Geology of Norfolk. 
461 
5. The Lower Drift— Till or Boulder Clmj. 
Above this marine deposit occurs the till or boulder clay, the 
lowest member of the northern drift. It is proved to be a marine 
deposit by the marine shells which it contains ; but they occur 
wherever I have observed it, in England, Wales, and Ireland, in 
a very different condition from those of the Norwich crag, or of 
the marine bed last mentioned, consisting rarely of anything but 
fragments, distributed with the utmost irregularity through the 
mass — species of different habits, the inhabitants of sandy and of 
muddy bottoms, of deep and shallow water, being mixed confusedly 
together and associated with fragments of various rocks, often 
little waterworn, but much scratched, derived from a number of 
distant localities intermixed largely with local detritus. The 
base through which these fragments of shells and rocks are dis- 
tributed is often not very dissimilar in other respects from the 
London clay. The foreign detritus consists of blocks and pebbles 
of granite, gneiss, mica-slate, clay-slate, greenstone, porphyry, 
and other rocks which occur no nearer than Scotland or Norway. 
They constitute the tall of a stream of detritus, traceable over 
a large portion of the north of Germany, the blocks diminishing 
in size and quantity with their distance from the Scandinavian 
peninsula. Besides these fragments, those of Kimmeridge clay 
and lias are met with, containing tiieir characteristic fossils. Those 
of the London clay and carboniferous strata are more rare. The 
most abundant detritus consists of fragmentary chalk and chalk- 
flints. These are found under a variety of conditions : 1, as de- 
tached masses of chalk enveloped in the clay ;* 2, as masses of 
fragmentary chalk collected together with little or no intermix- 
ture of other matter, and free from attrition by aqueous action ; 
3, as fragments sometimes quite angular, sometimes more or less 
waterworn, dispersed irregularly through the clayey matrix, in 
juxtaposition with the detritus derived from greater distances. 
On the flanks of those masses of chalk which have been brought 
together without attrition and unmixed with other matter, their 
materials have been rearranged by marine action after being finely 
comminuted and intimately combined with clay, thus producing 
proper marls, of various shades of white, grey, blue, and yellow. 
The insulated masses of chalk and transported chalk rubble are 
most abundant in the cliffs of Cromer, between Trimingham 
and Weybourne. Towards the termination of the cliffs near 
Happisburgh the thickness of the clay diminishes. Its upper 
surface sinks to below the level of the sea, and disappears be- 
* One of these transported masses of solid chalk at Old Hythe Point is 
nearly 80 feet high. 
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