GLACIAL MAEINE DELTAS. 371 



would in time form the pool. The problem is closely connected with the 

 general subject of the discontinuous osars, and will be refen-ed to again 

 later. 



GLACIAL MARINE DELTAS. 



Before proceeding to the discussion of tlie discontinuous osars it will 

 be of advantage to consider the general characteristics of the delta-plains 

 deposited by glacial rivers in the sea. They are here named "glacial marine 

 deltas." (See PI. XXVII, B, opposite p. 336.) They were of two kinds. 



I. Those deposited in front of the ice in the open sea. This class 

 spread outward in rounded or irregular fan shape when deposited over 

 broad and rather level plains where they were free to expand in all direc- 

 tions. In narrow valleys their shapes were necessarily determined in part 

 by the adjacent hills. They conspicuously show the characteristic hori- 

 zontal transition of sediments from coarse at the north to finer toward the 

 south — that is, away from the mouth of the glacial river. The surface 

 slopes gradually downward and outward radially to the outer edge of the 

 delta, but in tidal waters this slope is much more gradual than on the land. 

 The sand of the delta passes by insensible gradations into silt and silty 

 clay, which in turn merges into fine fossiliferous clay. In the region of 

 transition between the sand and clay the two deposits have the same siu-- 

 face level. Thus the proof is conclusive that they are contemporaneous 

 and that the clay is a continuation of the coarser parts of the delta. But 

 while logically and genetically the clay is part of the delta, yet since the 

 sediments of the glacial streams are so much more largely composed of 

 sand, gravel, and lai-ger stones and bowlders, I here include under the 

 term "deltas" only that portion composed of coarser matter. Moreover, the 

 clayey parts of the deltas are so mixed with clay derived from wave 

 erosion of the till, also with the clay brought down by the swollen rivers 

 of land drainage at the close of the Glacial period, that it is difficult to 

 distinguish the glacial from the other clays. So also the delta clays, being 

 scattered up and down the coast, often blend with one another, and the 

 separate deltas are indistinguishable. It is noticeable that the clays are 

 thicker near the mouths of the glacial rivers, a.nd doubtless the spirit level 

 will sometimes reveal where the mouths of the rivers were in cases where 

 to the unassisted eye the deltas are confluent. In mapping the marine 

 deltas it has been a matter of difficulty to determine where sand ends and 



