108 GLAOIAL GRAVELS OF MAINE. 



tliey are very prominent landmarks from every direction, and are locally 

 known as "mountains." Their material is rather coarser than the average' 

 of the osar, and shows the usual sprinkling- of stranded bowlders. 



In Greenfield this osar unites with the Howland tributary. Near their 

 junction are extensive sand-and-gravel plains having- a gently rolling surface. 

 I once supposed that the sea had washed down the original ridges as depos- 

 ited by glacial streams and had redeposited them with a nearly horizontal 

 stratification. As shown elsewhere, the power of the sea to erode till and 

 glacial gravel Avas very limited excej)t on the most exposed coasts. These 

 plains in Greenfield were deltas deposited by the glacial waters near where 

 they poured into the sea, or possibly into a large glacial lake. 



The gravel plain continues on southward along a branch of the Sunk- 

 haze Stream. Soon the plains are left behind and we find an osar of ordi- 

 nary type, often of very large size. This is a treacherous wilderness, and 

 the explorer must not let the osar get out of his sight if he can help it. 

 Just as he approaches the head of the Sunk-haze, he reaches a particularly 

 aggravating swamp. With many misgivings, he concludes to trust the osar 

 for just a few minutes and flank the swamp. Arrived at the other side of 

 the swamp, it is just as he had a right to expect. The osar has vanished. 

 Before him is the top of the divide, dreary with bare ledges and an endless 

 array of roches moutonn^es sprinkled with large bowlders. But really we 

 are dealing with rivers, and the gravel is only a symbol. A mighty osar 

 river certainly came from the north to this place. What became of it"? It 

 must have swept over that divide with velocity sufficient to enable it to cai'ry 

 all loose matter before it except the large bowlders. Still we must seek 

 field evidence that it passed over this divide. Going east, we soon descend 

 to the Morrison Pond, a long narrow body of water situated between two 

 high granite hills which slope steeply down to the pond from each side. 

 Within a half mile the osar reappears. Round cobbles and bowlderets 

 soon appear, and in the jaws of the pass take the form of a large windrow 

 of polished bowlderets and bowlders situated on the south side of the pond. 

 Then for a mile or two on a steep down slope there is but little sediment to 

 represent the osar. The osar river crossed the west branch of the Union 

 River, and immediately we find a broad series of sand and gravel plains 

 in Aurora known as the Silsby Plains. These are about 5 miles long and 

 from 1 to 3 miles wide. They extend about 1 mile north of the outlet of 



