752 GEOLOGY OF OHIO. 
two or three hundred feet high, as in the vicinity of Boston, and in 
Central New York. In other places, especially in Ohio and other 
western States, it forms a more uniform covering of considerable depth. 
By damming up old water-courses the irregular deposition of till has 
formed nearly all the smaller lakes of the country. In many cases these 
lakes, as well as the peat-bogs (so abundant over this region), are 
“kettle-holes,” which, as Col. Whittlesey first suggested (see Smithsonian 
reports for 1866), were probably formed by the burial of great masses of 
ice beneath glacial debris. When in such situations the ice melted, 
depressions would be formed without any outlet. 
3. THe TERMINAL MORAINE. 
A third evidence of the existence of a glacier in North America, 
continental in its dimensions, is the sharpness and continuousness of the 
southern boundary of glacial phenomena, and the special accumulations 
of glaciated material along portions of this boundary. For a good 
portion of the distance, south of New England and westward to the 
Pennsylvania line, and at frequent intervals from there to Illinois, the 
accumulations at the border of the glaciated area are worthy of the name 
of terminal moraine. This terminal moraine consists of a line of hills 
varying from fifty to three hundred feet high, and composed chiefly of 
a compact, unstratified mixture of clay, sand, gravel and striated 
pebbles. Where the movement was over regions favorable to the in- 
corporation of mych earthy material, and where the conditions were 
such as to maintain the southern margin of the glacier a long time at a 
given point, large terminal accumulations would naturally result. The 
warm currents of air from the south here met the point of the slowly- 
advancing glacier, and for a long while held it at bay—melting back 
each summer as much as it had advanced during the winter. How 
extensive this terminal accumulation would be depends on a variety of 
causes. It would vary in amount at any particular point directly as the 
length of the period through which the ice rested on or oscillated over 
a given line, and also according to the amount of earthy material in 
that portion of the glacier whose motion terminated at the point. The 
amount of earthy material in the ice is determined by the nature of the 
rocks over which it was moved, and of the height of mountains past 
which it was led. | 
