8 GEOLOGY OF OHIO. 



yet filled with water. This water drained off gradually through various 

 outlets opened by the removal of the great ice-dam formed by the re- 

 treating glaciers, by the cutting away of barriers, or the warping of the 

 earth's crust. The older outlets in Ohio have been enumerated. There 

 are others which lead from Lake Michigan to the Mississippi. The de- 

 scent of the water level in the lake-basins took place very slowly, and 

 it remained for long intervals stationary at various points. These are 

 distinctly marked by old shore lines which traverse the slopes that sur- 

 round all the lakes. Along these shore lines we now find terraces where 

 the shore was abrupt and hard ; lake-ridges, where it was sloping, and 

 composed of soft material. 



In the Old World distinct traces are found of a return of arctic condi- 

 tions after the first great glaciers had melted away, and a milder climate 

 had supervened. In this country we have not yet detected any certain 

 proof of the return of the glaciers to the area which they had before 

 occupied and abandoned, although in southern Ohio the sheet of pebbly 

 clay which overlies the Forest Bed seems to indicate a return in that 

 region of something like the condition in which the first bowlder clay 

 was deposited. Before this point in our Drift history can be considered 

 as settled, many additional and careful observations will need to be 

 made. 



The preceding synopsis of the phenomena and history of the Drift has 

 been made as brief and concise as possible, in order that the whole sub- 

 ject might be considered at one view, and thus the relations of its parts 

 be made more apparent than would be otherwise possible. A fuller pre- 

 sentation of the facts, and of the deductions drawn from them, will be 

 found grouped under different heads on the succeeding pages. 



GLACIATED AREA IN OHIO. 



The area over which glacial scratches and grooves occur is, for several 

 reasons, not so well defined as that of the distribution of the Drift. In 

 many of the rock exposures more or less decomposition and atmospheric 

 erosion have taken place, and the traces of glaciers have been removed, 

 where they once undoubtedly existed; and also over much the largest 

 part of the territory once occupied by an ice-sheet, the Drift deposits 

 cover and conceal the surface of the rock. The number of localities 

 where glacial scratches are visible is, however, so great that we can 

 trace with a good degree of certainty the reach of the ancient glaciers 

 by the inscriptions which they have themselves made. From these we 

 learn that the space covered with ice-marks coincides in a general way 

 with that covered by the Drift deposits. The coincidence is not, how- 



