C. K. Wenhvorth— Wedge Work of Pebbles. 313 



Aet. XXIII. — A Note on the Wedge Work of Pebbles; 

 by Chestee K. Wentwoeth. 



Tlie phenomenon here described is one of those very 

 simple processes which are so obvions as ordinarily to be 

 considered nnworthy of mention. But one reference to 

 the process is known to the writer^ and it is not so far as 

 he is aware mentioned in any textbook of geology. 



The wedge work of pebbles first came to the attention 

 of the writer several years since at numerous localities 

 along the gorge of the Potomac River below Great Falls. 

 The action of pebbles in this fashion has since been 

 noticed at other places where the conditions are similar 

 and it will suffice to describe the Great Falls occurrence 

 as typical. Below these falls the Potomac River flows 

 between rock walls which range from 20 to 80 feet in 

 height above low water level. The rock is gneiss which is 

 extensively cut by joints. The rock walls are deeply 

 fissured by diiferential weathering along lines controlled 

 by the jointing and by the unequal resistance of different 

 zones of the gneiss. On both sides of the present gorge 

 of the river are the rock cut benches of the outer valley 

 on which are strewn sands and gravels in thin, irregular 

 patches. During flood the river rises 20 to 30 feet within 

 the inner gorge and by occupying a number of channels 

 which are dry at other times separates several rock 

 islands from the mainland. In the vicinity of the river 

 the surface of the gneiss is commonly fresh and the rock 

 hard and compact. Within the wedge-shaped open joints 

 are numerous rock fragments, some of which are angular 

 blocks and others well rounded pebbles and cobbles from 

 the gravel. These have lodged in their present positions 

 in part by falling from the level of the rock bench above 

 and in part by deposition during flood stages of the river. 

 The notable feature is that a very large proportion of the 

 pebbles and blocks are wedged tightly in place in the 

 cracks which narrow downward. Pebbles of one or two 

 inches in diameter are more commonly than otherwise 

 held between comparatively smooth rock surfaces so 



^Wade, A., Some observations on the eastern desert of Egypt, Quart. 

 Jour. Geol. Soc, vol. 67, p. 249. 



