10 The Geology of Cambridgeshire 



Several outliers of Lower Cretaceous rocks rest upon the 

 Jurassic rocks to the west of the main line of outcrop of the 

 Lower Cretaceous group, and outliers of Chalk repo>.- <>n lower 

 members of the Cretaceous System to the west of the main 

 development of the Chalk. The Jurassic rocks, with tin- 

 exception of a local development near Upware, consist of 

 slightly consolidated clays or muds, while the Cretaceous rocks 

 present a greater variety of sediments. At the base is ,-ui 

 iron-stained sand, sometimes compacted into sandstone. This 

 is succeeded by a thick deposit of clay, and at the top is 

 the Chalk. No Tertiary sediments occur in the county, so our 

 knowledge of events during that period depends entirely on 

 the record contained in the rocks of other areas. 



The only accumulations newer than the chalk are of post- 

 Tertiary date. They consist of Boulder-clay, accumulated 

 during the 'Great Ice Age,' and of subsequently formed 

 gravels of fluviatile and marine origin, and of the alluvia of 

 the river- valleys and the peat and silt of the Fenland. 



For works on the geology of the county the reader may be 

 referred to : (i) the Rev. T. G. Bonney's Cambridgeshire 

 Geology, published in 1875 ; (ii) F. R. C. Reed's ILunllnntl- 

 to the Geology of Cambridgeshire, which appeared in 18!>7 : 

 (iii) The Geology of the Neighbourhood of Cambridge, being 

 the explanation of Quarter- sheet 51 S. W. and part of 51 N.W., 

 by W. H. Penning and A. J. Jukes-Browne, published in 1881 ; 

 and (iv) The Geology of the Fenland, by S. B. J. Skertchly, 

 which appeared in 1877. The two latter are official publications 

 of H.M. Geological Survey. The list of geological survey maps 

 and sections which illustrate the geology of the county will be 

 found in Mr Reed's bibliography, which forms the appendix to 

 his work on Cambridgeshire geology. This bibliography also 

 gives a very full list of the various hooks and pamphlets which 

 have appeared at different times, containing references to the 

 geology of the county. 



We may now proceed to describe the various deposits of 

 the county in order, commencing with the oldest strata. 



