I. Bowman — Atlantic JPregladal Deposits. 317 



eter of 6 feet. The two classes of deposits are in uncon- 

 formable relation to each other, and along the entire cliff face 

 are seen to be in strong unconformity with the beds on which 

 they rest (see B, C, D, fig. 1). The detailed structure of the 

 till is shown in fig. 3, great blocks of bowlder clay being 

 separated by a " filling " of horizontally stratified sand which, 

 beiug without the slightest marks of disturbance since deposi- 

 tion, was deposited in water either contemporaneously with the 

 till or later as a secondary de- 

 posit between the blocks of till 

 as fast as the ice between the 

 blocks was melted out. At 

 Fourth Cliif similar structures 

 are seen with a considerable 

 part of the material derived 

 from the preglacial beds eroded 

 by the ice sheet in its advance. 

 At Indian Hill, seven miles 

 southeast of Plymouth, a good 

 85-foot section shows the 

 further complexity of a layer 

 of clay entirely different than 

 any in the Third Cliff section, 

 with a thickness of 20 feet and 

 underlaid by erratic sand and gravel. 



This layer may represent an interglacial deposit or a deposit 

 formed during a temporary recession of the ice. Its homo- 

 logue does not exist in the Third Cliff section, nor is there 

 any indication whatever at the present time of deposits or 

 structures developed contemporaneous with those at Indian 

 Hill. In all three sections there exists a thin layer of iron- 

 stained reddish and apparently older till just above the strati- 

 fied material of aqueo-glacial origin. 



Fig. 3. Showing structure of till 

 just beyond left margin, fig. 1. Sand 

 between till blocks is cross-bedded 

 and undisturbed. Figure represents 

 area about 5 feet square. 



Succession of Events in the Deposition of the Third Cliff Beds. 



The points to be considered in the determination of the age 

 of the beds below those of glacial relationships will be better 

 understood after their interpretation in terms of the events 

 which they represent. 



It appears that we have represented in the 40 or more feet 

 of yellow clays and sands first described, marine conditions 

 with a steady shallowing of the basin of deposition either 

 through uplift or the gradual upbuilding of the floor through 

 sedimentation. These conditions are marked particularly by 

 the sand inclusions which occur on the top of the clays and 

 the gradual transition of clay to sand with the upper part of 



