THE STORY OF OHIO'S ROCKS 



27 



would say that the glaciers - there are still a few in the mountains - are growing, 

 that, huge ice fields would slowly form in places where there is no ice now. 



Besides 



As the edges of glaciers reach warmer regions or lower levels they begin to melt. In 

 this way, glaciers are always being fed at the source, moving outward from it, and melting 

 along the edges. 



Cool climate of this sort lasted for a long time during the Pleistocene so that glaciers 

 formed in known centers to the north of us and gradually spread until they covered most of 

 Canada and the northern United States (fig. 27). In Ohio they reached across more than half 

 of the state, along a line roughly from Brown County in the southwest to the middle of Colum- 

 biana County in the northeast. One ice invasion of this sort would be spectacular enough, but 

 the record shows that during Pleistocene time the ice advanced and melted back at least four 

 times. 



In their advance across the land, the glaciers picked up sand, soil, pebbles, and boulders 

 which they moved great distances. For example, many 

 of the boulders in glacial material in Ohio are completely 

 different from the bedrock of the state but are identical 

 with bedrock far to the north in Canada. When the glaciers 

 moved over the land, the thousands of stones frozen into 

 the bottom of the ice scratched the surface of the rock. 

 In places, these "scratches" are so large, 3 or more feet 

 wide and as deep, that they are called grooves. The ma- 

 terial picked up by the glaciers was dropped when the ice 

 melted. 



The glaciers scoured and altered the surface of 

 Ohio as far as they went, but their effects were felt even 

 farther. The meltwater from the ice poured down the 

 river valleys in much greater quantities than the rivers 

 were accustomed to carry. This increased the erosive 

 power of the streams, deepening and widening them. 

 Moreover, these waters bore along with them quantities 

 of clay and silt which were deposited far from the glacier's 

 edge. This explains why some Ohio rivers seem too 

 small for their valleys and why some of these valleys are 

 floored thickly with deposits of dirt into which the modern 



Streams are Cutting. Fig. 27 Pleistocene glaciers 



Some of the scenic features of Ohio are due to the work of glaciers and glacial streams. 

 Their story need not be repeated here as it has been told in detail by Carman (1946), Hall 

 (1953), and Marple (1954). The pre -Pleistocene drainage of Ohio was considerably modified 

 by the passing of the glaciers. Some valleys were buried deep under glacial materials; in 

 others, the direction of flow was completely reversed. Some streams were dammed, forming 

 lakes which have long since disappeared. 



The most radical changes of the drainage resulted in the formation of the Great Lakes. 

 Stout, Ver Steeg, and Lamb (1943, pp. 44-48 and map opposite p. 44) have described the 

 several stages of Lake Erie and given a map showing lake beach ridges in Ohio. For details on 

 other drainage changes, see the same reference, chapter 2, and Merrill (1953) and Marple 

 (1954). 



Judging by the fossil record, the animals of the Pleistocene, with very few exceptions, 

 were much like those of today. The exceptions are discussed in detail in chapter 10. 



Only one feature, other than the ice sheet, set the Pleistocene landscape apart from ours. 

 This was the presence of a different group of animals. Some of these are now extinct. Some 

 had migrated into the region. During an ice advance northern animals moved southward 



