472 
THE FRESH-WATEE LOCHS OF SCOTLAND 
maximum extension may thus be briefly summarised: — (1) The 
grinding up of the cover of rotted rock and loose debris due to 
subaerial waste in the later divisions of Tertiary time ; (2) the 
scooping out of loosened material along shatter belts or great lines of 
fracture ; (3) the differential erosion of the rocks entering into the 
geological structure of the country, dependent upon the variation in 
the powers of resistance of the strata, on the thickness and slope of the 
ice, and on the grinding power of its basal layers charged with sand, 
clay, and stones ; (4) the consequent steepening of opposing rock-faces, 
of escarpments, of mountain sides, the deepening of valley floors, the 
planing of cols, and the general lowering of the rocky plateaux. 
Thus we And in those cases where the trend of the valley coincided 
with the direction of ice-flow during the maximum extension, that 
V-shaped valleys became U-shaped — a form characteristic of glaciated 
mountain regions. The projecting spurs were removed, and a lack of 
adjustment was produced between the over-deepened trunk valleys 
and their tributaries. The latter are termed hanging valleys, owing 
to the steep gradient at which they enter the trunk valleys. Another 
result of the abrasion by the ice, which will be more fully dealt with 
in the sequel, was the production of one or more rock-basins along 
the course of the valley, dependent upon the topographical features, 
the geological structure, and the erosive power of the ice in each case. 
The distribution of the ice in the region situated to the north- 
west of the Great Glen, where the ice-shed lay to the east of the 
watershed over part of the area, caused a drainage of ice across the 
low cols in the old transverse valleys. Hence, owing to the excessive 
erosion which they experienced, these low cols form flats often 
studded with lochans, many of which are true rock-basins. Where 
the side streams debouch on the cols they form deltas, which deflect 
the drainage to the east or west, or impound the waters and thereby 
give rise to lakelets. 
The glacial accumulation characteristic of this period is the 
boulder clay, with lenticular sheets of sand and gravel, which forms 
an extensive covering in the Lowlands, and stretches up the 
valleys of the Southern L^plands, and to a limited extent in the 
Highlands. Its remarkable thickness in certain localities, and its 
continuity in the Lowlands, furnishes impressive testimony of the 
modification of the country during the period of maximum extension 
of the ice ; but though the area which it covers on the mainland seems 
large, it is in reality small compared with the covering which must 
have been spread over the Continental Shelf 
During the retreat of the great mer de glace ^ marginal lakes were 
formed between the ice-front and the slopes of the hills from which 
the ice had melted away, thus giving rise to terraces of sand and 
