Dec, 1908.] Rock Terraces. 403 



far cut into the rock that its valley shall be ornamented with 

 terraces as are its larger neighbors. Its problem will then, 

 however, be somewhat complicated, if its alluvial terraces shall 

 persist that long, for today the Big Darby has many fine terraces 

 of this more ephemeral type. 



In conclusion, a few facts and deductions may be noted. 



1. The grade of the rivers has been reduced since the 

 terraces were made. Almost every terrace top is nearer the 

 river level at its downstream end than at the other. And 

 further, an analysis of the slopes of the various terrace tops 

 shows that the higher ones descend downstream most rapidly, 

 and the lower ones but little faster than the present water level. 

 The average fall of the present Scioto across the quadrangle is 

 65 feet per mile; that of the Olentangy is by2 feet per mile — bee 

 line distances. The upper terraces fall 20 feet per mile and some 

 of the lower ones about 10 feet per mile. 



2. This relation just noted confirms a statement made 

 earlier in the paper, that the terraces, with two possible excep- 

 tions, were made by the present streams. The same statement 

 is further confirmed in the fact that all terraces descend south- 

 ward or with the present drainage. 



8. It has been pointed out that in harmony with the slight 

 eastward dip of the rock and the customary streamward slope 

 of the terrace tops, those on the west side are usually structural 

 plains, while those on the east side are usually not. 



4. The terraces are confined to country w^hose surface rock 

 is limestone, or limestone with a very little shale cover. The 

 shale alone seems not to be adapted to terrace formation. It 

 might be added here that in one side stream good terraces were 

 found consisting of Ohio shale from which the thick drift cover 

 had been removed. The shale proving a much tougher material 

 than the drift the stream has, to date, been unable to make 

 nearly as wide valley in it as was made while still cutting in the 

 drift above. Hence is found a large shale terrace symmetrically 

 disposed on each side. 



5. The terraces are of marked economic importance, offer- 

 ing good roadwavs above flood waters, good building sites above 

 the fertile flood plains but not so far away as sites on the uplands 

 must needs be. Springs frequently occur at the back side of the 

 terrace, making them still more desirable for residences. Finally 

 most of the quarries are in these terraces, because of the excel- 

 lent opportunities to get at the rock. 



