Boulder-clay i 37 



Pleistocene followed Pliocene and the rigour of the climate 

 still increasing culminated at last in the period of the Great 

 Ice Age. 



Glacial Period, 

 (a) Boulder-clay. 



The accumulations of the Ice Age must at one time have 

 covered the whole of Cambridgeshire. They still occupy a 

 greater area of it than any other series except the fen, so are 

 of enormous importance to the agriculturist. Being only 

 indifferently exposed and very complex in character they 

 have received but scant attention at the hands of geologists. 



Glacial accumulations generally may be divided into two 

 types, Gravel and Boulder-clay', and in Cambridgeshire it is 

 the various modifications of Boulder-clay which are dominant. 



The Boulder-clay as we see it is a heterogeneous mass 

 of coarse and fine material containing pebbles and boulders 

 of many sorts of rocks. No stratification is apparent, and 

 the included rock fragments may be of all sizes and shapes, 

 rounded or angular. Soft and compact rocks may have their 

 surfaces smoothed, scratched, striated, and often facetted, 

 and harder rocks, if homogeneous, generally bear evidence 

 of similar treatment. 



The constitution of the Boulder-clay varies greatly from 

 place to place, and with changes of constitution there are also 

 great changes of texture. 



The constitution of the Boulder-clay at any place is 

 closely related to that of the solid rock beneath it, but is also 

 partly determined by the composition of the rock floor along 

 some definite line, which stretches more or less northward 

 from the place in question. 



The matrix of the clay is almost entirely the crushed soft 

 material of rocks near at hand, but the boulders include the 

 survivors of hard and tough rocks from further afield, and the 

 size and abundance of the different boulders is a measure of 



