736 MR. F. W. HARMER ON THE CRAG OF ESSEX. [JNoV. 19OO, 



The Upper Crag deposits are not of uniform age, as believed by 

 Prestwich, but arrange themselves in zones, characterized by a 

 gradually diminishing percentage of extinct and southern, and an 

 increasing percentage of recent and northern mollusca. 



Although these zones contain faunas sufficiently distinct to justify 

 their separate classification, they all form part of a more or less 

 continuous and closely connected series ; they group themselves in 

 horizontal, and not in vertical sequence, the older deposits occurring 

 invariably towards the south of the Crag area, and the newer 

 towards the north. 



The term Red Crag is too- comprehensive, and, when we attempt 

 to correlate the East Anglian deposits with those of Belgium and 

 Holland, inconvenient. While retaining it for general use, it seems 

 desirable to adopt for its different horizons some more definite and 

 distinctive names. The classification tabulated on p. 708 is therefore 

 proposed. 



The Oakley zone is a new horizon of the Crag, intermediate in 

 age between the Walton Eed, with its southern fauna, and the Suffolk 

 Crag in which northern mollusca are more or less common ; it 

 represents the period before the southern shells had begun to die 

 out, when a few boreal forms, invading the Anglo-Belgian basin 

 from the north, were establishing themselves in greater or less 

 abundance in the Crag sea. 



The earliest indication of the conditions obtaining during the 

 Bed Crag Period is afforded by an un stratified bed at the base of the 

 Walton cliff-section, not now visible, originating in comparatively 

 shallow water, where a colony of mollusca lived and died. JS"o trace 

 of this bed has been met with in deposits of Waltoniau age in 

 other parts of Essex. 



A slight upheaval of Northern Essex afterwards took place, causing 

 the Walton area to form the edge of a land-locked bay, which was 

 gradually siltcd-up by material, brought down possibly by a river 

 entering the Crag sea at no great distance. To some extent the 

 silting process went on from north-north-east to south-south- 

 west, the shelly sand being obliquely bedded against the northern 

 shore of the Waltonian bay, or deposited in the form of banks 

 or shoals within it (see map, fig. 3, p. 714). A subsequent but 

 slight submergence permitted the deposition of horizontal beds 

 upon the obliquely-bedded Crag, 1 and carried the sea over a gently 

 sloping shore of London Clay to the west towards Beaumont, and 

 afterwards northward to Little Oakley, some erosion of the Eocene 

 beds taking place as the sea encroached upon the land. 2 The bed of 

 pebbly gravel underlain by clay, forming the upper part of the 



1 A similar state of things is to be seen in the stack-yard pit at Chillesford, 

 where horizontally- bedded Crag rests upon beach-Crag, indicating, as at Walton, 

 the gradual subsidence of the beach and the encroachment of a shallow sea over 

 the adjoining land. 



2 The fact that phosphatic nodules are principally found towards the base of 

 the Crag seems to show that it was when the position of the Red Crag sea was 

 shifted, and the sea invaded a fresh area, that denudation of the Eocene strata 

 took place ; and that this denudation ceased when the new bay began in its turn 

 to be choked by the deposition in it of shelly sand. 



