136 SURFACE GEOLOGY. 



appears that a thick deposit of sand underhes a thin surface of till upon 

 a hillside between the heights of 100 and 200 feet above the lake. These 

 localities have become noticeable because a great depth of sand has been 

 excavated by rivulets. Probably thinner deposits of sand exist in many 

 places underlying till, but not having an economical value like the clay, 

 they have escaped notice. 



At the north-west ends of Rattlesnake and Davis islands deposits of 

 clay are found similar to that of Clay point, and in former times it has 

 been excavated at both these places for brick-making. The same abrupt 

 bank from 20 to 30 feet high forms the shore, and the surface of coarse 

 till slopes gently upward from its top. The underlying clay-beds are free 

 from pebbles and plainly stratified. 



Review and Conchtsio7is. 



These numerous examples make it probable that many other similar 

 deposits exist about these lakes, since their presence is not usually indi- 

 cated on the surface. In considering the question of their origin, we 

 notice that these beds of stratified clay and sand are uniformly overlaid 

 by a comparatively thin covering of unstratified glacial drift, which in 

 every instance is probably wholly made up of the loose, sandy, and very 

 coarse material which we have called upper till. It is remarkable that 

 no similar deposits are found on the hillsides without this thin covering 

 of till. The examples found, however, do not lie in the pathway of any 

 stream which could be supposed to bring the modified drift, or to exca- 

 vate and carry it away if any had been left on the surface; instead of this 

 they occur on the rounded slopes of hills at all heights from the lake 

 shore to 200 or 300 feet above it. If any clay beds had been left in such 

 situations without a covering of till, they would remain to the present 

 time, and would be worked in preference to others for brick-making. It 

 is also remarkable that these deposits frequently extend in a stratum of 

 varying thickness over a considerable area of hillside, sometimes appear- 

 ing to be continuous upon a slope which rises 100 feet or more in vertical 

 height. In all cases, however, where the stratification has been seen, it 

 is approximately horizontal and riot conformable to the slope. 



The section observed near Squam river in Ashland (p. 132) indicates 

 the probable position and mode of formation of these deposits of clay 



