GLACIAL MAEINE DELTAS. 373 



these broad channels in which the narrow deltas were deposited would 

 indeed be estuaries. On further reflection I find they can also be accounted 

 for as having been deposited in broad ice channels at a time when the sea 

 stood not at its highest elevation but at the level of the delta itself, or per- 

 haps at the place of transition from gravel to sand. The ice front then 

 stood at or near the place of transition from sand to clay, where the clay 

 of the local delta merges in the general sheet that covers all the coast. On 

 this conception we have in the narrow deltas a type of the sediments 

 poured into the sea or formed at or near sea level during a rise of the sea 

 accompanied by melting of the ice as it advanced. If so, we could expect, 

 on sloping shores, and where the flow of the glacial stream continued 

 marine, a delta to extend backward from the outer one, up to the level of 

 230 feet. Such a recession would show finer sediments overlying the earlier 

 and coarser ones, since at any given point the water would be growing 

 deeper and the distance to the shore greater as the sea rose to its highest 

 level. It is doubtful if the facts thus far observed make it possible to 

 decide positively between the two hypotheses, and indeed narrow marine 

 deltas may have been formed in both of these ways. On either hypothesis 

 the delta as we go southward was bordered laterally by ice until we reach 

 the place where the delta clay merges into the broader clay sheet of the 

 coast. As the ice retreated and the delta channel broadened, clay was laid 

 down over the valleys at the sides of the original ice-bordered delta. The 

 currents would naturally be swifter in front of the main channel. For this 

 or some other reason the later-deposited clay was thin or lacking on top of 

 the sand-and-gravel portions of these as well as on the broad or fan-shaped 

 deltas. 



That the clay into which the marine delta-plains pass by insensible 

 gradations is a true marine sediment is evidenced by the following facts: 



1. The clay extends continuously from the delta-plains to the present 

 seacoast, a distance of 10 to 30 miles, and in a few cases even a greater 

 distance. 



2. This clay thus is continuous with the clay that surrounds the beach 

 gravels. We can not separate them. 



3. In many places this clay contains marine fossils. Near the belt of 

 transition between the glacial delta sands and the clays I have not been 

 able to find fossils, but within 2 or 3 miles south from that point fossils 



