804 THE JOURNAL OF GEOLOGY. 



ing to it, and when hardened with a thin solution of glue and 

 dried it is ready for the taking of a paper positive. This papier- 

 mache model is a close representation of the original clay. 



3. A late glacial deposit. — A glance at the first model will 

 show the typical fofm of these delta deposits, the esker like an 

 arm, and the sand-plain like a hand with its finger lobes. The 

 esker rises in height as it approaches the head of the plain. The 

 top of the sand-plain slopes very gently downward from the head 

 to the top of the lobes, but the front slopes of the lobes are much 

 steeper, about twenty degrees. 



The sand and gravel are so little disturbed that the deposit 

 cannot be pre-glacial. That the deposit was not made by marine 

 or fiuviatile action is shown by the three following considerations. 

 First, an aqueous deposit of gravel, composed of fragments from 

 the crystalline high-lands between two and three miles to the north, 

 should have extended originally from its source outward ; but 

 the amount of denudation and transportation required to cut out 

 these delta deposits from a continuous sheet extending across the 

 Charles river to the crystalline highlands on the north, whence 

 a large part of the fragments come, would be greater than the 

 post-glacial denudation that has been measured elsewhere. 

 Second, the delta front and the even sloping delta-plain imply 

 standing water, and if this water level existed for so long a time 

 as would be required to form such an extensive deposit, we 

 should expect to find more evidence of its shore line in other 

 localities than now exists. Third, the constructional forms, cusps, 

 hollows, kettle-holes, at the head of the sand-plain are so marked 

 that one cannot believe them to be the product of erosion. 

 The kettle-holes and marshy depressions show that the plateau 

 tops did not extend much farther than at present. 



The dwindling New England ice-sheet, whose existence is 

 proved by other facts, supplies all the conditions necessary for 

 the construction of such discontinuous deposits. The ice-sheet 

 could not have advanced over the plain after its deposition, for 

 the sand and gravel would have been easily carried away. There 

 is no gullying of the sides of the sand-plain ; therefore it was 



