456 WATER SUPPLY AND SEWAGE DISPOSAL 



in a general north and south direction. The line of highest 

 land does not usually rise more than about 100 ft. above the 

 level of the two lakes. Since the average distance of the water 

 shed is about 100 miles from the lake on either side and since 

 the shed has an average height of 100 ft., it is evident that the 

 slope is about 1 ft. to the mile. 



Much of this peninsula is covered by a thick layer of fine 

 soil, or GLACIAL DRIFT. In many places this drift is several 

 hundred feet deep. It is composed quite largely of clay, al- 

 though, in places, considerable sand and gravel is mixed with it. 



FIG. 279. Cross section of the rock underlying the Lower Michigan 

 Peninsula. 



The clay is generally fine grained and water passes through it 

 slowly. Beneath this drift, lie beds of sandstone and other 

 rock, all of which hold large quantities of ground-water. It 

 happens that these layers of rocks do not lie in horizontal 

 layers but that they are depressed at the center of the state 

 as shown in Fig. 279. 



The average yearly RAINFALL over this region is between 30 

 and 40 in. (see Fig. 217). The average yearly RUN-OFF, that 

 is, the amount of water which is known to run off in the open 

 streams, is between 10 and 20 in. (Fig. 280). The average 

 yearly rate of EVAPORATION of water standing in an open 



