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NEW YORK STATE MUSEUM 



that it disappears by thinning", either bringing the layers above and 

 below it in contact or giving way to a bed of shale. 



A careful examination of these individual beds will show the pres- 

 ence of ripple marks in many of them. This indicates moderately 

 shallow water during the accumulation of these sands; for ripple 

 marks are found only down to the depth to which wave action pene- 

 trates. These ripples vary greatly in size, a bed about 10 feet above 

 the concretionary layer showing examples in which the crests are 

 from one to one and a half or more feet apart. 



The fossils found in these sandstones are the characteristic Medina 

 pelecypods, and the common Medina L i n g u 1 a cuneata. 



5 The thin bedded sandstone layers are followed by 12 or 15 feet 

 of massive sandstones in beds from one to several feet in thickness, 

 and varying in color from reddish to grayish. This rock generally 

 shows strongly marked cross-bedding structure on those faces 





Fig. 2Ca Cross-bedding in Medina sandstone, Niagara gorge. 



which have been exposed for some time. This structure illustrated 

 in figure 20a, copied from a ledge of this rock, indicates diverse cur- 

 rent and wave action in the shallow water in which this rock was 

 forming. While the deposition of the strata was essentially hori- 

 zontal, the minute layers made up of the sand grains were for a 

 time deposited at a high angle, much after the manner of deposition 

 of the layers in a delta. After a while the activity of the current 

 changed to another direction, and the layers already deposited were 

 in part eroded, or beveled across the top, and new layers, inhar- 



