266 GEOLOGY OF OHIO. 



The black slate is exposed at this place in the bed of the creek, and 

 the above shows nearly the full thickness of the Drift. This conspicu- 

 ous bed of gravel has been excavated for use in the neighborhood, and 

 its position (below a heavy bed of unassorted Drift) resembles that of a 

 similar bed of assorted materials seen near Upper Sandusky. (See 

 Geology of Wyandot County, p. 639, Vol. I.) 



The appearance of the Drift in Morrow county also indicates a greater 

 age than in counties further north. This is one of the most noticeable 

 features of the Post-Tertiary deposits of north-western and central Ohio. 

 In traveling south from the northern, and especially the north-western 

 portions of the State, the observing geologist sees the increasing coarse- 

 ness of the surface of the Drift, the deeper and wider excavations of the 

 river valleys, and the deeper oxidation from the blue color to the yellow- 

 ish brown. He sees the superficial lamination of the fine clay that is 

 found in the valleys of nearly all the streams that flow northward into 

 Lake Erie entirely disappear long before reaching the summit of the 

 watershed. The streams are plainly older in the central part of the 

 State than in the north-western. Instead of only occasional exposures 

 of the bed rock in their valleys, the rock is almost constantly seen along 

 their banks. Instead of having bluffs composed entirely of loose Drift 

 material, their banks show sometimes an erosion in the rocks to the 

 depth of forty or fifty feet. This difference can not be due to the occur- 

 rence of more fragile and destructible formations in the central and 

 southern portions of the State, for the formations lie in outcrop in belts 

 running across the State from north to south. Moreover, instead of a 

 deeper erosion in the softer rocks, where a stream passes from one forma- 

 tion to another, in certain cases the greater erosion is in the harder 

 formation, as when it happens to lie further to the south or south-east. 

 (See Keport on the Geology of Delaware County.) 



There is another striking phenomenon connected with the subject of 

 the relative ages of the Drift deposits in the north-western and the cen- 

 tral portions of the State. In those streams which flow northward and 

 join the Lake Erie valley there is a steady diminution in the exposure 

 of the bed-rock, due to the denuding action of the streams on the Drift, 

 in going north from the watershed. The most frequent exposures are 

 in the valleys of the tributaries, spite of the greater erosive power of 

 the larger volume of water further north. The exposures of the rock 

 that occur over large tracts in north-western Ohio are either confined to 

 the lake shore, where the power of the waves has removed the Drift, or 

 to the "lacustrine area," where the same power has washed the Drift 

 from the rock at points outside of the river valleys. On the contrary, 



