48 A CHAPTER IN THE PHYSICAL GEOGRAPHY OF THE PAST. 
been thrown into a series of folds is planed down on its upper 
surface by the action of the sea, forming what is called a plain of 
marine denudation, it is evident that the folds which are convex 
upwards must be planed off before the concave portions or the 
troughs can be reached. Moreover, when such a plain of marine 
denudation becomes again dry land, and subjected to atmospheric 
influences, the trough and saucer-like portions of the folds, owing 
to the inclination of the strata towards each other, will be more 
stable than the convex portions in which the strata incline 
outwards. In one case the force of gravity will sefard denuda- 
tion, in the other it will faci/tate it. And thus it may, and often 
does come to pass that the summit of a hill is coincident with the 
trough of one of the folds, or the synclinal as it is called, whilst 
the valley runs along the anticlinal. Another reason for the 
difference is to be found in the frequent fracture of the tops of 
anticlinals allowing the freer access of water, and thus hastening 
the destruction of the arch. 
It now becomes easier to understand what I have said about 
the saucer-like shape of the western portion of our coalfield 
conducing to its preservation. This is a structure common to 
nearly all our coalfields, and is merely a geological instance as 
applied to strata of the “survival of the fittest” to withstand 
denudation.* 
Having now briefly glanced at the physical features and 
geological structure of the Pennine Range and of its southern 
extension under the newer rocks of the Central Midlands, we 
must try to ascertain something about the conditions under which 
* T do not know any better instance of the comparative stability of synclin- 
ally curved strata over strata curved in the opposite direction than is shown in 
a portion of our coast which most of you know well. The great Orme’s Head at 
Llandudno is a bold hill of massive Mountain Limestone connected with asmaller 
hill of the same rock, the Little Orme, by a very narrow neck of low lying 
ground. When the two hills are seen from Llanfairfechan on the west, it can 
be clearly seen that the strata of the Great Orme are bent upwards in a syn- 
clinal or saucer-like form, and it is perfectly evident from the lines of the curved 
strata that they once bent over again in the form of an arch to the mainland, 
and joined those of the Little Orme. The result of denuding forces acting 
equally upon this once continuous mass, has been entirely to remove the com- 
paratively weak arch or anticlinal portion, and to leave the saucer-like synclinal 
untouched. 
