REVIEW OF GEOLOGICAL STRUCTURE. 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 isgescentially an unstratified 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. IJ, 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 county, 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 
difficulty 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 Chio 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-aneu- 
lar, scratched and planed fragments either of indigenous or of exotic 
rocks, the former largeiy preponderating. It is usually yellow or brown 
at the surface and blue below. In our former actices 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 have been so buried and lodged as to escape complete attrition, and 
leave rolled fragments in the terminal moraine. 
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