WOOD AND ROME LINCOLNSHIRE AND S.E. YORKSHIRE. 169 



attention *. Unless wo suppose the forest at Grimsby to have grown 

 down to the very level of high water, we must allow an even greater 

 depression than the 52 feet of the Grimsby borings ; and it will, we 

 think, occur to all how material an alteration, and in some parts re- 

 versal, of the earlier lines of Postglacial drainage such a depression 

 must involve, if, as the position of the / beds in fig. 1 seems to show, 

 it were not equal over the area. 



Leaving the sands and gravels on the Wold-top and along its 

 scarp-foot, and those of the Oolite ridge of Lincolnshire to be de- 

 scribed in reference to the denudation, we pass to the structure of 

 southern and central Lincolnshire. 



III. The Structure op Southern and Central Lincolnshire. 



The scarp of the chalk Wold extends as a continuous cliff-like f 

 slope from the north-eastern edge of the formation at Speeton, 

 where it is buried in the purple clay, until it reaches the Glacial 

 clay-tract of mid Lincolnslure, which begins near Castor, and is 

 throughout formed of the chalk alone. South of Castor, however, 

 the western edge of the chalk ceases to maintain this feature, and 

 the cliff-like and regularly continuous scarp changes into ridges of 

 Glacial clay, chalk, and subcretaceous beds together, that rise to 

 elevations equal, and even superior, to that possessed by the cliff- 

 like scarp itself for the southern sixteen miles of its stretch. From 

 the part where the Glacial clay begins to set in, there is not only no 

 cliff-like scarp, but the edge of the chalk, in common with the 

 Glacial clay, and in some instances with the subcretaceous series 

 also, is denuded into a series of ridges and valleys which run out in 

 various directions, both parallel with, and at right angles to, the 

 subcretaceous outcrop. This feature has a most important bearing 

 upon the Postglacial structure of the region. 



The following sections follow the line of the Great Northern and 

 the Manchester, Sheffield, and Lincolnshire Railways. The first of 

 them, fig. 4, starts from the north-eastern edge of the large insular 

 mass of Upper Glacial clay which occupies the principal portion of 

 the counties of Huntingdon and Bedford, and part of those of 

 Buckingham and Cambridge (being, except that it is divided by 

 narrow channels of denudation, the second in point of size of the 

 tracts of Glacial beds in England), and is carried north-westwards 

 to the outcrop of the Trias in the valley of the Trent at Newark. 

 The second, No. 5, is carried from the Trias outcrop higher up the 

 Trent valley, north-eastwards to the Holderness coast. These two 



* Geol. Mag. vol. iv., p. 519. 



t In making use of the term " cliff-like " we would be understood as distin- 

 guishing by it the continuous scarp, and not as implying that the chalk escarp- 

 ment ever formed an actual sea-cliff. It will be seen that we regard the southern 

 part of it as a Postglacial, and the northern part of it as an Intra- as well as 

 Postglacial margin of denudation, partly (at least) marine. The slope of the 

 escarpment, although, from the disproportion of the vertical scale, unavoidably 

 represented in the sections as precipitous, is, like all the other escarpments, far 

 less steep than the most sloping sea-cliff. 



