330 GEOLOGY OF OHIO. 



cent date than it does generally in Delaware county. It appears very 

 similar to the Drift in the north-west corner of Delaware county, the 

 characters of which are sufficiently discussed in the report on. the geol- 

 ogy of that county. This evidence is of two kinds : (1st) that which 

 pertains to the rock ; (2d) that which pertains to the Drift itself. 



(1) The streams of the county have not excavated channels in the rock, 

 and but very rarely expose it in their beds. This is not strictly true in 

 the south-eastern part, in the area of the Corniferous, where^ there is some 

 erosion in the rock, like that *een throughout the most of Delaware 

 county. This indicates that in the south-eastern corner the erosion by 

 streams has been longest continued, although that part of the county 

 has at the same time less elevation above Lake Erie. In other words, 

 that the overspread of Drift in the south-eastern part of the county was 

 earlier than in the rest of the county. 



The rock, where exposed in the south-eastern part of the county, has 

 the same long-weathered appearance, even when freshly uncovered by 

 the removal of the Drift, that is observable in Delaware county. The 

 marks of glacial action are dim. The natural jointing and planes of 

 separation between the bedding are loosely filled in with the effects of 

 oxydation and decomposition to a greater depth than in the rest of the 

 county. 



(2) If we revert to the appearance of the Drift itself, the most striking 

 contrast is presented in the general smoothness of the surface through- 

 out the county, compared to the surface of Delaware county. This is 

 partly due to the effect of less erosion on the Drift by the streams, and 

 partly to the evenness of the rock surface. With a single exception, the 

 Drift seems to have been very uniformly and gently deposited in Union 

 county. The uniform direction of, and the regular intervals between 

 the main streams, may all have been at first determined by slight differ- 

 ences in the thickness of the Drift deposited, but such differences are 

 now so obscured that they can not be detected by the eye, except in the 

 interval between the Big Darby and Mill Creeks. 



Besides this general flatness of surface, the yellowish color, caused by 

 the formation and infiltration of hydrated oxides from above, does not 

 extend so far downward in Union county as in Delaware. In the latter 

 county the light colored clay extends downward to the depth of fifteen 

 or twenty feet, and sometimes as much as- twenty-five feet. In the 

 former the blue clay is usually met within ten feet. It sometimes rises 

 within eight feet of the surface, and occasionally the yellowish color ex- 

 tends to twelve or fifteen feet. The depth of such superficial coloring 

 seems to vary not only with the length of time the Drift may have been ■ 



