shallows where it enters the 
low ground of Wigtownshire. As in the case of Loch Doon, there is 
here clear evidence of differential ice-erosion on the shores and rocky 
islets of the lake. 
Loch Girvan Eye, at the head of the river Girvan, is a small rocky 
tarn evidently due to ice-erosion. Several plateau rock -basins occur 
on the floor of the central cauldron, as for instance Lochs Macaterick, 
Lochricawr, and Enoch, which drain into the Doon, and Lochs 
Neldricken and Valley, which discharge into the Trool. The last of 
these is ponded by moraines, but the granite is exposed not far below 
the outlet. 
Rannoch Moor, embracing an area of about 180 square miles, 
also appears to have acted as an ice-cauldron, radiating ice through 
gaps in the surrounding high ground. It now forms a plateau with 
a general height of about 1000 feet, composed mainly of granite, with 
encircling mountains rising to a height of over 3000 feet, consisting 
chiefly of crystalline schists of sedimentary origin. Several lines of 
fault or shatter belts traverse the granite and surrounding schists in 
a north-east and south-west direction. Situated about the middle of 
the Central Block, it is drained by streams that have breached the 
mountain barriers and have base-levelled large areas of the Moor. 
The river Tummel — a tributary of the Tay — has accomplished more 
work in this direction than any of the other streams. The geological 
history of the Rannoch plateau closely resembles that of the Galloway 
cauldron just described. From its situation also it served as a reservoir 
for the accumulation of ice during both glaciations. 
It is a remarkable fact that rock-basins are situated in many of 
these gaps, where the volume of ice issuing from the central cauldron 
would be greatest and its erosive power, subject to local conditions, 
would be increased. Loch Rannoch, situated in the widest gap, is a 
fine example of a rock-basin ; for though at the lower end the river 
Tummel on issuing; from the lake flows along; an alluvial flat for a 
