480 THE FRESH-WATER LOCHS OF SCOTLAND 
flankiDg ridges Blathbheinn on the east, and Druim an Eidhne, 
SgLirr an Eidhne, and Sgurr na Stri on the west. 
Among Scottish rock-basins, perhaps the most convincing ex- 
amples of the relation of differential ice-erosion to topography are 
those that radiate from what may be described as ice-cauldrons. The 
first group to which attention may be directed occurs in the 
mountainous district of Galloway, where a cauldron-like hollow re- 
presenting a drainage area of sixty square miles is situated between 
the Kells and Merrick ranges of hills. The hollow is due to the 
differential weathering of the granite mass extending from Loch 
Doon to Loch Dee and its surrounding aureole of altered Silurian 
sediments which have been indurated by contact metamorphism with 
the granite. The lofty hill ranges bounding the central granite mass 
are composed of these altered sediments, and these are breached 
by the rivers Doon, Dee, Girvan, and the Trool, a tributary of the 
river Cree. 
The Doon, an obsequent stream in a through consequent valley 
still partly drained by the consequent Dee, has base-levelled a large 
part of the interior granite mass, and has formed a watershed with the 
Dee in a deep valley, upon a low flat col studded with lochans. The 
Trool though breaching the barrier at a lower level than the other 
streams, has not been able to remove so much of the interior granite, 
and hence drains a higher part of the plateau. The river Girvan 
enters the cauldron on a higher level than the Doon, and has therefore 
been beheaded by tributaries of the latter stream. Hence by the 
action of these streams the granitic detritus has been removed at a 
quicker rate than the debris of the altered Silurian sediments. I^och 
Doon, the chief outlet, drains nearly two-thirds of the central plateau, 
while the remainder of the catchment basin is about equally shared 
by the Dee and the Trool, the part drained by the Girvan being 
extremely small. 
In the description of the glaciation of the Southern Uplands we 
pointed out that this mass of high ground formed an axis of dispersion 
during both periods of ice extension, when a large reservoir of ice 
must have accumulated in the central cauldron, which discharged deep 
streams by the respective gaps. 
Loch Doon, occupying the floor of the largest gap, has been 
described as a typical rock-basin showing clear traces of glaciation 
round its shores, on its rocky islets, and at its outlet, where well- 
striated roches moutonnees appear. The deepest sounding (100 feet) 
occurs where the valley is constricted by the northern range of altered 
Silurian sediments abutting against the loch east of the Wee Hill of 
Craigmullach. Below this point the lake widens, and its floor there 
forms a shallower basin, where it emerges from the higher hills on to 
