34 GEOLOGY OF OHIO. 
examination ‘since made of many exposures of the Bowlder clay in 
northern Ohio has failed to show any constant difference, except in color, 
between the yellow and blue portions. Soéfar as observed the pebbles 
are the same in both, and there is no distinct line of separation between 
them. In some places the yellow color is seen to penetrate the blue 
irregularly, tv affect the sides of fissures to a considerable depth, and to 
pervade the exterior of blocks of clay of which the central portions re- 
main blue. Mr. M. C. Read, whose attention was called to the question, 
and who has had good opportunities for observing, reports that he also 
has been unable to find any constant difference, except in color, between 
the two phases of the Bowlder clay. Hence, until facts shall be observed 
which invalidate the conclusion stated in our former article—that the 
yellow is the leached and oxidized portion of the blue clay—this will 
remain unqualified. 
In the examination made with Prof. Torell beautiful exhibitions of 
“contorted drift”? were found on the shore of Lake Erie, just west of the 
mouth of Rocky River. Here the upper portion of the Bowlder clay has 
evidently been thrust forward and much folded and twisted by a power- 
ful lateral pressure. So far as observed, this appeared to be simply a 
change in the physical condition of the Till. The character of the ma- 
terial and the enclosed pebbles seemed to be the same here as below. 
Such examples are pot uncommon, and they appear to illustrate the 
manner in which the Bowlder clay was formed; portions of the mass 
which had before accumulated having been impinged upon and crowded 
forward by a temporary advance of the glacier. In such advances the 
edge of the ice sheet over-rode a part of the Bowlder clay, but crushed 
and contorted another part by vertical and lateral pressure.* 
In the description of the Hrie clay given in Vol. I. it is said to be the 
material ground up and transported by the great glacial sheet in its 
passage from the Canadian highlands to southern Ohio, and that it was 
moraine matter unwashed and unassorted, thrust out and left behind by 
the retreating glacier. It was also said that it did not accumulate beneath 
_* the glacier, because the rock surface on which it rests is planed down, 
*Mr. Searles V. Wood, Jr. (Geol. Mag., Sept., 1871, p. 3), attributes the conterted 
drift of Cromer, to the bumping and dragging of icebergs on submerged Bowlder 
clay, and Prof. Geikie (Great Ice Age, pp. 122, 258) gives examples of what he considers 
both glacier and iceberg contortions of the Drift clays. Of these the first are more 
regular and general, and the foldings of the clay are in the direction of the glacial 
scratches; the second local and irregular. Most of our contorted Drift has probably 
been produced by glaciers, but some, folding of the stratified Till which has been noticed 
- ean hardly be due to any other cause than icebergs. Doubtless the Bowlder clay was 
also sometimes crushed and folded by the grounding of icebergs. 
