628 STKATTGKAPHICAL GEOLOGY 



nests and patches of sand, TMina balthica, Cardium edule, Cyprina 

 islandica, and Mya arenaria being tlie commonest species. 



Near Cromer the thickness of the Contorted Drift is sometimes 

 as much as 170 feet, and the contortions are very marked, but 

 southward and south-westward the deposit passes into a set of 

 loams, sands, and fine gravels. Moreover, the underlying Cromer 

 Till appears to thin out, so that over large parts of Norfolk, 

 Suffolk, and Essex the Lower Glacial Group consists only of sands 

 and gravels, with occasional intercalations of laminated loam or 

 brick-earth. These beds were termed " Middle Glacial " by Mr. 

 S. V. Wood. As they are followed to the higher ground in the 

 western parts of these counties, and to the Chalk-escarpment in 

 Cambridgeshire, they thin out and are overlapped by the Chalky 

 Boulder-clay, which then rests directly on the Chalk. 



The Chalky Boulder-clay is an extension of that already 

 described, and has been formed partly from the detrition of the 

 Chalk and partly from that of the Jurassic rocks, materials from 

 the Oxford and Kimeridge Clays being numerous, and derived 

 fossils, such as Gryphcea arcuata, Gryphwa dilatata, Cardinia 

 Listen, and Belemnites abbreviatus, are not uncommon. In this 

 respect it contrasts with the boulder -clays of the Lower Glacial 

 Group. 



This boulder -clay exhibits a fairly uniform character over 

 Norfolk, Suffolk, Essex, Cambridge, Herts, and Middlesex, the 

 most southerly outlier being that on Muswell Hill, north of 

 Highgate ; here the clay is of the usual pale colour, and contains 

 occasional fragments of red chalk, which must have journeyed a 

 distance of some 100 miles, for the nearest place whence they 

 could have been derived is Hunstanton in Norfolk. The thick- 

 ness of this great sheet of boulder-clay is very variable ; sometimes 

 only a few feet can be seen, and in other places borings have gone 

 through 100 or 150 feet of it. 



Another fact of some importance in connection with the 

 formation of this clay is the occurrence in it of pebbles which have 

 been carried up to a level much higher than that of their source ; 

 thus phosphatic nodules derived from the Cambridge greensand 

 have been found at a level of 500 feet above the sea, whereas the 

 outcrop of the parent stratum to the north and west is everywhere 

 under 150 feet. 



5. Tlie Midland Area 



In the counties of Derby, Stafford, Warwick, and Leicester we 

 find the meeting ground of four different ice -lobes (see map,. 

 Fig. 203). Around Derby and Nottingham, and thence south- 



