BLAKE : EAST YORKSHIRE. 
23 
After the long continuance of the colonies of corals on the 
Pickering area, those changes began to take place which led 
to the alteration in character between the Kimmeridge and 
Oxford clays ; and the corals became choked with inroads of 
sands and clay, coming apparently from many directions, and 
therefore local and variable. In the ups and downs, the 
shifting and changing of currents, local beds were no doubt 
denuded, and their remains deposited in shoaly water. But 
after these minor preludes, that great continued era of 
depression set in steadily which enabled the great thick- 
nesses of Kimmeridge clay to be here and elsewhere all alike 
deposited. The widespread common features of this forma- 
tion, as seen not only in Yorkshire but in Lincolnshire, and 
still further to the south, forbid us to look to any local source 
for the sediment required ; rather we must go back once more 
to the coal measures, which throughout their range must now 
have been well exposed to the action of the rivers, draining 
into an open sea, and limiting them ultimately to nearly their 
present boundaries. The great masses of argillaceous matter 
thus derived sank slowly and uniformly in an ever-deepening 
sea, at first shallow enough to encourage the growth of the 
great deltoid oyster of the period, but soon becoming the 
resting-place of the more pelagic cephalopods, and compara- 
tively barren of life. Thus at least 500 feet and more, 
probably 1,000 feet, were left behind. But just as the coral 
reefs of Yorkshire were formed, whilst many other places 
were receiving only the debris of carboniferous shales in their 
open sea, so now, by a change of turn, Yorkshire continued 
to receive its clay deposits while Portlandian reefs were 
formed in the south. These clays we find at Speeton, a 
locality of great importance to our history. In this way, 
then, Yorkshire continued without change to the close of the 
Jurassic period. 
We now enter upon a portion of the history which is 
