Upham.] : Pyles [April 2, 
highlands, and groups of mountains. Boulders and detritus were 
thus gathered into the ice-mass, which became more or less filled 
with the materials of the drift, at least to the height of the ridges 
and peaks which it crossed. The worn surface of the ledges also 
shows that many boulders and pebbles were rolled and dragged along 
at the bottom of the ice-sheet, breaking up and grinding themselves 
and the underlying rock into gravel, sand, and even the finest clay. 
At the end of the glacial period, the material which had been thus 
gathered, mingled, and swept along by the moving ice, was left in 
three different classes of deposits, namely, modified drift, upper till, 
and lower till. Under a warmer climate, melting and evaporation 
finally triumphed, so that there was no longer an annual excess of 
snow-fall which they could not remove. The glacial sheet then 
ceased to increase in depth, and its previously nearly level surface 
became hollowed with basins of drainage and channelled by rivers. 
The materials contained in the ice had been exposed near its termin- 
ation to the washing of many rills and small streams through every 
summer; but when its decline and departure came, this melting was 
extended over a very wide area, and immense floods laden with clay, 
sand, and gravel flowed from the surface of the retreating ice-fields. 
In the lower part of the channels of these rivers, while they were 
still walled on both sides by ice, the heaviest portion of their burden, 
that is, the coarsest gravel and occasional layers of sand, were de- 
posited, and when the ice-walls disappeared these remained as gravel 
ridges or kames. Oftener, however, the conditions favorable for the 
formation of these did not exist, and the whole load of the glacial 
floods was borne forward beyond the ice-margin and there spread in 
nearly level plains, especially along the valleys of rivers that flow 
southward, which appear to have been thus filled to the level of their 
highest terraces. 
This modified drift lies above the other two glacial formations, 
which are generally classed together under the common name of un- 
modified drift or till. A large portion of the material held in the 
ice-sheet, including multitudes of large and small angular boulders, 
escaped transportation by its streams, and were dropped loosely 
upon the surface. This deposit is usually from one to five feet thick, 
but sometimes is found ten or even twenty feet in depth. Its-char- 
acteristics are comparative looseness of its mass; its gravelly and 
sandy detritus, which has a yellowish color owing to the exposure of 
its iron to oxidation; and the abundance, large size, and prevailing 
