190 THE BIVER VALLEYS OF 



cliffs ; while during the winter these shelly deposits 

 are covered over by a drift, more or less clayey, 

 containing numerous ice-dressed blocks and frag- 

 ments. These glacialoid drifts have led to innumer- 

 able mistakes ; as they have been supposed, as in the 

 Counties Dublin and Wexford, to be a newer glacial- 

 drift overlying a shelly gravelly drift ; while in fact 

 these are both members of the same drift, formed 

 under different circumstances, and in places graduat- 

 ing into one another. 



If a valley were excavated solely by rain and 

 rivers, the gravels could only contain blocks and 

 fragments of the subjacent rocks, or of rocks 

 situated higher up the valley, pieces of which might 

 be carried down to any lower level. Sea-formed 

 gravel, on the other hand, may contain fragments 

 from both higher and lower levels, which are either 

 carried down or driven up by the waves and marine 

 currents; but the detritus from the upper rocks of 

 the valley ought to preponderate, as they would 

 always be coming down in rivers, or by rolling from 

 heights. 



Estuary gravels may, in some places, contain only 

 fragments and pebbles of rocks from a higher level, 

 as a stream flowing -into the estuary may supply all 

 the materials of which such gravels are formed. 

 From this it is evident that river and estuary gravel 

 may be nearly similar, as rivers may supply the 



