332 ME. TV. HILL ON THE LOWER BEDS OF THE FPPEE 



"Wolds seems to me the soulhern edge of a transverse valley, the 

 northern side of which has been entirely removed, so that the strata 

 exposed may be regarded as a nearly transverse section of the Cre- 

 taceous basin. 



To the well-known unconformity between the Chalk and the 

 underlying strata I need not further allude. 



As in Lincolnshire, I separate the Hunstanton Limestone, dealing 

 with it alone ; but it will be more convenient, to save repetition, if 

 the whole of the Lower Chalk to the Belemnite-marls is dealt with 

 at each of the principal exposures, taking the minor evidence of the 

 continuance of the beds as we proceed from south to north. 



The Condition of the Chalh on the brow of the Wolds. 



In the consideration of the thickness and sequence of the Lower 

 Beds of the Upper Cretaceous series in Yorkshire, a formidable diffi- 

 culty presents itself in the broken and disturbed condition of the 

 Chalk along the brow of the Wolds. Por the greater part of their 

 length the stratum underlying the Chalk is a clay, and as the base 

 of the Chalk becomes elevated, this occupies a proportionally larger 

 part of the slope at the base of the Wolds. 



Everywhere the effect of the weather on this clay is apparent ; 

 for the rain, helped probably by frost, destroys its cohesion for a 

 short distance from the surface, and a mass, usually not very ex- 

 tensive or deep, begins to move down the hillside. In the central 

 and northern part of the Wolds, where the base of the Chalk reaches 

 its highest elevation and the clay-slopes are mostly grass land, this 

 general downward motion is evidently now going on, and its action is 

 seen in the fact that the roots of herbage and grass are parted more 

 or less perfectly along the outer edges of the slip and also in the 

 wave-like roll which the mass assumes at its lower boundary. 



Such slipping masses can be seen in any railway-cutting through 

 clay near London, and are evidently a constant trouble to the 

 district engineer. 



But the effect of this constant removal of clay from a higher to 

 a lower level has, I think, been somewhat underestimated. It 

 appears to me that the gradual removal of the supporting material 

 from the base of the Chalk would first cause the outer edge of the 

 solid chalk to incline towards the valley, and ultimately the 

 higher beds would slip over the lower ones in their descent on the 

 yielding surface of the clay, or a large part of the hillside would 

 slip down bodily, the more solid beds retaining their normal rela- 

 tions to each other. That considerable masses of chalk have slipped 

 over the lower beds must be evident to anyone who will examine 

 the Chalk along the brow of the Wolds. 



This overslipping of the beds may be the reason why for so long 

 a distance no Eed Chalk can be seen, the actual outcrop being 

 buried under the debris of higher beds. Slips of considerable im- 

 portance have taken place in comparatively recent times ; for in- 

 stance, a large part of the hillside near Wharram Grange has 



