594 ON THE UPPER CRETACEOUS IN WEST SUFFOLK AND NORFOLK. 



the interval being here marked by the sudden change from Gault to 

 Chalk-marl, and from " Red Rock" to white limestone. 



The diagram, fig. 6, is drawn with the view of illustrating the 

 normal succession in the counties referred to, and the manner in 

 which this is broken by the erosion of the upper part of the Gault 

 over a certain space. From this it will be seen that we are pre- 

 sented, as it were, with the two ends of the stratigraphical chain, 

 and that, in the interval beween Totternhoe and Stoke Perry, the 

 total thickness of the Gault has diminished from about 230 feet to 

 60, the difference, 170 feet, being entirely the result of thinning- 

 out. The additional information which has been obtained since the 

 publication of the paper on " The Relations of the Cambridge Gault 

 and Greensand," makes it now possible to estimate more accurately 

 the width of the area over which erosion took place, and the 

 maximum amount of the Gault which was removed from the central 

 portion of this area. 



In the first place we must point out that, though there is a steady 

 decrease in the total thickness of the Gault, yet between Tring and 

 Hitchin there would appear to be an increase in the thickness of 

 the Lower Gault from 150 to 204 feet, so that if the Upper Gault 

 had maintained the thickness it presents near Tring, namely, 80 

 feet, the formation would have had a thickness of at least 284 feet 

 near Hitchin and Arlesey. There is, however, good reason to believe 

 that the Upper Gault becomes rapidly thinner as it is traced north- 

 ward, and could never have been more than 20 or 30 feet thick at 

 Hitchin, so that the original thickness of the Gault there was pro- 

 bably about the same as near Tring, though the relative thicknesses 

 of the two subdivisions were different. 



North of Arlesey the Lower Gault diminishes in thickness, and it 

 is assumed that this is mainly due to the erosion it has undergone ; 

 it is not improbable, however, that this division attained its greatest 

 thickness at or near Arlesey, and that it thinned northward as 

 rapidly as it did and does to the southward. On this supposition it 

 would have had a thickness of 150 feet at Cambridge, and to this 

 must be added a certain thickness (say 20 feet) for the Upper 

 Gault. 



The diagram, fig. 7, shows the actual surface of the Gault be- 

 tween Cheddington (near Tring) and Bersingham in Norfolk, and 

 the broken lines show what we suppose to have been the original 

 surfaces of the Lower and Upper Gault respectively. Prom Arlesey 

 to Cambridge these are drawn in accordance with the inference that 

 the Gault was originally 170 feet thick at the latter place, and, as 

 we believe that the Gault at Stoke Perry has not suffered from 

 erosion, the diagram is easily completed by joining the lines between 

 Cambridge and Stoke. It will be seen that the slope thus obtained 

 agrees very nearly with that of the actual surface between Stoke 

 and Hersingham. 



If the data on which this diagram is founded are correct, the 

 maximum amount of Gault removed from any part of the eroded 

 area appears to be 50 feet. The base of the Gault is drawn as a 



