520 GEOLOGY OF OHIO. 



ridge along the line of Knox county the Lower Coal, connecting the 

 county at this point only with the true Coal Measure rocks. 



SURFACE DEPOSITS. 



Around the borders of Savannah Lake, and of the valley in which it 

 is situated, are a series of water-washed, sandy ridges, in places filling 

 the valley to unknown depths, and in others working upon finely lam- 

 inated lacustrine clays, which come near to the surface. Southward 

 from the lake the sand hills become smaller and gradually disappear, and 

 the valley expands into a broad water-plain, bounded by the ordinary, 

 drift hills. Savannah Lake, covering a surface, as estimated by Dr. 

 George W. Hill, of Ashland, of about one hundred and sixty acres, and 

 a smaller lake adjacent, of about eighty acres, which has been partly 

 drained, occupy the highest part of this old channel, the present drain- 

 age being toward the north, and a low ridge of sand alone preventing 

 draining at the south into the head-waters of Jerome Fork. These, to- 

 gether with several other lakes and swamps in the State similarly situa- 

 ted, illustrate the occurrence of lakes and lakelets with two outlets, and 

 in opposite directions. Both of these are quite deep, the depth of the 

 larger being reported at one hundred feet, and the borders show that the 

 water once stood at a considerably higher level, and that then there was 

 also an outlet at the south. These lakes, draining but a small area of 

 the table land to the east and west, can not receive a large supply of 

 water. Until comparatively hard material is reached, the channels of 

 the outlets are carried deeper, and the lake recedes, until only one chan- 

 nel remains on the side where the descent is most rapid, or the drift-bed 

 is most easily removed. In the meantime, the bottom is being silted up 

 from the wash of the hills, and a growth of vegetation accumulates in 

 the shallow water, until in time the lake becomes a marsh, and this ulti- 

 mately arable land. In all these old channels which have cut through 

 this table land, separating the waters of the Lake from those of the Ohio 

 River, at the highest point lakelets or marshes, or the clear indications 

 of their former presence, can be seen. The bottom below the vegetable 

 debris is, in some places, bowlder clay ; in others, laminated clay. The 

 ridges on each side are usually formed of water-washed sand and gravel, 

 the outlets in each direction passing over the debris of the drift, which 

 has filled, to the depth of from one hundred to two hundred feet, the old 

 channel of drainage now covered with alluvium. On the northern slope 

 this debris, where the alluvium is removed, is largely laminated clay, 

 containing occasionally large, angular, and frequently striated bowlders; 

 southward it is gravel, rolled and water- worn, with ridges and pockets of 



