PROCEEDINGS OF THE COTTESWOLD CLUB 83 



Windsor, Uxbridge, Watford, Hertford, and Bishop 

 Stortford to Dunmow occupies an area within the 

 watershed of 1560 square miles. The area south-east of 

 this is occupied by the Woolwich and Reading Beds and 

 the London Clay, or Lower Tertiaries series of the 

 London Basin. 



The surface soil of the Oolitic Rocks of the Cotteswold 

 district, unless drift-covered, is seldom more than a few 

 inches thick, and little or no true bcmldei' clay occurs. 

 Quaternary Gravels of variable thickness rapidly absorb 

 the rainfall, rendering the surface-flow small, except in 

 heavy rains ; the rivers being mostly supplied from 

 springs, thrown out by faults, and gathered from the 

 impermeable floors of the Lias and the Fuller's Earth, 

 according to their geographical distribution, both at and 

 below the surface; the Lias receiving and throwing out 

 those waters after percolation through the Inferior Oolite 

 above ; the Fuller's Earth constituting the second natural 

 reservoir from rainfall, percolating through the Great Oolite 

 and Forest Marble, and in places the Cornbrash. 



These two impermeable rocks are therefore the im- 

 portant factors in the development of springs, over the 

 eastern flank of the Cotteswolds, supplying eight streams, 

 draining 320 square miles, and contributing to the Thames 

 above Lechlade and St John's Weir from 30,000,000 to 

 50,000,000 gallons per day, ordinary flow. 



The Inferior Oolite extends over an area of about 120 

 square miles, furnishing many important perennial springs; 

 notably those of Syreford and Dowdeswell, which yield 

 from 3,000,000 to 4,000,000 gallons per day from the 

 surface of the retentive Lias below. 



From the impermeable rocks, the rainfall, after allowing 

 for evaporation and vegetation (or absorption), flows away 

 at once, and whenever in excess gives rise to the extensive 

 floods in the Upper and jMiddle Thames, whereas the 

 rainfall on the porous Limestones, is at once stored 



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