LOWER STRATUM OF VALLEY DRIFT. 485 



but there appear to be local reasons for the j)eculiar alluvial drift of this 

 valley, such as its direction, its passing through so many lakes that would 

 arrest its sediments, its large ravines or gorges of postglacial erosion both 

 in rock and till, with terraces composed of the coarser eroded matter extend- 

 ing for some miles below the gorges, etc. 



CAUSES OF THE RELATIVE FINENESS OF THE LOWER STRATA OF THE VALLEY 

 DRIFT AND THE MARINE BEDS OF THE INTERIOR VALLEYS. 



The valley drift passes into the marine beds by not easily distinguish- 

 able gradations. They are here treated together in order to avoid the 

 necessitv of absolute determination or distinction of one from the other in 

 the field. 



THE LOWER STRATUM, COMPOSED OP OLAY, SILT, OR FINE SAND. 



1. We have alread)' given proof that this sediment was chiefly of 

 glacial origin. 



2. The average composition of the till is such that great quantities of 

 fine glacial sediment demand the existence of great quantities of the coarser 

 matter also, although it must be admitted that in some of the interior 

 regions, as the upper Kennebec Valley, the local slates would cause the till 

 to have a finer than average composition. 



The inference follows that at the time the finer basal clays and silts 

 which cover the bottoms of the valleys were being- deposited, there was 

 also a body of coarser sediments being deposited higher up in the valleys, 

 or in part, perhaps, in channels within the ice. The smaller glacial streams, 

 perhaps, then carried little beyond the ice front except Gletschermilch and 

 the finer debris. 



3. Fineness of sediment implies the presence either of the sea or of a 

 lake, or, if above their level, a very gentle slope. Some of these basal fine 

 sediments pass above any level of the sea that now appears at all admissi- 

 ble. The interpretation is thus preferred that the land slopes were very 

 gentle at the time the basal fine sediments were deposited. Such low 

 gradients must have marked the time of deepest subsidence of the land, 

 and I see no other assignable cause — remembering that the subsidence in 

 northwestern Maine was three or more times that of the coast, or, rather, 

 that the postglacial elevation has been such. 



