L98 



w 



M. DAVI: GLACIAL SAND PLAINS. 



/ idena from Cross-bedding. — Normal oblique deposition may be typified 

 in fig. "_', from which it appears that every bed presents ;i convex upper 



portion, a />, and a concave lower portion, c d, joined 1>\- a tangent, l> e. 

 When a change of current carries away the upper part of such a deposit 

 abov( the line e /, tin- upper convex curve is destroyed ami only part of the 

 tangent and the concave curve remain : and when a later change brings 

 additional deposits, these lie with their concave lower curve.- tangent to the 

 surface of truncation of the earlier beds. It does not appear that the forms 



Figure 2. — Ideal Si I ling. 



and relative position of such beds would be changed, whether they are laid 

 down on a descending or an ascending surface : the only essential condition 

 of their growth is the presence of a stream of varying power and load, but 

 on the whole of greater load than power. Back-set and fore-set beds should, 



their lore, t u in the concavity of their cross-beds in the direction of the stream 

 that formed them — that is, in the direction from the head to the front of the 

 plain. 



Figs. •'! and 1 are from sketches made of the back-sel beds at the head of 

 a -ami plain in Newtonville, and of the fore sel beds at the extremity of a 







