THEORETIC DISCUSSION 605 



Knolls stand above the plain, as evidence of the failure of the waves to 

 complete their work. Other plains in the immediate neighborhood have 

 different levels. The list of 30 plains or leveled tracts given by Clapp 

 (68, page 202) emphasizes the fact of great discordance in altitude of 

 plains in the same district. That they were not laid in glacial lake 

 water is proven by the following negative characters : Xo relation to any 

 lines of land drainag^e; no relation to any passes across divides; no 

 correspondence in altitude; no similarity in form; no definite structures. 

 They are wave-smoothed kame or other deposits which were sensitive 

 to the agitated oceanic waters (see further description, page 609). 



As land uplift progressed and the rivers extended seaward, intrench- 

 ing their earlier deltas, they swept detritus over the lower ground within 

 their reach; but, as no heavy inferior deltas are found, it is concluded 

 that the rise of the land was comparatively steady, and that the rivers 

 spread their later loads, which were finer detritus than at the higher 

 levels, mostly under the sealevel waters. 



A criterion of the 23rimitive water level is the unmodified form of the 

 kames and moraines in areas above the theoretic marine surface and 

 the washed down forms beneath that surface. A good locality is the 

 morainal tract between Foxboro and Wrentham. 



SUBMERGEXCE PhEXOZ^IEXA 

 1. SUMMIT DELTAS 



These deltas are not the stream-contributed deposits at inferior levels, 

 but the plains built at the initial and summit level of the sea by the 

 streams (rarely glacial outwash in this region) flowing from the higher, 

 unsubmerged ground. These summit deltas, as valley filling, are rep- 

 resented on practically every stream examined in northeastern America 

 lying within range of the theoretic submergence. They may be confi- 

 dently predicted for every stream. 



Deltas produced during a single phase must have similar altitude ; but 

 deltas built far inland, in the deep valleys, might be laid after som»3 

 rise of the land had occurred, and hence such deltas do not register the 

 full amount of land uplift. In the area under present discussion any 

 variation of this kind must be very small, if present at all, and certainly 

 is negligible. 



Heavy deltas built in constricted valleys by vigorous and well loaded 

 streams might displace the estuary waters for some miles, so that the 

 line of standing water becomes far distant and much below the head of 



