PLEISTOCENE GEOLOGY OP MOOERS QUADRANGLE 



17 



The explanation depends on a simple consequence of the retreat 

 of the ice sheet from high ground sloping toward its front. In 

 the larger valleys blocked by the ice margin, contemporaneous- 

 glacial lakes will result. These lakes will have their outflow 

 along the lowest point in their borders; thus the discharge may 

 take place across the divide at the head of a valley, the usual con- 

 dition; or it may take place along the ice front; or it may cut into 

 a ridge which separates this valley from the next one. Where 

 the land is relatively smooth, the drainage from the ice or that 

 11 owing toward the ice may be compelled to flow for miles along 

 the front before discharging into the open country or a static body 

 of water. All of these discharge ways along the ice front are 

 denominated spillways by Dr Gilbert. 



Jericho spillway 

 The first signs of a spillway in this district appear in Moose 

 and Jericho rock, above mentioned, on the northern slope of 

 Dannemora mountain at an elevation of about 1500 feet. When 

 the ice sheet had disappeared from the crest of this massif, its 

 front, probably extending northwest and southeast at right 

 angles to the general line of motion, would have allowed the dis- 

 charge of the waters confined along the northern slope of the 

 mountain through the pass of Stillwater brook. The scouring of 

 the rocks would thus be accomplished. 



Great Flat Rock system 



That the flat rock areas extending from Altona to " the Gulf " 

 at Covey hill in Canada belong to a single great stage, is shown 

 by their approximate agreement in range of altitude between 

 620 feet and 920 feet, their alinement along the same general 

 slope, and by their approximate continuity. 



Though the accompanying map shows considerable intervals of 

 fill-covered rock between. the several bare rock tracts within the 

 area, it is probable that a different mapping would somewhat ex- 

 tend the area assigned to the water-swept rocks. Between the Big 

 Chazy south of Altona and the vicinity of Irona the original area 

 of bared rock is undoubtedly much greater than is represented 

 on the map. The district has been overgrown by forests and 

 covered with vegetal debris. Here and there are certainly 



