80 R. S. TARR 



the wells passed through coarse material at this depth, and in 

 several of them the coarse materials reach a much greater depth 

 than this. On the other hand, the records of four of the wells 

 show that the sands and gravels were passed through before that 

 depth was reached. In his letter Dr. Gilbert makes this further 

 statement : 



From Richland northward to Adams Center the gradient is 4.3 feet per 

 mile, and north of Adams Center it. is still steeper. So it is possible that a 

 correction might advantageously be applied to southward flattening of gra- 

 dient. 



Some such correction, the exact amount of which is not clear, 

 would seem to harmonize better with the distribution of the gravels 

 as revealed by the well sections. 



Several facts, as follows, seem to warrant correlation of these 

 deposits with the Iroquois stage: (i) the difficulty of otherwise 

 explaining the sand-gravel series ; (2) the evidence of coarseness 

 of material, and of plant and animal remains, all of which point 

 either to land or shallow-water conditions; (3) the position of 

 the series, resting on deposits which appear to have been com- 

 pleted just before the Iroquois stage; (4) and the fact that their 

 position is approximately at the level to be expected on the 

 theory of formation during the Iroquois stage. No facts oppos- 

 ing this correlation are known, and no other rational explanation 

 suggests itself. It is therefore proposed as an interpretation of 

 the phenomena. 



SUMMARY OF EVENTS, 



On the basis of these interpretations, we have revealed the 

 following postglacial history of the Ithaca delta: First moraine 

 was formed while the ice-front stood in a deep lake, in which 

 the morainic material was largely assorted. After this stage 

 there was a long period of lake clay deposit in an ice-dammed 

 lake whose area was expanded, while its level fell by successive 

 drops to lower levels as the steady melting back of the ice-front 

 discovered lower and lower outlets to the north. In this lake 

 there was floating ice, but little, if any, animal life. When the 

 Mohawk outlet was finally discovered the ice-dammed lake nearly, 

 if not quite, disappeared from the site of the artesian wells. The 



