1866.] SEELET DRIFT OP THE FENLAND. 471 



And the great level itself is the Fen-clay, in which are a few isolated 

 reefs of Coral-rag, and some stone-bands in its Oxford-Clay member, 

 which clay covers nearly the entire area. 



There are in this region three kinds of drift — namely, a Boulder- 

 clay covering the high land, a coarse gravel which caps the hiUs, 

 and the fine gravel of the plains. 



The Boulder-clay is widely spread to the west of Cambridge. It 

 rarely, if ever, forms hills, though frequently capping them. Yet 

 it is thick ; for, near Caxton and Longstow, wells in it have been 

 sunk 160 to 180 feet: and the clay seems sometimes to fill up 

 valleys ; for a well in the village of Caxton, half a mile north of the 

 deep sinkings, found the drift reduced to a thickness of 14 feet. 



From. March to Longstowe it is generally a dark-blue deposit 

 wholly unstratified, and more or less abundantly charged with frag- 

 ments of Chalk and Septaria and Limestone -rock of the ?Kim- 

 meridge Clay ; while fragments may be found of almost any other 

 rock, though not everywhere. They abound at Longstowe ; but a 

 little north, scarcely anything is found in it but chalk ; while at 

 March the clay is almost free from fragments, and largely used for 

 brickmaking. 



In this district, as elsewhere, it is rare to find gravel over the 

 Boulder- clay, though at a lower level gravel abounds. Thus the 

 high land of Elsworth, Papworth, and Abbotsley, is Boulder-clay ; and 

 the high land north of St. Ives, at Bluntisham &c., is Boulder-clay ; 

 yet all the valley between, as at Fenstanton, St. Ives, Hemingford 

 Abbot, &c., is filled with a fine flint-gravel. Where the Boulder- 

 clay ceases, at Elsworth, there the gravel begins, and thickens as it 

 joins the valley of the Ouse. 



At Bluntisham the Boulder-drift presents the usual feature — 

 capping a hill. From it I obtained : — 



Ammonites serpentinus. 



bifrons. 



Marine. 



— — cordatus. 



biplex. 



Achilles. 



Lamberti. 



serratus. 



fimbriatus. 



oxynotus. 



Pleurotomaria, sp. 

 Gryphsea incurva. 



Grryphaja cymbium. 



dilatata. 



, sp. 



Ostrea deltoidea. 



laeviuscula. 



Cardinia Listeri. 

 Belemnites tornatilis. 



abbreviatus. 



, Liassic sp. 



Belemnitella mucronata. 

 Terebratula subimpressa. 



these species being characteristic of the Lias, Oxford Clay and Kim- 

 meridge Clay, and Upper Chalk. 



The Boulder-clay at Ely is brown, and remarkable for show- 

 ing, in places, regular courses of curved lines making a rough stra- 

 tification. But the chief importance of the section is in the fact 

 that, while the country above is level, the Boulder-clay has been let 

 down some 30 feet by a fault, thus evidencing considerable denuda- 

 tion since the Boulder-clay was deposited. It is also worth stating. 



VOL. XXII. PART I. 2 k 



