ORIGINAL ICE STRUCTURES IN SANDS 227 



these conditions probably the interstitial ice became an efficient 

 binder between the individual sand grains and caused the whole 

 to behave as a rock mass, yielding slowly to gradual stresses and 

 breaking or shattering or faulting like a true rock when subjected 

 to sudden changes. 



Subsequent melting of the ice left everything in place, and the 

 whole complex mass suffered so little distortion from shrinkage 

 with the loss of its ice matrix that these secondary structural 

 features are still preserved. 



With this statement of the thesis some of the minor structures 

 may be described. All of them occur in the finer sand deposits 

 exposed in the excavation at 135th Street and Broadway. 



On the south wall there is a low regular arch several feet across 

 in a bed of soft fine clayey sand, perhaps eight inches thick. It 

 is overlain and underlain by much coarser sands. In the upper 

 part there are three narrow vertical cracks some three or four 

 inches deep where the fine sand has been pulled apart, as if by 

 stretching, and the coarse material from above has filled the open- 

 ings. In this case, the difference in the constitution of the two 

 beds seems to have been a controlling factor and the two types 

 of material behaved somewhat differently, the fine-grained sand 

 layers moving as a unit, and cracking on the upper stretched 

 surface of the arch. The fine sand is now so soft as to be easily 

 pared with a knife and the coarse material runs out from above 

 and below it by its own weight. 



At one point on the north wall of the excavation, and less per- 

 fectly at several other places, the sand preserves what seems to 

 be the original shattering and cracking of the frozen mass. It is 

 in very fine-grained sand, easily trimmed down without dulling 

 a knife blade. The fracture lines are decidedly darker than the 

 mass of the sand, sufficiently so to bring out the structures in the 

 accompanying photograph. The original sedimentary structure 

 of the sand is preserved and shows distinctly to what extent the 

 whole was crushed and faulted. It is in fact a sand deposit brec- 

 cia of extremely complicated structure. 



A few feet distant from the last, a wholly different structure 

 is preserved. This is in quite fine-grained, clayey sands which 



