140 G. H. Stone — Glacial Sediments of Maine. 



flowed, not even the swollen rivers of the valley drift epoch. 

 These conditions prove that the clay was deposited in a channel 

 between ice walls. The central gravel ridge proves that 

 originally a glacial stream flowed in a narrow channel and 

 deposited a kame or osar. Later the channel broadened so as 

 to become an eighth of a mile wide or somewhat wider. The 

 border-clays were deposited in channels resembling those of 

 the osar-plains, but they all belong to the shorter gravel sys- 

 tems. In the case of the longer glacial rivers the flow of water 

 was moderately swift, so that a plain of sand and gravel was 

 laid down in' the broad channel. When the supply of water 

 was small the flow became so slow in the broad channels that 

 the finer sediment was dropped. This clay flanks and overlies 

 the sides of the central ridge, or in some cases appears to pass 

 into by degrees and to stand at the same level. 



As there are some advantages in confining the names kame 

 and osar to ridges of the coarser sediments, I give this bordering 

 clay a special name, though in fact it is genetically a broad- 

 channel deposit like the osar-plains. In both, the central part 

 of the plain is coarser than the material at the sides. 



In Madison and Anson a plain of the border-clay is strewn 

 with quite a number of bowlders up to six feet in diameter, or 

 somewhat more. These bowlders show no sign of water-polish 

 on their surfaces and have the till-shape. But there is no 

 sheet of till overlying the clay, only these occasional erratics. 

 They were probably dropped from small bergs or ice-floes 

 floating on the glacial stream. 



11. Frontal Plains. — These are plains of sediment brought 

 by glacial streams down to the extremity of the glacier and 

 then spread sub-aerially over the land in front of the ice. 

 The sediment extends across the bottom of the valleys in 

 which it is found, and thus with respect to these valleys, is a 

 form of valley drift. In Maine the frontal plains or deltas are 

 found in valleys having a southward slope and at an elevation 

 of more than 230 feet. They form the southern terminations of 

 rather short osars. The osar ridge widens as we go southward 

 and soon expands into a plain that extends across the whole 

 valley. The stones are very much water-worn, and thus have 

 the shapes found in the glacial gravels but not in the beds of 

 streams, except the very steep mountain streams. Within a 

 few miles we pass beyond the gravel and coarse sand and find 

 the valley covered with a deep sheet of silt and clay. These 

 so-called frontal plains are found in only a few places in Maine 

 and are situated sixty or more miles back from the coast. 

 They must have been formed late in the ice age when the ice 

 had melted over the coast region. Both the frontal plains and 

 the marine and also the lacustrine deltas, were deposited in 

 front of the ice, but under different circumstances. 



