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- 
