﻿12 MR. W. HILL ON A DEEP [Feb. I908, 



the surface. South of Hitchin the ground rises and becomes more 

 uneven, the natural drainage having cut valleys which run in 

 sinuous courses far back into the escarpment. These valleys are 

 bordered by ridges rising in some instances to 290 feet above sea- 

 level. Most of these ridges appear to be mounds of sand and gravel 

 (possibly with a core of Chalk), but some are bosses or discontinuous 

 ridges of the bare Chalk cropping out through the mantle of 

 sand and gravel which masks a pre-existing contour of the Chalk- 

 escarpment. From the ground between the headlands to the gap, 

 there is, however, a gradual ascent from 200 to 320 feet. 



The present drainage of the district remains to be noticed. 

 Again referring to the 300-foot contour-line, it will be seen that on 

 its eastern limb is a deep indentation which carries back the 

 escarpment to within half a mile of Stevenage, from which point it 

 makes a bold curve to the eastward to Graveley. This is the valley 

 of the Purwell, as indicated on the maps of the Ordnance Survey. 

 Though the water takes a well-defined but a sinuous course, the 

 first 3 miles of the valley-way must be considered as only a tem- 

 porary water-course flowing in the winter or during wet weather, 

 for in the summer months it is not infrequently dry. No spring 

 occurs in the valley until it has passed under the Great Northern 

 Kailway at Nine Springs, near Hitchin, where a great deal of water 

 comes to the surface. 



Separated from the Purwell by a well-marked ridge, which can 

 be followed in almost a straight line down the Y nearly to Hitchin, 

 and dividing it almost in its centre, is another line of drainage. 

 This commences at the water-parting within the Gap. In wet 

 weather a considerable body of water draining from the ridge 

 intervening between the two waterways, and also from the main 

 ridge lying to the westward, descends in a north-westerly direction 

 along a narrow well-marked channel, and is lost in a swallow-hole 

 about 2 miles distant from the divide within the Gap. A mile lower 

 down the valley, at Ippollitts, some of this water is probably seen 

 again in the springs that rise in swampy land, which together form 

 a constantly-running rivulet known as Ippollitts Brook. This 

 brook, taking first a northward direction for three-quarters of a mile, 

 turns to the east, and seems to have cut through the ridge separating 

 the two waterways. Joining the Purwell at Nine Springs, the two 

 flow together as the Purwell northwards towards the Ouse basin. 



The channel of Drift now to be described follows this line of drain- 

 age to the point where the Ippollitts Brook changes its direction, its 

 course then seeming to be north-westward straight out between the 

 Headlands. 



Another drainage-line is that of the Eiver Hiz. This takes its 

 rise in a strong spring known as Well Head, a mile and a half 

 west of the town. The water increased by minor springs along its 

 course flows nearly parallel with the escarpment, that is, in a north- 

 north-easterly direction, for 2 miles, when its water joins that of 

 the Purwell at Grove Mill. Thence the combined water, now called 



