ZONES OF THE MAINE lOE-SHEET. 295 



found ill much of Greenland. This would favor a low surface gradient of 

 the ice-sheet. The slope being southward would favor a higher gradient. 

 It is probable that on a uniform slope the gradient is chiefly determined b}^ 

 the ratio between snow precipitation and waste. During the advance and 

 retreat of an ice-sheet over transverse hills and valleys the surface gradient 

 must often change with some corresponding change in the positions of the 

 crevasses and in the boundaries of the zones of superficial and subglacial 

 waters. 



The ice flowed over Mount Desert Island to an unknown depth. From 

 there to Mount Katahdin the distance is approximately 110 miles, and they 

 are nearly in the same lines of glacial motion. Prof. C. H. Hitchcock, in 

 his report on the geology of Maine, estimated that the top of Mount Katah- 

 din rose above the ice surface. I visited the mountain in 1870 and found 

 fossiliferous drift fragments to within a few hundred feet of the summit, 

 just as Professor Hitchcock did, but there has been so much surface 

 weathering and sliding toward the top that drift debris would long since 

 have disappeared, even if it had once been there. However, without 

 insisting on the doubt, if we assume the highest limit of the ice at 4,500 

 feet at Katahdin and 1,500 feet at Green Mountain, Mount Desert, we have 

 a surface gradient of 27 feet per mile. If the gradient was as moderate as 

 this, or near it, we have reason to estimate the zone of subglacial waters 

 as })retty broad. 



The western part of Maine must have been overflowed by the ice 

 from the St. Lawrence Valley and Hudson Bay. How far east this north- 

 ern ice overflowed Maine is at present uncertain. Without assuming the 

 correctness of Mr. Chalmers's hypothesis of a divergent flow in eastern 

 Quebec and New Brunswick as applying to Maine, we must at least con- 

 sider it a possibility. Obviously the breadth of the zone of subglacial 

 waters of an ice-sheet fed from the far North will be much greater than of 

 a local ice-sheet covering the peninsula south of the lower river and Gulf 

 of St. Lawrence. Until the doubt as to the condition of northeastern 

 Maine is removed it will be unsafe to attempt an estimate of the ^Dosition 

 <if the n^ve line at any stage of the glaciation. 



