ISf) 



(}. K. aiLBERT UJIM'LK-MAKJCS AND CROSS-niCDDING 



ocean bed, and the unconformities record tlie readjustment of the sand- 

 ripple pattern when the controlling water movement assumed a new 

 direction. 



In width (distance from crest to crest or trough to trough) these giant 

 I'ipples range from 10 to 30 feet ; in height, from 6 inches to 3 feet. Their 

 material is a sand of medium grain. Their exposure in complete form 

 is comparativel}^ rare, but the cross-bedding with which they are con- 

 stantly associated is exhibited in a majoritj-- of the visible sections of the 

 sandstone lenses between the Niagara river and Medina, a distance of 

 33 miles. In Lockport they are found in lenses of three different hori- 

 zons, and the opportunities for ex- 

 amination are excellent. The3' are 

 also well exhibited in the principal 

 sandstone ledge at Lewiston, the 

 ledge named by Hall "the Quartz- 

 ose Sandrock." 



Figure 1, in plate 13, shows an 

 exposure in a small disused quarry 

 on the south bluff of Eighteenmile 

 creek, in Lockport, near the Indu- 

 rated Fiber -works. It includes two crests and an intervening trough. 

 The depth of the trough is 29 inchess and its width (from crest to crest) 

 23 feet. Figure 2, plate 13, shows a trough fragment laid bare by the 

 side-hill cut of the New York Central railroad in the gorge of the Niagara 

 river. The portion observed meas- 



ures 15 feet across and 16 inches in 

 depth, but neither of the limiting 

 crests remains. A larger pattern is 

 indicated by a crest and part of the 

 adjacent trough seen in a ravine 

 about 2 miles east of the village of 

 Pekin (figure 1). A triangular prism 

 of sandstone 3 feet high, embedded 

 in shale, has been partly bared by 

 the cutting of the ravine. Its form 

 and relations show that it is a de- 

 tached crest of the sort produced 



Figure 1. — Section of Prism of Sandstone. 



The locality is 2 miles east of Pekin. The 

 sandstone is represented in the diagram by a 

 dotted pattern ; the enclosing shale by lines. 

 Broken lines distinguish the inferred part from 

 the visible portion of the section. 



FiGUKE 2. — Typical Profiles of Sand-ripplcs. 



a, the ordinary form with angular crests, h, 

 imperfect development of form a when the sup- 

 ply of sand is small, c, rounded form supposed 

 to lie derived from a by subsequent gentler agi- 

 tation of the water. 



when the supply of sand is not sufficient for the modelling of the fully 

 developed ridge. In such cases the sand is gathered in a series of wind- 

 rows corresponding to the crests of fully formed ripples (see h, in figure 

 2). As the ordinary ratio of width to height in sharp-crested fossil ripple- 

 marks is 10 to 1, the thickness of this prism implies a ripple width of 30 

 feet or more. 



