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. So far 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, to aflect 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 not 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 crushfed 

 and contorted another part by vertical and lateral pressure.* 



In the description of the Erie 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 contorted 

 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 ; tl'O secoud 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 

 can hardly be due to any other cause than icebergs, Doubtless the Bowlder clay was 

 also sometimes crushed and folded by the grounding of icebergs. 



