MODEEN VIEWS OF DENUDATION. 
45 ? 
terraces, we have evidence of an agent which acted along a 
horizontal plane, such as the waves of the sea, an estuary, or 
a lake. Steep- sided valleys, on the other hand, afford prima, 
facie evidence of vertical erosive action, such as that of brooks 
or glaciers. I think we may safely say, that rivers never 
form terraces jutting out upon a plain, nor terraced surfaces 
of any kind except when composed of the gravels or silt 
which they themselves bring down. On the other hand, there 
are instances of deep ravines which cannot be regarded as the 
result of river action, inasmuch as they contain no rivers or 
brooks ; while (as I hope to be able to show) wide and shallow 
valleys, containing others smaller and deeper, are generally 
referable to marine denudation. 
According to these principles, such valley-plains as those of 
Gloucester, between the Malvern and Cotteswold Hills (hap- 
pily called by Sir R. Murchison “the Ancient Straits of 
Malvern ”), must be regarded as the result of marine denuda- 
tions. Likewise the terraces of marlstone, which jut out 
from the base of the Cotteswold escarpment, as well as those 
of the lower chalk, green sand, and other formations along 
the outcrop of the Chalk formation. It is impossible to con- 
ceive solid materials to have been swept away from the surface 
of these platforms by streams which have no existence, even 
with the utmost, latitude as regards time. To our mind, the 
opinions once expressed by Mr. Jukes — which possibly he 
might now repudiate — are most truthful and appropriate, 
when he says “ Isolated crags and precipices, or long lines of 
cliff, and steep slopes looking down upon broad plains, must 
have, in like manner, been, formed by the sweeping power of 
the sea.” * 
I now come to speak of valleys without brooks, to which 
allusion has already been made. If an imaginary plain be 
stretched across the crests and summit-ridges of many of our 
mountain groups, it will be found to form a surface either 
gently sloping or slightly undulating. This has been shown 
by Professor Ramsay to be the case in regard to South Wales, 
and is remarkably clear in the district south of Cader Idris. 
Such a plain as this was doubtless once a sea-bottom, and is 
termed by the writer above mentioned, “a plain of marine } 
denudation.” When this surface was first lifted from beneath 
the sea, it began to be acted on by the agents of atmospheric 
denudation. The rains, seeking a passage to the sea, would 
follow the slight irregularities of the ground, and innumerable 
rills, uniting into larger streams, would form channels for 
themselves, which, becoming deeper and wider, in the lapse 
* “ Student’s. Manual of Geology.’ 
