BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 10, pp. 135-140, PL. 13 March 23, 1899 



RIPPLE-MARKS AND CROSS-BEDDING 



BY GROVE KARL GILBERT 



(Presented before the Society December SO, 1898) 

 CONTENTS 



Page 



Giant ripples of the Medina 135 



Interpretation in terms of physical condition 137 



Associated cross-beddino- . . 139 



Giant Ripples of the Medina 



The Medina formation consists chiefly of red shale. In the type dis- 

 trict, about Medina, New York, the thickness is about 800 feet, and there 

 are beds of sandstone in the upper hundred feet. Most of the sandstones 

 are argillaceous and soft, but there are a few lenses comparatively free 

 from clay. These are usually white or gray, and afford a strong, dural)]e 

 stone, extensively quarried for structural purposes. 



From some quarries flags and blocks of large dimensions can be ob- 

 tained, but others yield only small and irregular blocks, because the rock 

 is traversed by cross-bedding. The oblique structure is of a peculiarly 

 intricate type, often exhibiting dips toward all points of tbe compass in 

 the same quarry, and associated with it are maii}^ unconformities. There 

 are places in the floor of the Whitmore quarry at Lockport where the 

 strike of a dipping layer can be traced through an elliptic arc, like the 

 end of a spoon, for 150 degrees, the dip swinging with the strike. There 

 are places where oblique partings of opposite dip are seen in section to 

 unite at the top, making angular anticlines, and other places where they 

 unite below, making smoothly curved synclines. 



Studying these structures in the summer of 1898, 1 was at first greatly 

 puzzled, but atJast became satisfied that they are phenomena of sand- 

 rippling, differing in no respect except size from the familiar rip])le-mark 

 of the bathing beach and the museum slab. The anticlines and syn- 

 clines are crests and troughs of sand-ripples, the cross-bedding is a re- 

 sult of deposition during the maintenance of a rippled surface on the 



XXI— Bui.i,. Geol. Soc. Am., Vol. 10, 1898 (135) 



