THE CAMBRIDGE GAULT AND GREENSAND. 261 



One instance will suffice ; and this is from a pit near Horningsea, 

 four miles north of Cambridge. Fig. 2 (p. 260) represents the section 

 as seen at the time I visited it. The average depth at which they 

 were working the nodule-bed was about 12 feet ; but so irregular 

 was the surface of the Gault that in one spot the head of a man 

 standing in the hollow was on a level with the top of the neigh- 

 bouring roll or ridge, which was therefore about 5 feet high, and 

 ran for some distance in an east and west direction. 



The Chalk-marl above was in no way disturbed ; and the thickness 

 of gravel, or " trail," was very slight. It seems clear to me there- 

 fore that these and similar inequalities existed before the deposition 

 of the Chalk-marl, and that they cannot have been produced by the 

 pressure of ice ; I think, moreover, that land-ice would hardly acquire 

 sufficient power to make such marks over so flat a district as this 

 part of Cambridgeshire. 



Wow these relations of the Gault, Coprolite-bed, and Chalk-marl 

 continue the same throughout Cambridgeshire, from Soham, near 

 Ely, on the north, to Guilden Morden in a southerly direction. Along 

 this line also it is interesting to find a rapid increase in the thick- 

 ness of the Gault. In the neighbourhood of Cambridge this formation, 

 though variable in thickness, ranges only between 100 and 120 feet. 

 At Horningsea it is only 103, at Cambridge itself from 105 to 115, 

 at Barton and Grantchester about 120 feet. These details were 

 given me by Mr. N. W. Johnson, of Chesterton, who has had great 

 practical acquaintance with the Gault and Greensand of Cambridge- 

 shire. 



Information from the same source, and deduced originally from 

 the well-sections in various coprolite-pits along the line, has enabled 

 me to construct the following table of thicknesses presented by the 

 Gault. 



ft. ft. 



Horningsea and Cambridge 103 to 120 



Grantchester ■. 115 „ 120 



Haslingfield 140 „ 160 



Barrington 150 „ 160 



Wimpole ; 170 „ 185 



Bassingbourne 150 „ 175 



Guilden Morden 175 „ 190 



Ashwell , 185 „ 205 



Arlesey and Hitchin 204 „ 214 



From these figures it will be seen that, while the thickess of the 

 Gault varies much locally, there is an increase of 100 feet between 

 Cambridge and Arlesey. Now I consider that this is not wholly due 

 to a westward thickening of the formation, but chiefly to the coming 

 in of higher and higher beds of clay. In the absence of bedding or 

 fossiliferous bands it is of course very difficult to prove this ; but 

 certain considerations, to be mentioned hereafter, render it at least 

 very probable ; and one fact may be noticed here which Mr. Johnson 

 has observed, viz. that the clay of Ashwell and Guilden Morden is of 

 a dull slate- colour, while that of Cambridge is more decidedly blue ; 

 this may indicate the presence of a higher horizon. 



t 2 



