DEVELOPMENT OF CLIFF PEOFILES. 
251 
which this fragmental matter takes and its reaction upon the rate and mode 
of weathering. We may reach the matter most easily by discussing a 
hypothetical arbitrary case. 
Let us suppose a series of strata consisting of four groups (Plate XL), 
the uppermost group (A) being obdurate or very unyielding to the attack 
of weathering ; the next below (B) being notably softer or less obdurate ; 
the third (C) being hard ; and the fourth (D) soft.* Conceive a stream cor- 
rading a gash nearly but not quite through the hard upper stratum. The 
quantity of fragmental material furnished will vary {ceteris paribus) inversely 
with the hardness of the rock, and since the hardness is assumed to be great, 
the quantity supplied will be small. On the other hand, the fragments fall- 
ing into the stream are quickly dissipated by corrasion. In other words, 
the removal of debris by transportation is for a time at the extreme of ra- 
pidity. Corrasion, moreover, while a country is rising, is a very rapid pro- 
cess in comparison with the rate of recession of a hard massive wall. Very 
little talus, therefore, can form at this stage ; the obduracy of the wall rock 
prevents its recession ; little material is supplied which could serve to form 
a talus, and that little is quickly swept otf by the stream. Hence the canon 
will be naiTOw, with vertical or nearly vertical walls, and no appreciable 
talus will accumulate. 
Conceive now the corrasion to go on until the stream has cut nearly 
but not quite through the soft group B. The supply of ddbris is greatly 
increased. The soft beds weather easily and undermine the hard beds above. 
Let us recall here that the width of the cut is no greater tlian the width of the 
water surface. As the cliff recedes the fragments begin to find a lodgment at 
its base, and though some of them roll into the stream and are devoured, yet 
another portion must await the slower process of solution and gradual decay 
before disappearing. Thus the rate of transportation slackens by the 
decreasing declivity of the river, while the rate of supply of ddbris in- 
creases. As the cliff further recedes the talus mounts higher and higher 
up the breast of the softer part of the wall. The faces of the softer strata 
become in due time a slope capable of supporting a talus. 
*I am unable to think of a pair of adjectives which will suitably express the great and small 
degrees of obduracy of rocks against weathering. I shall use the terms “ hard” and “soft,” therefore, 
in this sense. 
