46 The Geology of Cambridgeshire 



the character of the gravels is well seen. Lenticles of coarse 

 and fine material alternate capriciously, and surfaces of local 

 erosion separate most of the beds. Flints, which form most of 

 the pebbles, are a little worn but are by no means all rounded. 

 Some of the finer beds consist almost entirely of re-sorted 

 chalk, and these, in suffering solution by percolating water, 

 have often caused contortion of the beds above them. Piping 

 along ancient tree roots is very common and also complicates 

 the bedding planes. Small fossils occur only in the marly 

 lenticles. Cyrena fluminalis was at one time abundant but 

 the mass containing it is now quarried away. Bones of 

 mammoth, rhinoceros, Bos primigenius and deer are often met 

 with in pockets where they have escaped solution, especially 

 at the base of the Gravel. The gravel is from fifteen to 

 twenty feet thick. 



Other interesting gravel pits occur at Shelford and Staple- 

 ford, also near Sawston and at Comberton, and from all of these 

 large mammalian bones have at various times been collected. 



Much more interesting are the peculiar beds which occur 

 at Barrington. These are on the whole much finer in texture 

 than the usual Barnwell gravels and seem to have been 

 deposited in a lake or backwater behind the Haslingfield chalk 

 ridge. 



The best exposure is that of the above mentioned pits 

 belonging to the Royston Cement Company. There, resting 

 in a hollow, eroded in the Chalk Marl, is a variable deposit of 

 fine and very chalky silt which contains enormous numbers of 

 slightly worn bones and boulders. The association is truly 

 remarkable, and it is noticeable that wherever large boulders 

 are most abundant there too are the best of the bones. The 

 surface of the Chalk Marl is hard, quite un weathered, and very 

 pockety, and the best preserved bones always rest directly upon 

 it. The thickness of the chalky silt may be anything from 

 a few inches to several feet, and above it come some three 

 or four feet of ordinary sandy loam with few fossils. A few 

 pebbles then come in and then another bone-bearing bed which 



