THEORETIC DISCUSSION 603 



floored by sands and silts similar to the materials forming the plains. 

 The creases do not have the definite form and character of land streams, 

 but agree with the supposed form and nature of tidal scourways. Along 

 lines parallel to the sea the plains are very flat and smooth, except for 

 the intercepting creases, and are too smooth for the conception of a 

 series of united fans. The slope seaward is too steady, and with too low 

 gradient to represent subaerial gravels. The composition of the plains 

 is too uniform, and on tlie low ground too fine, to be the deposition of 

 land streams from glacial outwash. Further description of these plains 

 is given in writings 27 and 77. If they were the product of subaerial 

 glacial outwash, then similar plains should have been formed on uplaiub'. 

 in unconfined areas, along the face of the retreatal moraines. 



GENERAL STATEMENT 



The third postulate, as given above, must be ruled out, admittedly 

 for the mainland. The wide silt and clay plains of the low grounds are 

 recognized as products of deep water. The great number of kettles 

 and kettle lakes in southeastern Massachusetts among level plains of 

 sand and silt can not be explained by mechanics of subaerial agency. 



The suggestion that the morainal belt and border of the mainland 

 received its uplift previous to the last ice-invasion has no support in 

 fact. Over all the area the phenomena of standing water are superfi- 

 cial and unaffected. There is no evidence of any glaciation of Xew 

 England subsequent to that which built the great terminal moraine. 

 Any late ice-sheet would have more or less obliterated the features 

 made by the submergence and should have left a distinct terminal 

 moraine and other conspicuous evidence. 



The second postulate, that of shallow waters facing the receding 

 ice-front, is also disproved. The heavy deltas and extensive cobble 

 plains that must mark the edge of the standing water are absent on 

 the low ground, where we find the clays and other proof of deep water. 

 The Merrimac Eiver will serve as an illustration. If the area of eastern 

 Massachusetts had stood near its present level at the time when the ice- 

 sheet melted, the river would have built a huge gravel delta in the dis- 

 trict of Newburyport, and its aggraded coarse deposits would have 

 buried all the low ground along its course in Massachusetts; but its 

 earliest, heaviest, and coarse deposits were dropped far north in New 

 Hampshire. Over the lower ground, in Middlesex and Essex, it spread 

 only finer detritus, contributing to the low marine filling among the 

 hills about Lowell and Lawrence and the wide plains about Newburypovt. 



