888 STRATIGRAPHICAL GEOLOGY. [Boox VI. 
the prominences reached by the ice were rounded off and smoothed 
over, the pre-glacial soils and covering of weathered rock were ground 
up and pushed away, the valleys were deepened. and widened, and the 
plains were strewn with ice-borne debris. It is obvious that the in- 
fluence of the moving ice-sheets has been far from uniform upon 
the rocks exposed to it, this variation arising from the differences in 
powers of resistance of the rocks on the one hand, and in the mass, 
slope, and grinding power of the ice on the other. Over the low- 
lands, as in central Scotland and much of the north German plain, 
the rocks are for the most part concealed under glacial debris. But 
in the more undulating hilly ground, particularly in the north and 
north-west, the ice has effected the most extraordinary abrasion. It 
is hardly possible, indeed, to describe adequately in words these 
regions of most intense glaciation. The old gneiss of Norway and 
Sutherlandshire, for example, has been so eroded, smoothed, and 
polished, that it stands up in endless rounded hummocks, many of 
them still smooth and curved like dolphins’ backs, with little pools, 
tarns, and larger lakes lying between them. Seen from a height 
the ground appears like a billowy sea of cold grey stone. The lakes, 
each lying in a hollow of erosion, seem scattered broadcast over the 
landscape. So enduring is the rock, that even after the lapse of so 
long an interval, it retains its ice-worn aspect almost as unimpaired 
as if the work of the glacier had been done only a few generations 
since.» The connection of the abundant ice-ground and lake-filled 
rock-basins of glaciated regions with the erosive work of land-ice was 
first pointed out by Sir Andrew C. Ramsay (p. 417). The pheno- 
menon of “ giants’ kettles” (p. 415) is another mark of the same pro- 
cess of erosion. 
Ice-crumpled Rocks.—Not only has the general surface of the 
land been abraded by the ice-sheets, but here and there more yield- 
ing portions of the rocks have been broken off or bent back, or cor- 
rugated by the pressure of the advancing ice. Huge blocks 200 
yards or more in length, as in the case of the chalk erratics in the 
cliffs of Cromer, have been bodily displaced and launched forward on 
glacial detritus. The lamin of shales or slates are observed to be 
pushed over or crumpled in the direction of ice-movement. Occa- 
sionally tongues of the glacial detritus which was simultaneously 
being pressed forward under the ice have been intruded into cracks 
in the strata, so as to resemble veins of eruptive rock. 
Detritus of the Ice-sheet—Boulder-clay—Till—Older 
Diluvium.—Underneath the great ice-sheet, and perhaps largely 
incorporated in the lower portions of the ice, there accumulated a 
mass of earthy, sandy, and stony matter (till, boulder-clay, “ grund- 
morane,” “moraine-profonde”) which, pushed along and ground up, 
was the agent whereby the characteristic flowing outlines and 
smoothed striated surfaces were produced.? This “glacial drift’’ 
' Some of these roches moutonnées are of Paleozoic age (Nature, August 1880). 
* When the formation of the till began the materials may have consisted largely of a 
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