IN NORTH HERTFORDSHIRE. 
35 
The ridge upon which this line runs is of considerable elevation, 
rising, at its highest point, at Therfield, to 550ft. above sea-level. 
Standing on this ridge at Kelshall (about a mile W. of Therfield), 
and looking to the IS", and X.W. into Cambridgeshire and Bedford¬ 
shire, the ground from one’s feet falls rapidly away over the outcrop 
of the various lower beds of the Chalk for about four miles, until, 
at the bottom of the shallow valley, a line of springs is reached, 
brought out by the Totternhoe stone, at an elevation above the sea 
of about 150ft. The average slope, therefore, of this side of the 
valley is about 100ft. per mile. The slope is, of course, very 
irregular and diversified by hills and valleys and transverse ridges, 
of which latter, one more marked than the rest, capped towards its 
northern termination by a bed of boulder-clay, divides the head¬ 
waters of the Cam or Rhee at Ashwell from the streams which run 
to the Ivel more to the west. This ridge joins the main ridge at 
Kelshall. A similar transverse ridge, but very ill-defined for a 
considerable distance from the main ridge, runs down from Wal- 
lington to Bygrave, where it forms a considerable hill, also capped 
with clay. Generally in this narrow strip of Chalk country there 
is no surface-drainage, the exception being where patches of 
boulder-clay throw off the water; all the valleys, and the lower 
and some of the higher hills, are bare chalk. Patches of clay and 
gravel lie here and there on the higher hills. There are, however, 
occasionally, crossing this band of bare chalk, water-courses, 
which, under ordinary .circumstances, are dry ditches, but which 
fill with water after heavy rain. These water-courses drain off 
the patches of clay, and especially the small area of clay, perhaps 
consisting of washings from the clay originally capping the hills, 
which lies along the highest ridge just within the limit formed by 
the water-parting, that is to say within the valleys of the Cam and 
Ivel. These ditches draining into the Cam and Ivel bring down 
at times of heavy rain a good deal of water. There are thus two 
sources of river-supply; the permanent flow from the springs 
brought out by the impervious bed of the Totternhoe stone at its 
outcrop, which contribute the main volume of water to the streams, 
and the occasional flow of surface-water from the boulder-clay, 
which, after passing over a strip of bare chalk, finds its way into 
these same streams. 
One of the conspicuous features of this district is the position of the 
boulder-clays and gravels capping the hills, the valleys between 
these hills being free from clays or gravels, except such trifling 
deposits as are clearly due to the intermittent streams which pass 
through them. It is also to be observed that it is only on the 
higher hills that the clays lie ; and that the summits of these clay- 
capped hills lie in the plane of one general slope from the main 
ridge of the water-parting between the Thames and Ouse valleys. 
This plane seems to represent in a general way the surface of the 
country at the time of the deposition of the glacial beds, which 
were, no doubt, spread pretty uniformly over the whole district. 
The clean-cut valleys which we now have, probably represent in 
