THE ORIGIN OF CROSS-BEDDING. 199 



frontal lobe of a sand plain near Wakefield, both in eastern Massachusetts. 

 It is manifest that both were built by a stream moving to the right in the 

 figure. This corresponds to the direction from head to front of the plains, 

 and indicates that the back-sets were built by an ascending stream, rising 





Figure 4.— Cross-bedding at the Front of a Sand Plain. 



from beneath the ice to the top of the delta plain, while the fore-sets were 

 built by a descending stream, flowing from the plain into the water at its 

 front. 



Ratio of Sand-plain Groivth to Ice Melting. — The ratio of fore-set and back- 

 set beds is of interest, for, as already stated, it indicates the ratio of the for- 

 ward growth of the delta to the backward melting of the ice. The sections 

 thus far examined do not furnish final numerical results ; but enough has 

 been seen to make it clear that the fore-sets are from ten to forty or fifty 

 times as extensive as the back-sets, and from this it appears that the melting 

 of the ice was slow compared to the growth of the delta plain. 



Origin of Depressions in Sand Plains. — This conclusion is of value in ex- 

 plaining the pits, kettles, and irregular depressions that frequently interrupt 

 the otherwise level surface of the plain. The theory has long been current 

 that these pits were the sites of isolated blocks of ice, around which the sands 

 of the plain were deposited ; but it has also been currently objected to such 

 an explanation that it involved an improbable and unproved rapidity of 

 sand-plain growth. The conclusion just gained from the ratio of the fore- 

 sets to the back-sets overcomes this objection. No satisfactory section of the 

 slopes of a pit has, however, yet been found to give more direct evidence on 

 this question. 



