370 GLACIAL GRAVELS OF MAINE. 



the surface appearances at a few small excavations, the table-land consists 

 of sand, gravel, and cobbles mixed in alternating layers, but the northern 

 and southern parts of the plain do not vary much in degree of fineness. 

 The transition between the somewhat lenticular mass of gravel and the 

 marine clay is quite abrupt, proving that they were not formed simultane- 

 ously. Whereas in the delta deposited in the open sea the coarse sediments 

 are stratigraphically continuous with the marine clays, one passing into the 

 other by insensible degrees, in the case of the plain under consideration 

 the glacial currents were, within the area of deposition, not checked suffi- 

 ciently to cause them to drop their clay and silt. The gravel is overlain by 

 the clay, but they are plainly of different origin and dates. Such a mass 

 as this must have been deposited in a gradually enlarging pool or lake 

 within the ice. The inflowing stream did not flow into a body of water as 

 large as the whole plain. I conceive that it first flowed into a small pool, 

 which it partially filled with sand, gravel, and cobbles. Subsequently, as 

 the ice was melted and eroded, the water of the glacial stream continued to 

 flow in the space between the enlarging central mass of gravel and the 

 receding ice. Thus the flow was never checked, as it would have been if 

 it flowed into a lake as large as this one finally became. This sort of 

 structure is substantially the same as that of many of the massive plains 

 that make the discontinuous systems of osars and the discontinuous portions 

 of the osars. The more important features of the class are their solidity 

 (freedom from kettleholes and reticulations) and their coarseness of mate- 

 rial, which is in marked contrast with the horizontal passage into the finest 

 sediment characteristic of the true delta. The top is somewhat convex, but 

 not always conspicuously lenticular. They are found in a part of the State 

 where subglacial streams abounded. They could be accounted for as being 

 formed in the pool where a superficial stream fell down a crevasse, or where 

 a subglacial stream entered a pool or lake within the ice. We know that a 

 superficial stream can make such a pool. In case of a subglacial stream, it 

 is more difficult to account for the pool. It is possible that a subglacial 

 river of fresh water pouring into the sea, or having its channel obstructed 

 for any other reason, would under some conditions be forced to rise up the 

 crevasses, and when the ice became thin enough it could outflow upon the 

 ice, or such a rise of water could be caused by a gorge in the tunnel. This 

 water, now being exposed to the sun, would become warmed; and if so, 



