GLACIAL BOUNDARY IN OHIO. 751 
and was undisturbed by ordinary hills, and at its height was not deflected 
in its upper portions even by low ranges of mountains. Upon the sum- 
mits of the Green Mountains, in Vermont, and upon such isolated peaks 
as Monadnock, in New Hampshire, and Mounts Tom and Holyoke, in 
Massachusetts, the direction of the scratches is diagonal to that of the 
adjacent valleys. The ice-stream was no more disturbed by such 
obstacles than the moving water of a deep stream is by a pebble. Much 
of the exceptional variation in the direction of the strie is probably 
due to the fact that the earlier striz are erased by the later action of 
the ice. Doubtless the retreat of the ice-front was far from continuous, 
but there were successive oscillations, the ice often regaining ground 
from which it had withdrawn. Evidently, the markings leit on the 
ice (except where specially protected), would be those made by the 
very last forward movement; and that naturally would be more in- 
fluenced by local topography than previous movements would be when 
the ice was deeper. In Appendix I, will be found an Abstract of 
Glacial Striz and Grooves in Ohio, prepared for me by Col. Charles 
Whittlesey. The comparative absence of observed strize over a large 
portion of the State arises partly from the great depth of the overlying 
glacial deposit, described in the next section, and partly from the softness 
of certain strata, which, in consequence, do not retain the markings. 
2. GROUND MoRAINE, oR TILL. 
A second evidence that the southerly movement of ice was glacial 
in its character, and not like that of icebergs, is to be found in the 
existence of a true “‘ ground moraine” all over the northern part of the 
United States. 
The material resting upon the striated surface of the rocks in 
that region is not a stratified water deposit, but coarse pebbles and the 
finest clay are indiscriminately packed in one mass. The enclosed 
pebbles also are scratched, the scratches upon them usually running 
parallel with their longest diameter, showing that the overlying mass 
was shoved along upon the rocks by an unyielding force. The stones 
that did the grooving were themselves striated in the process. This 
ground moraine, or “till,” as it is technically called, closely corresponds 
to what accumulates under present glaciers, and is spread pretty generally 
over the whole glaciated region of America, though it varies greatly in 
depth in different localities. Sometimes the till is heaped up into hills, 
