282 Paleo-Geological and Geographical Maps of the British Islands. 
Prats XXXII. 
The Cretaceous Period. 
At the close of the Jurassic period, the bed of the sea was elevated into dry 
land over the British area, and re-distributed into lakes and estuaries, with 
surrounding tracts of lands formed of Jurassic and older formations. During this 
epoch, denudation of the strata proceeded, while beds of shale, sandstone, and 
limestone (representing the Purbeck formation), were deposited over the floors of the 
lakes. Later on, these conditions gave place to others, when the Wealden beds, 
restricted to the south-east of England, were deposited at the mouth of a river, or 
rivers, draining the lands lying towards the north and west. 
The interval of the Purbeck and Wealden epochs may be considered as a sort of 
interregnum between the great Jurassic period on the one hand, and that of the 
Cretaceous on the other. It was, however, one of considerable duration; and on 
the Continent is partly represented by the “ Maestritch Beds” of Belgium, and in 
Western America, by the “ Laramie Beds” of Colorado.* 
Upon the commencement of the Lower Cretaceous epoch, beds of sand and 
gravel, now known as the “ Lower Greensand ” formation, were deposited in a 
shallow sea, and at no great distance from the land, which lay—both to the 
westwards (in the region now forming Cornwall, Devon, and Wales), and also 
along the line of the Thames valley—where the old ridge of Silurian, Devonian, 
and Carboniferous rocks was still uncovered by sediment.+ 
After the deposition of the Lower Greensand, there was another shght elevation 
of the sea-bed, and much of this formation, with portions of those below it, were 
swept away ; but upon the commencement of the Upper Cretaceous epoch, subsi- 
dence again set in, which continued till the close of the Cretaceous period, at which 
time the south and centre of Europe, ard all but the very highest elevations of the 
British Isles, were submerged beneath the waters of an ocean which must have 
extended eastwards from the Atlantic into Asia, and which not only occupied the 
basin of the Mediterranean, but the plains of France, Germany, Italy, Spain, and 
of Northern Africa,t This epoch of greatest submergence is represented in Plate 
XXXIL, Figure 2. 
The interval of land, lacustrine, and estuarine, conditions between the Jurassic 
and Cretaceous periods, together with the concomitant denudation, has resulted in 
* According to Dr. Hayden, and Prof. Cope. Bull. U.S., Geol. Surveys, vol. v., No. 1 (1879). 
+ The absence of the Lower Cretaceous beds in the Crossness boring is evidence of this, as also the beds 
of conglomerate occurring in the Lower Greensand of Oxfordshire and Wiltshire. In the fourth edit. of 
the ‘ Coalfields of Great Britain” (p. 354), I have given a section showing the position of this ridge 
under London, from which it will be seen that the Palsozoic rocks were not completely covered till the 
period of the “ Gault Clay.” 
tIt is probable that only a core of Paleozoic rocks of the Alps and Pyrenees were left un- 
submerged at this period, but Scandinavia was probably a land area. 
