Z A. SUTTON — THE KIVER-STSTEM 



I do not put forward the theory that the rivers under 

 consideration originally had their rise so far to the north-west ; 

 indeed, their valleys are not large enough, and they are too close 

 together to warrant such a supposition, but there can be no 

 doubt that, as was pointed out by Sir John Evans in an early 

 number of the Society's ' Transactions,' they had their rise far 

 to the north of their present sources. Lord Avebury refers 

 to them in his recent book on ' The Scenery of England ' as 

 " beheaded rivers " ; that is to say, rivers whose upper portions 

 have disappeared owing to the action of rival river-systems, and 

 of subaerial denudation generally, in wearing down the drainage- 

 area which formerly fed them, and converting it into an area of 

 contrsbTj inclination feeding the rival streams. 



The upper portions of the former area of drainage of our 

 rivers has thus been captured by the Upper Thames and the 

 Ouse, owing to the more rapid action of these larger streams 

 in eroding their valleys in the softer material of the Lower 

 Cretaceous beds, and the atmospheric influences which have 

 worn back the escarpment of the Chalk. We see, therefore, at 

 the heads of the valleys of all oiir rivers, gaps in the Chalk 

 escarpment through which the rivers formerly ran. These gaps 

 are, however, at such an elevation as to preclude the idea that 

 the rivers ever rose beyond the original escarpment, like some of 

 those in the Wealden area, which rise beyond the Chalk and cut 

 their way through it. Their sources, it would seem, must always 

 have been south of the escarpment, and must have receded with 

 it. Mr. Hopkinson has pointed out, in the Introduction to 

 Pryor's ' Flora,' already referred to, that they are all formed by 

 overflow from the level of satiu-ation of the Chalk, which forms 

 an immense spongy reservoir, never entirely emptied, and I think 

 that they were always fed almost exclusively in this way. 



In considering the development of any river-system, we must, 

 as Lord Avebury says in his book already mentioned, carry our 

 minds back to a time when the streams composing it flowed at 

 a considerably higher level than at present. When our rivers 

 first commenced to flow, the whole of the area over which they 

 ran was probably covered by the Tertiary beds, outliers of which 

 still occur far from the main formation, and the sxirface was no 

 doubt several hundred feet higher than at present. The 

 superincumbent material which once formed the soil of Hert- 

 fordshire has been carried away and deposited in the German 

 Ocean, except the stony fragments which now constitute the beds 

 of gravel and " clay with flints " overlying the Chalk nearly 

 everywhere, the larger blocks of Hertfordshire conglomerate 

 (pudding-stone), and the Sarsen stones or " grey wethers," 

 which are scattered over the country. At this early period the 

 general level of the country was probably higher than the 

 summits of the ridge which now runs along the Middlesex 

 border, rising at Bushey Heath and Stanmore Common to an 

 altitude of over 600 feet above sea-level, and our rivers possibly 



