THE NEWTONVILLE SAND-PLAIN. 807 



5. Comparison of models. — Turning from Fig. 1, which shows 

 the deposits as they exist to-day, to Fig. 2, which shows the 

 theoretical conditions of formation, it will be seen that the north- 

 ern half is covered with ice, from which is issuing an esker river. 

 The ice in the second is represented as fitting into the inter- 

 cuspate hollows shown at the head of the sand-plain in the first 

 model, and is from one hundred to three hundred and fifty feet 

 thick. Toward the rock hills on the east and west it falls off, as 

 would be the case where the ground was higher. The ice has a 

 convex curving surface in front, with contours softened by melt- 

 ing, while on top it is approximately level with here and there 

 surface streams, moulins, and perhaps a little lake. 



The three little knobs of older date than the sand-plain 

 standing near its front margin, can be seen in both models. The 

 till-covered hills of bed rock are also the same in the two, but in 

 the second the water stands higher up on their sides. The 

 second model being a trifle larger, a little more of them is shown 

 on the edges. The group of hummocky kames, shown to the 

 southwest of the sand-plain in the first model, is covered in the 

 second by the body of standing water into which the esker river 

 flowed. 



6. Esker river. — Professor Chamberlin has given us the very 

 helpful distinction between " kame " and "esker" (osar), from 

 the use of the words in Scotland and Ireland respectively. The 

 former is used by the Scotch for their irregular mounds and hil- 

 locks, so typically shown in that country, and which, if developed 

 at all in lines, have their axes at right angles to the direction of 

 ice flow ; and the latter for the Irish ridges of sand and gravel, 

 beds of former glacial rivers, which have their axes parallel to 

 the lines of motion in the ice. This terminology is here followed. 



In the first model the esker, a ridge of sand and gravel, fairly 

 stratified, may be traced from the middle of the northern end, 

 where it is some ten to twenty feet high, curving eastward and 

 then southward again, gently rising to some seventy feet above 

 the alluvial plain shown on the northwest corner of the model of 

 the sand-plain, and one hundred and thirty feet above mean tide. 



