PLIOCENE PEEIOD Ilf EN^GLAIfD. 519 



Clay already referred to, wherever this rises above the beach. Its 

 elevation in this part does not exceed 100 feet. On the line dividing 

 Sheets 94 and 95, however, it suddenly rises with the chalk and 

 covers that point of the Wold which projects to form Mamborongh 

 Head, and near this it envelops the crest of the escarpment at an 

 elevation of 400 feet. It is here confined to a narrow strip of land 

 skirting the coast, which varies from half a mile to three miles in 

 breadth * ; but to the south, in Sheets 94 and 85, it spreads out over 

 the lower ground so as to reach Hull, in excavating the docks of 

 which town it was found to descend to 70 feet below the sea-surface, 

 and to rest at that depth on gravel, as in some places it did at 

 Grimsby; but between Hull and Hessle, a space of four miles, it 

 disappears, and as the chalk rises at the latter place, the Hessle sand 

 and clay (which overlay the Purple Clay in the dock-excavations) only 

 are present. Its western edge is mostly overlapped and concealed 

 by the Hessle Clay, resting against the eastern slope of the Wold. 

 From the narrow coast-strip which envelops the Wold escarpment 

 it extends continuously northwards along the coast, lying irregularly 

 on the slopes, and filling old valleys and hollows of the Jurassic 

 rocks, and so enters the valley of the Tees on the borders of Sheets 

 103 and 104, up which valley it extends, skirting the northern 

 slope of the eastern moorlands, and covering it up to elevations of 

 about 400 feet. Pilling the Tees valley, it passes over the watershed 

 between that valley and the depression of central Yorkshire, which 

 is drained by the many rivers which converge to the Humber, and 

 which depression it occupies as far south as the low grounds skirting 

 the Humber in the north-east corner of Sheet 87, beneath which it 

 disappears, but shows itself again in the north-east of Sheet 82 ; 

 in the cuttings between Bawtry and Retford clay like it overlies sand 

 and gravel. Throughout Sheets 85, 86, and 94 it is purple in colour 

 towards its base, where it contains a moderate proportion of rolled 

 chalk, but is far more copiously charged with small angular and 

 subangular rock-fragments; and it gradually parts with the chalk 

 debris upwards, becoming, as it does so, more of a red colour ; and 

 it does precisely the same thing in a horizontal direction as it rises 

 over the Wold along the coast-belt, the clay enveloping the Wold 

 and its scarp being similar to that which forms the uppermost portion 

 only of the clay in the lower grounds of Sheets 94 and 95. In the 

 buried ravines of the chalk intersected by the cliff about Plam- 

 boroughHead it is underlain by clay of a blackish colour, which, again, 

 is underlain by (and sometimes interbedded with) beds of chalky 

 sand and of gravel, and by beds of rolled chalk only, all of which 

 seem to be of the age of the Holderness Basement Clay. At Dim- 



* Mr. Eome and I mentioned the occurrence of some very small patches of 

 clay on the higher parts of the Wold near the dividing-line of Sheets 93 and 94, 

 and we referred them to the Purple Clay. One of these, at Huggate, was after- 

 wards, with the object of seeing if it contained shells, excavated under the 

 direction of Sir Charles Lyell. The clay was fuU of chalk, but no moUuscan 

 remains occurred in it. Some small patches also occur on the Lincolnshire 

 Wold, and they aU appear to me now to be remnants of moraine formed during 

 the accumulation otbS. • 



