REVIEW OF GEOLOGICAL STRUCTUEE. 33 



process of deposition at a time and under circumstances quite different 

 from those in which the Bowlder clay was formed. 



The Erie clay in Ohio is essentially an unsfcratified formation. It is 

 true that in certain localities it is divided into thick layers or beds, sep- 

 arated by sheets of sand and gravel, as will be seen by the interesting 

 section given on page 24 of Vol. II, but these subdivisions are quite 

 local, and, as a whole, the Erie clay shows as little evidence of aqueous 

 stratification as does the Till of England and Scotland. 



No unquestionable case of the occurrence of fossils in the Erie clay— 

 except as transported pebbles — has yet come to our knowledge. A piece 

 of wood was found by Mr. M. C. Read, in Lake copnty, in what he sup- 

 posed to be the Erie clay, and it is probable that he is correct in that 

 supposition, but there is room for doubt in regard to this case, from the 

 difiiculty of distinguishing, on the Lake shore, where the clay beds are 

 constantly slipping, between the true Bowlder clay and the more recent 

 laminated clay which overlies it. The latter contains many fragments 

 of floated wood, and these are sometimes brought down by slips below 

 the surface of the Erie clay, and where they would very naturally be 

 supposed to have been derived from it.* 



Over nearly all the counties of Ohio that lie within the lake basin the 

 Bowlder clay forms a continuous sheet from ten to a hundred feet in 

 thickness. It has every where the same general character, although 

 toward the north-west corner of the State the stones it contains are 

 larger, and it is underlain by a water-bearing sheet of gravel and sand. 

 The pebbles contained in the Bowlder clay are generally small, sub-angu- 

 lar, scratched and planed fragments either of indigenous or of exotic 

 rocks, the former largely preponderating. It is usually yellow or brown 

 at the surface and blue below. In our former notices of the Erie clay 

 this difference of color was represented as due to the oxidation of iron at 

 the surface. Prof. Otto Torell, who examined, with the writer, some of 

 the exposures of the Bowlder clay in northern Ohio, was inclined to re- 

 gard the upper and yellow portion as a distinct formation, and as corre- 

 sponding with a yellow Till found overlying the blue Bowlder clay in 

 Scandinavia and Germany, reported to be separated from it by a well- 

 defined line of demarkation, and to contain different pebbles. A careful 



* It is by no means impossible that sticks and logs should be found in the Erie clay, 

 although it is purely morainic material, for when the ice period began, all the country 

 between Lake Erie and the Arctic Sea was covered with a luxuriant forest, and in the 

 advance of the glacier which removed the soil and all other superficial material some of 

 the tree trunks may haye been so buried and lodged as to escape complete attrition, and 

 leave rolled fragments in the terminal moraine. 

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