442 



GLACIAL GRAVELS OF MAINE. 



While the sediments of broad osars are prevailingly finer than that 

 of narrow osars, yet these plains show decided variations in coarseness 

 of material in different parts of their courses, just as the osars do. Thus 

 the great Portland system on a north slope in Rumford and Milton consists 

 of fine gravel and a large amount of sand. Approaching the top of the 

 divide at North Woodstock, we find gravel and cobbles, while on the south 



slope from North Wood- 

 stock past Bryants Pond 

 to North Paris it consists 



Fig. 34 Diagrammatic section across osar-plain; Woodstock and Milton. iii iin i i 



of pebbles, cobbles, bowi- 

 derets, and many bowlders 2 to 4 feet in diameter, all very much waterworn. 



In many places the osar-plains have been nuich eroded by small 

 streams and boiling springs. Invariably the erosion has been most rapid 

 toward the sides of the plain, leaving a central uneroded ridge resembling 

 an osar in external form. In some cases the central ridge is composed of 

 much coarser material than the ground on each flanli, but I have not been 

 able to find sections satisfactorily showing the nature of the stratification of 

 the central and lower parts of the ridges. At the tops of the ridges and in 

 the plain at their sides the strata are nearly horizontal, or somewhat cross- 

 bedded, dipping a little toward the south. 



Fig. 34 shows a section across the osar-plain in Rumford and Milton. 

 The central ridge is here known as the "Whalesback," and has about the 

 same height as the uneroded terraces at 

 the sides of the valley. 



■Fig. 35 shows a section across the 

 valley of Bog Brook in Canton and 

 Livermore. The broad osar has here been eroded by two brooks, one on 

 each side of the central ridge, and they flow in opposite directions. The 

 central ridge here rises several feet higher and the material is much coarser 

 than the terraces of sand and gravel that are found on each side of the 

 valley. 



There must be a reason why the central ridge invariably resists erosion 

 better than the matter at its sides. In most cases the ridge is plainly com- 

 posed of much coarser matter. I have found no sections showing that an 

 ordinary narrow osar with arched cross section lies along the axis of the 

 osar-plain, though this is probably the case where the central ridge rises 



Diagrammatic section Jicroas oaai'plain ; valley 

 of Bog Brook, Canton. 



