316 GLACIAL GEAVELS OP MAINE. 



that the courses of the surface streams are determined by the accidents of 

 the Avinter snow drifts. Many observers report seeing domes and rounded 

 ridges on the ice, presumably formed by some buried obstruction.^ Instru- 

 mental surveys might reveal shallow anticlines or synclines where to the 

 eye there was a plain. Where ice flows over a ridge that is parallel to its 

 motion there will be a bulging at the stoss end of the ridge, and probably 

 then to leeward there would be a shallow valley on the top of the ridge for 

 a coxisiderable distance, caused by the retardation of the flow at the bulg- 

 ing. But in Maine the hills were mostly transverse, and the transverse 

 billows of the ice-sheet would be more numerous and higher than the 

 longitudinal ones. It is certain that the osar rivers penetrated the higher 

 hills by low cols and passes. In many cases, especially in western Maine, 

 they must have been subglacial rivers. It is as yet uncertain whether we 

 are to attribute the courses of these subglacial rivers wholly to conditions 

 existing within or beneath the ice, or whether we can trace additional links 

 in the chain of causation and can declare that the courses of the subglacial 

 were in part determined by those of the superficial streams, and that these 

 in turn were determined to the low passes by the undulations of the surface 

 ice as it flowed over the adjacent hills. Such an investigation could not 

 proceed far without the aid of a topographical map. The facts in the field 

 certainly seem in numerous cases to favor the hypothesis. The topic will 

 be referred to later. 



FORMS OF GLACIAL CHANNELS. 



Observation proves that the subglacial and many of the englacial 

 channels have arched roofs. This is chiefly due to the fact that the waters 

 are always in contact with the lateral walls, but only in time of flood can 

 they reach to the roofs to melt them, and partly because water of 39.1° tends 

 to sink to the bottom. In case of a roaring stream this would have little 

 effect, but it might be an important element in case of a quieter flow, as 

 when a stream enters an eidargement of its channel or goes up and over a 

 hill. That the melting is most rapid near the bottom of a cavity that con- 



' Lieutenant Peary, Bull. Am. Geog. Soc, vol. 19, p.287, 1887,says: "As to the features of the 

 interior beyond the coast-line, the surface of the 'ice hlink' near the margin is a succession of 

 rounded hummocks, steepest and highest on their landward sides, which are sometimes precipitous. 

 Farther in, these hummocks merge into long flat swells, which in turn decrease in height toward the 

 interior, until at last a flat, gently rising plain is reached, which doubtless becomes ultimately level." 



See also Prof. I. C. Kussell, Nat. Geog. Mag., vol. 3, pp. 106, 107, 132, May 29, 1891. 



