300 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 seen 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 difter- 
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 
