in East Anglia. 413 



although, so far as the writer is aware, there is as yet no map giving 

 direct information, concerning the variation in thickness of the 

 formation. 



We are in possession of a certain amount of information, from 

 deep wells and borings, concerning the level of the base of the Chalk 

 at various points throughout the area. "While wishing that this 

 information were more abundant we need not be deterred from 

 prcfceeding with the investigation. Committing our data to paper 

 and considering the items of information correlatively, we find that 

 the base of the Chalk appears to slope away in a general east-north- 

 easterly direction Avith remarkable uniformity, dropping at the rate 

 of about 24 feet to the mile, throughout the whole of the eastern 

 portion of the area. Further west the contours take on a slight 

 sinuosity, assume a more or less north and south direction, and 

 become somewhat more closely spaced, indicating an increase in the 

 gradient. Since in the construction of this map we are able to use 

 the information supplied by only about half a dozen levels, it follows 

 that in our use of it we must not expect a greater degree of accuracy 

 than it is capable of yielding. But our confidence in the utility of 

 the map for the purpose which follows is maintained when we 

 observe that it so happens that the available levels are well 

 distributed, and further, that the contours over a great part of the 

 area are practically straight lines. 



Having represented cartographically our information concerning 

 the level of the base of the Chalk, we next require a similar 

 representation of our knowledge of the present level of its surface, 

 and this we find to our hand. The Chalk-surface contours of this 

 area have been mapped by Professor P. G. H. Boswell,' and we are 

 able to avail ourselves of the results of his labours. If the base of 

 the Chalk were " corrected " in such a way as to cause it to occupy 

 a horizontal plane at sea-level, and the Chalk surface were modified 

 correspondinglj% then lines joining points at which the surface of 

 the Chalk is at the same height above the base of the formation 

 would constitute the isopachyte system which we seek. We therefore 

 superimpose the Chalk-surface map upon that showing the level of 

 the base of the formation. At points where the contours intersect, 

 the amount of correction necessary to hring the base of the Chalk to 

 sea-level is indicated by the one map, and the appropriate correction 

 is applied to the other. The completed series of lines thus obtained 

 constitutes our pi'ovisional idea of the variations in thickness of the 

 Chalk in East Anglia, and is shown in the map here presented. The 

 sources of error are manifold, but every incorporated item of 

 additional information Avill bring the -final result nearer to the 

 truth — and however inaccurate the present map, it at least provides 

 us with a notion of the general character of the isopachyte system of 

 the East Anjilian Chalk which we could never liave conjured up had 

 the attempt not been made — and, further, by its aid certain salient 

 cliaracteristics of the denudation undergone by this Chalk area jump 

 at once before the eye. 



1 Boswell, Q.J.G.S., vol. Ixxi, pi. I. 



