260 DR JAMES GEIKIE ON 
than now—a time when the streams flowed in sufficient body to flood their 
valleys, and to prevent the undisturbed accumulation of débris-banks at the 
base of their cliffs. It is quite impossible that the valleys could have been 
excavated to their present breadth by the small streams of to-day—even with 
all the aid of springs and frost. These streams are now busied in digging 
narrow trenches in the flat bottoms (as shown in Plate XV. fig. 22), forming as 
it were valleys within valleys. 
7. Glacial Erosion of Valleys, &c.—There are many appearances, however, 
which cannot be explained by aqueous erosion even on the supposition that the 
rainfall was formerly more excessive. The width of many of the cirques, the 
form of the valley-bottoms, the presence of rock-basins, and other phenomena 
all testify to powerful glacial erosion. The rounded and somewhat undulating 
contour of the valley-bottoms, and the smoothed and bevelled appearance of 
the cliffs are conspicuously glacial. The valleys have been glacially deepened — 
and widened, and the harsher features which the cliffs must have presented in 
preglacial times have thus been softened down. The demolition of the rocky 
cols between two valleys is also unmistakably the work of the ice. As with 
the valleys so with the amphitheatric cirques, large and small alike, and whether 
forming the head of a valley or opening abruptly upon a valley from a mountain- 
slope—all have been considerably modified by glacial action—many containing 
rock-margined lakelets. When local glaciers occupied these cirques, the 
recession of the cliffs by which they are surrounded would proceed at a rapid 
rate—the débris, mstead of gathering at their base and forming a protecting 
mantle, being carried outwards and drifted over the rock-ledges to some lower 
glacier, or glacier remanié, upon the surface of which they might be carried 
down to sea. 
8. Weathering of Glaciated Surface.—But throughout all the islands the 
features impressed by former intense glacial erosion are now, as I have said, 
being more or less rapidly effaced. Roches moutonnées are breaking up and 
disappearing ; ice-worn cliffs are being chipped and shattered by frost ; great 
taluses of débris are accumulating at the foot of scaur and precipice; rock- 
basins are being tapped and silted up ; streams are digging deep gullies in the 
flat glaciated bottoms of cirques and valleys ; and thus erelong the characteristic 
ice-worn outlines will vanish, and those features which must have characterised 
the islands in early preglacial times will come more and more prominently into 
view. 
9. Limited accumulation of Till upon Land.—When the islands were 
enveloped in their ice-sheet, the action of frost would be confined to such ridges 
and hill-tops as projected above the mer de glace, while severe glacial abrasion 
would go on below. This abrasion, carried on doubtless during a prolonged 
period, resulted in the more or less complete removal of all great banks of débris 
