DKIFTS OP THE VALE OF CLWYD. 



101 



it ; so it often is in the case of material deposited by currents at 

 the foot of cliffs. 



The stones of which the shingle is composed lie with their longer 

 axes parallel to the planes of deposition as a rule ; but sometimes 

 when the scour is great they have a subordinate arrangement ap- 

 proaching that next described in the case of river-gravels. 



River-gravels are left, as the velocity of the current decreases, in 

 the order in which, from their size, weight, form, and other condi- 

 tions, they first attain their position of rest; but the principal character 

 common to all such gravels is that the flattened oval forms lie packed 

 together obliquely, with their longer axes inclined to the bed of the 

 stream, the lower end pointing up stream, so that the flat stones 

 overlap one. another in such a manner as to present a face to the 

 descending current and throw the water up instead of letting it 

 get under the edge and lift the stone. 



Of course all gravels deposited by marine currents have this 

 character also ; but there are seldom marine currents of the force 

 and velocity of a mountain-river in flood. 



In the case of the run-of-the-Iiill, on the other hand, the stones 

 and other materials are more commonly carried down by movement 

 more akin to slips. The position of rest is not that in which the 

 material can best resist the force of a downward current urging it 

 on, but the position in which it can catch by friction, or by 

 arriving at a gentler slope, so that its own downward tendency is 

 overcome. The flat pieces therefore lie with their longer axes 

 parallel to the surface along which the debris is travelling. 



The material is roughly stratified because rain helps, and different 

 material catches in different circumstances. The travelling of 

 isolated boulders in the rain wash or marine drift or on the shore is 

 easily explained. Though gravel, consisting of stones of a given 

 size, requires water of a given velocity to move it, it does not follow 

 that one such stone lying on sand requires water of the same velocity 

 to carry it along. The fine material round and below it is removed 

 by the swifter current close to the obstructing mass ; it is set in 

 motion, and the water playing on the whole of one face rolls it along 

 over the even surface of sand. So the occurrence of single blocks 

 can often be -explained without calling in the agency of ice or any 

 exceptional condition. All around the Vale of Clwyd such debris is 

 everywhere found, sometimes derived from the solid rock, as in the 

 section above quoted (p. 100), near Tremeirchion, sometimes de- 

 rived from the drift, as up the road from Pontyralltgoch to Wigfair 

 Uchaf, sometimes red with the washings of the New Red and the 

 result of the decomposition of limestone, and sometimes grey where 

 the iron-oxides have been washed away or the rocks from which it 

 is derived contained originally a smaller proportion. 



In the Vale of Clwyd this crumbling of all loose material down 

 the hill-sides has been going on ever since the land emerged from 

 below the waves that left the St. Asaph Drift, through the long ages 

 while the sand-dunes and wave-driven shingle were travelling from 

 west to east and damming the streams that ran into the sea between 



