222 R. S. TARR — MORAINES OF SENECA AND CAYUGA LAKE VALLEYS 



So widespread is this' condition that it is evidently the result of the 

 operation of a law governing morainic accumulation along the margin 

 of valley tongues. This law seems to be that the least morainic develop- 

 ment is on the side which has the least supply from the ice and from 

 marginal drainage (O-D, figure 1). In lateral tongues it usually happens 

 that there is one long drainage slope and one shorter one. In the case 

 mentioned, for example, the distance from the terminus of the valley 

 lobe to a divide where marginal drainage tributary to this valley began 

 to accumulate is many times greater to the north (A-B, figure 1) than 

 to the south (OD, figure 1). The shorter marginal streams along the 



Figure 1. — Diagram illustrating Causes of unequal Development of Moraine in lateral Ice Lobes. 



southern side of the valley tongue evidently did not gather enough drift 

 for the construction of pronounced moraines, whereas the longer north- 

 ern streams did. 



For essentially the same reason the supply brought directly by the 

 ice and accumulated at its terminus is far greater on the side of the 

 longer than that on the shorter side. In the case just mentioned, for 

 example, ice movement and supply to the moraine on the northern side 

 was really dependent on the southward movement of the marginal por- 

 tion of the main valley lobe, but the ice movement and supply on the 

 southern side was merely that of the short tongue itself supplied from 

 the less heavily drift-filled ice away from the margin of the main valley 

 lobe. Therefore the moraine on the northern side received large contri- 

 butions from outside the valley, while that of the southern side came 





