38 The Geology of Cambridgeshire 



their resisting qualities and the distance from their point of 

 origin. Large cakes of soft rocks carried from some little 

 distance are also present. 



Where the rock underlying the Boulder-clay is moderately 

 hard the junction between the two is very sharp. In other 

 districts it is found that under similar circumstances the 

 surface of the lower rock is grooved, and often polished, but 

 in Cambridgeshire we know of no such 'glaciated pavements.' 

 Sharp junctions of Boulder-clay and Lower Greensand can be 

 seen at Great Gransden, and others are common wherever the 

 clay rests upon Middle or Upper Chalk. In many parts of 

 Cambridge the Boulder-clay rests on soft clays or marls whose 

 upper parts are so disturbed that it is well-nigh impossible to 

 determine any j unction. 



An exactly similar state of affairs has also been recently 

 described as affecting the Chalk of the highest part of the 

 escarpment near Royston. There, wisps of Boulder-clay under- 

 lie parts of the shattered and highly inclined chalk which 

 occurs along the celebrated Royston 'line of flexure,' and 

 Mr H. B. Woodward, who describes the phenomena, refers 

 the production of that 'line of flexure' to the agents which 

 formed the Boulder-clay. 



The rock beneath Boulder-clay is generally quite fresh 

 and unweathered, but if soluble, and at the same time porous, 

 it may have undergone considerable alteration by percolating 

 water whose direction of flow the impervious clay above often 

 determines. The rock most easily affected in this way is the 

 Chalk, and between it and the Boulder-clay there are, in some 

 places masses of unweathered flint rubble, while in other 

 places the chalk adjoining the Boulder-clay has been re- 

 cemented and is exceedingly hard. 



The general geography of the surface on which the Boulder- 

 clay rests was studied by Mr Jukes-Browne, who concluded 

 that the general form of the country was much the same as at 

 present, but that the chalk escarpment then extended about 

 a mile and a quarter further to the west. He states " that 



