Boulders in Boulder-clay 39 



the Cambridge valley, as such, is pre-glacial," and allows that 

 there are certain old valleys, as those of the Lin and Bourn, 

 which were completely choked with Boulder-clay. More 

 recent well-sinkings have shown that filled valleys are more 

 abundant than he supposed, and moreover, that a few of 

 them extend downwards considerably below sea level, e.g. 

 Impington sixty feet, Saffron Walden and Audley End, but 

 in Cambridgeshire none of these has yet been traced con- 

 tinuously. 



Having thus indicated the general relations of the Boulder- 

 clay we may now mention a few of the points of interest 

 presented by the boulders. 



The most abundant are those of chalk and flint, which, 

 being ever present, have led to the inclusion of the Cambridge- 

 shire Boulder-clay as a part of the Great Chalky Boulder- 

 clay. The chalk boulders of Boulder-clay which overlies 

 Chalk are often only the local chalk rubble, but those of the 

 clayey accumulations of the west are well rounded and often 

 scratched. They are, moreover, of a type unknown in East 

 Anglia, and strongly resemble certain of the hard beds of the 

 Flamborough chalk. A similar remark applies also to the 

 irregular masses of tabular flints which abound in the west, 

 and these are further interesting in that they are striated on 

 concave as well as on convex surfaces. A few of these foreign 

 chalk and flint boulders occur among the others in the 

 eastern area. 



Next in point of numbers come the boulders of the hard 

 members of the Lower Cretaceous and Jurassic series. Many 

 of these are very fossiliferous and lithologically quite charac- 

 teristic, but much work will have to be done before we are 

 able to refer them to their original homes. 



Nodules of Cambridge Greensand which has no known 

 outcrop above the two-hundred-foot contour can frequently 

 be collected from the Boulder-clay right up to the top of the 

 hills about Royston, but do not seem to occur east of Linton. 

 Red Rock like that of Hunstanton is found all over the 



