42 
MORTIMEK : ORIGIN OY CHALK DALES. 
cbalk area ? This is no easy question to answer. But it is fair to 
presume, that in the first instance, the outer or that part of the 
chalk area whose base is now the highest, was the first to rise, and 
probably somewhat suddenly, above the action of the cretaceous 
waves. The inner edge was just sufficiently elevated to catch the 
full denuding" force of the tides and storms of the cretaceous sea. 
Afterwards, probably, the chalk rose more slowly and consequently 
the inner margin would remain under the full and long continued 
action of the ocean waves. Such a probable state of things would, 
in a great measure, account for the very different degrees of 
denudation, which the two sides of the chalk wolds appear to have 
sustained. Since the formation of the chalk, denudation, submarine, 
and subaerial, has ever tended to plane down prominences, widen 
and heighten the bottoms of the main valleys, and fill up some of the 
smaller ones. I have also shown, in a paper published in the 
proceedings of the Yorksh. Geol. and Potyt. Society (Vol. VII., part 
IV., page 373 to 380.) the great probabihty that the inner edge of 
the chalk outcrops, suffered considerably more from denudation, 
during the glacial period than the outer edge, fiastly, in returning 
to complete m^^ line of section, we observe that the slipped masses 
at its west end, are such as occur all along the outer margin of the 
wolds, and are due to the action of water constantly removing the 
clay under the edge of the chalk. This removal of clay may have let 
down the beds and produced the general outward dip which we 
observe, close along the whole of the outer edge of the chalk range. 
In conclusion, I must repeat with emphasis, that, it is difficult to 
receive the erosion theory as sufficient to account for the formation 
of the grand system of the Yorkshire chalk valleys. But, if we 
admit that the numerous fractures, contortions and displacements 
above mentioned, which are everywhere visible must have 
been the natural result of such a non-expansive substance as chalk 
having to accommodate itself during periods of upheaval to a larger 
surface area, the origin of this system of valleys becomes clear. 
They are due originally to fractures in the crust. Their present 
rounded outlines have been moulded by subsequent denudation. 
