538 Surface Geology of the Merrimack Valley. [September, 
The oldest of our deposits of modified drift are the kames. 
From the position of these peculiar accumulations of gravel, which 
are overlaid by the horizontally stratified drift, the date of their 
formation is known to be between the period when the ice-sheet 
moved over the land and that closely following, in which this 
more recent modified drift was deposited in the open valleys from 
the floods that were supplied by the melting ice. We are thus 
led to an explanation of the kames which seems to be supported 
by all the facts observed in New Hampshire and Massachusetts, 
and which appears to apply also to the similar deposits that 
have been described in other parts of the United States and in 
Europe. 
The melting of the ice-sheet over New England advanced from 
the sea-coast towards the north and northwest. The lowest and 
warmest portions of the land were probably first uncovered, and 
as the melted area advanced into the continental glacier its vast 
floods found their outlet at the head of the advancing valley. 
This often took place by a single channel bordered by ice-walls, 
as was the case along the Connecticut kame; but in the Merri- 
mack Valley and in Eastern MasSachusetts, these glacial rivers 
also frequently had their mouths by numerous channels, which 
were separated by ridges of ice. In these channels were deposited 
materials gathered by the streams from the melting glacier. By 
the low water of winter layers of sand would be formed, and 
by the strong currents of summer layers of gravel, often very 
coarse, which would be irregularly bedded, — here sand and there 
gravel accumulating, and, without much order, interstratified with 
each other. Sometimes the melting may have been so rapid that 
the entire section of a kame may show only the deposition of a. 
single summer, which would then be very coarse gravel without 
layers of sand. When the bordering and separating ice-walls dis- 
appeared, these deposits remained in the long ridges of the kames, 
with steep slopes and irregularly arched stratification. Very 1t- 
regular short ridges, mounds, and inclosed hollows resulted igs 
deposition among irregular masses of ice. The glacial rivers 
which we have described appear to have flowed in channels upon 
the surface of the ice, and the formation of the kames took place 
at or near their mouths, advancing as fast as the ice-front re- 
treated. 
The extensive level plains and high terraces which border gn’ 
rivers, constituting the most conspicuous and by far the seg 
portion of our modified drift, were also deposited in the pee 
plain period. The departing ice-sheet was the principal sou 
