Page:The Journal of geology (IA journalofgeology11893univers).pdf/388
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-machè model is a close representation of the original clay.
3. A late glacial deposit.—A glance at the first model will show the typical form 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 fluviatile 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