202 ROCHESTER ACADEMY OF SCIENCE. 



debris. In numerous places the roof of this tunnel has broken in 

 and the tunnel itself is now deserted for some distance by the stream, 

 so that the debris is caving down into the bed of the old tunnel as 

 the edges of the ice melt away, thus forming a tortuous ridge, with 

 projecting knolls where the funnels into the tunnel are oldest and 

 largest. At the same time, the ice on the sides at some distance 

 from the tunnel, where the superficial debris was thinner, has melted 

 down much below the level of that which was protected by the 

 thicker deposits ; and so the debris is sliding down the sides as well 

 as into the tunnel through the center. Thus three ridges approxim- 

 ately parallel are simultaneously forming — one in the middle of the 

 tunnel and one on each side. When the ice has fully melted away, 

 this debris will present all the complications of interlacing ridges 

 with numerous kettle holes and knobs characterizing the kames ; 

 and these will be approximately parallel with the line of glacial 

 motion. The same condition of things exists about the head of the 

 subglacial stream on the east side, also' near the junction of the first 

 branch glacier on the east with the main stream, as also about the 

 mouth of the independent glacier shown on the map lower down on 

 the west side of the inlet" (126, pp. 65-66). 



Eskers may have been formed between hill slopes and the steep 

 edge of the ice. Upon the melting of the ice such deposits would 

 tend to slide down and in some places be preserved with decidedly 

 esker form. 



An esker near Polmont in the south of Scotland, described as 

 glacio-fluvial by Gregory, is unstratified, made up of angular mate- 

 rials, contains no bands of sand and appears to have been formed 

 as a bank of wash quietly deposited along the margin of a melting 

 glacier at one stage of its retreat. 



Ridges may also be formed subaerially as natural levees, the 

 current being bordered by slacker water. Likewise when streams 

 enter standing water they tend to build up ridges at either side. 

 Subaqueous ridges may be formed at the mouth of streams, and also 

 in the lee of islands or other obstructions in the midst of sediment 

 bearing streams (99). 



