420 Walter Keeping—The Upware and Potton Pebble-beds. 
The present westerly line of shallow water, sand and pebble beds, 
marks closely enough the ancient west coast-line. The Northern 
Neocomian Sea (Anglo-Germanic), extending HE. and W. from 
Yorkshire to Brunswick was freely open to the north, but gradually 
shallowed to the south over Norfolk and Cambridge, till it beat upon 
the shores of the ancient east and west ridge of Paleozoic rocks (the 
Harwich axis), now hidden under the Cretaceous and Tertiary deposits. 
By gradual depression and denudation this ridge came to form 
only an imperfect barrier, perhaps a scattered chain of islands 
marking off the Northern Sea from the Sonthern (or Anglo-Parisian) 
basin, the two seas surging together through newly-made channels." 
It is to this old ridge that I look for the origin of most of the more 
ancient pebbles found in our Neocomian deposits ; as well from the 
inadequateness of other theories as from the likelihood of such a 
source of supply in itself. 
The other rock surfaces of the period can be restored with some 
certainty. The rocks of the country to the west of the Upper 
Neocomian shore-line were of much the same general character as 
at present. The same series of Jurassic rocks overspread the 
country to the north-west and south-west, only ranging further west 
than at present; and the New Red Sandstone was also more wide- 
spread than now. The Mountain Limestone of Derbyshire and the 
ancient rocks of Charnwood were also to some extent exposed. 
Now, regarding this country as a source of pebble supply, we find 
a large area of Jurassic and New Red rocks, and single, isolated, 
and distant patches of primary rocks. But looking at the pebbles 
in the Upware and Potton sands, we find, after eliminating the 
Upper Jurassic fragments of local cliff origin, that the older Jurassic 
and New Red pebbles are but scantily represented, while Mountain 
Limestone chert, and other older Paleozoic rocks are abundant. 
The Derbyshire Mountain Limestone is, therefore, I believe, utterly 
inadequate as a source of supply for the quantity of chert in our 
Neocomian deposits, and still less was the Charnwood inlier capable 
of supplying the quantity of Lower Paleozoic pebbles. 
The pebble bed discovered in the Kentish Town deep-boring tends 
further to show that many of the more ancient rock fragments were 
not derived from any existing exposure. There, nearly thirteen feet 
below the base of the Gault, occurs a “hard red conglomerate, with 
pebbles of syenite, greenstone, trap, quartz, hornstone. red claystone 
porphyry, and fossiliferous schist from the size of a marble to that of 
a cannon ball.” Some of the lower beds also contained pebbles, 
but these are small. White quartz is named as the material of these 
in one case, and small angular fragments of chert (?) in another.* 
And these beds probably rest upon the old Palzozoic ridge. 
But little of the constitution of this ancient ridge and barrier has 
yet been directly revealed. Its date being Post-Carboniferous, we 
should naturally expect large exposures of Mountain Limestone 
1 Tn earlier Cretaceous times (Lower Neocomian) the two seas could have com- 
municated only by their common union with the Atlantic in the west. 
* Mem. Geol. Survey, vol. iv. p. 498, 
