The Rivers 3 



surface soil, are Boulder-clay, gravels, sands and loams, 

 and peat. 



The drainage of the rivers is to some extent determined 

 by the direction of strike of the strata, but in the Fens many 

 changes have taken place in recent times, and the meandering 

 courses of the river have again and again been altered. The 

 course of the Ouse through the county is, as a whole, that of 

 a strike stream, though the portion between Bluntishain and 

 Ely, known as the Old River, flows in the general direction 

 of dip of the strata. The Rhee, from its source near Ash well 

 to its junction with the Granta, and beyond this to Bottisham, 

 is also a strike stream. One important tributary from the 

 west, the Bourn, flows in the direction of the dip to the 

 junction with the Cam near Trumpington. Most of the tribu- 

 taries, however, flow in a direction opposite to that of the dip 

 of the strata, and drain the chalk tract. The most note- 

 worthy are the Granta, which flows from Saffron Walden, the 

 Lin, from Bartlow, the Lark, from Bury St Edmunds, and 

 the Little Ouse from Thetford. All of these have low passes 

 (wind-gaps) at their head, and may occupy valleys which 

 were once drained by streams flowing in a direction contrary 

 to that of the present streams. 



Though the pre-glacial courses of the principal streams 

 were much as now, many minor modifications were produced 

 during and after glacial times, but into this matter we cannot 

 enter here. 



We may now proceed to give a brief account of the 

 characters of the principal and minor areas which we have 

 previously defined. 



(i) The Chalk Tract. The main tract of Chalk occupies 

 that part of the county which lies east of the Rhee, and of 

 the Cam after the Rhee has joined it. It is bordered on the 

 west by wide sheets of gravel, and its higher hills are capped 

 by Boulder-clay. Several of the hills in the south-east of the 

 county rise above the four hundred foot contour-line, and much 

 of this district is relatively elevated. The district consists 



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