DEVELOPMENT OF CLIFF PROFILES. 
255 
Leaving for future consideration the characteristics of the great Red 
Wall group and the strata beneath it, let us endeavor to infer the typical 
forms of cliff profiles generated by the combined action of corrasion and 
weathering in the strata thus far described. Imagine first a stream corrad- 
ing its channel through the upper Aubrey limestones. The degree of ab- 
ruptness in the slopes descending to the stream would be dependent upon 
the rate of weathering relatively to the rate of corrasion. If the rate of 
corrasion were slow and the rate of weathering rapid the slopes would be 
gentle or at a low angle ; if the corrasion were rapid and the weathering 
slow the banks would be precipitous. Imagine the stream to corrade still 
further into and through the very obdurate cross-bedded sandstone. Upon 
this latter rock weathering has but little effect directly. It may stand for 
long geological periods with but a slight loss of substance, provided it is 
not undermined. Corrasion, however, may go on in it at a rate somewhat 
retarded by its obduracy indeed, but only a little less rapidly than in much 
softer rocks; for declivity and the amount of protection afforded by the 
clastic material in the bed of a stream are incomparably more potent factors 
than the hardness of the bed-rock in determining the rate of con-asion. But 
the rate of weathering is dependent upon the nature of the stratum itself. 
Hence the limestones above would be much less precipitous than the ada- 
mantine sandstone below. 
Imagine, further, the sinking of the channel deep into the very easily 
weathered shaly sandstones of the lower Aubrey. The problem now be- 
comes a little more complicated. As before, the quality of the newly-cut 
rocks does not necessarily imply any great increase in the rate of corrasion. 
But it does imply a modification in the rate of weathering and in the con- 
sequent form of the profile. A new factor now enters in the plan of opera- 
tions. In consequence of their very yielding character the shaly sandstones 
are rapidly dissolved and the adamantine stratum above is undermined. The 
rate of recession becomes in the harder stratum equal to that in the softer 
shales beneath it. So rapid at first is this decay that at a certain early stage 
of the penetration of shales by the corrading stream the cross-bedded sand- 
stone and the cherty limestones above often form a single ledge. The rate 
at which they are undermined is for a time greater than the rate of reces- 
