Page:VCH Hertfordshire 1.djvu/65

 GEOLOGY driving piles into the beds of lakes and building his dwellings upon them. By this time he began to take a pride in chipping his flint weapons more carefully than was necessary for the use to which they were put, and then, with infinite patience, finely polishing them. At this stage Geology merges into Pre-historic Archaeology. 1 HYDRO-GEOLOGY The Chalk is our great water-bearing stratum ; firstly owing to its pervious nature, the whole of it being permeable, but its permeability or water-bearing capacity decreasing towards its base ; and secondly because the water contained in it is held up by the Gault clay on which it rests, the Upper Greensand, which usually in other districts separates these two formations, either being absent or reduced to a thin bed. The permeable Chalk and the underlying impermeable Gault dip from north- west to south-east, the gradient being at least 60 feet to the mile in the north-west and 30 feet to the mile in the south-east. The inclination of our valleys, and consequently of our rivers and of the surface of the underground water in the Chalk, is less than this, varying from about 24 feet to the mile in the north-west to about 12 feet to the mile in the south-east. In each valley the plane of permanent saturation in the Chalk, whilst sloping downwards along the course of the river, or longitudinally, with an inclination at least equal to that of the river, also slopes downwards to the river from the limit of its watershed above its source and on either side, or transversely, with an inclination less, and usually much less, than that of the sides and head of the valley. Water therefore stands lowest in the Chalk along the rivers, wherever there are springs which feed the rivers, or where the plane of saturation is artificially lowered by the water being pumped up from wells, or, generally speaking, wherever there is an outlet for it, and highest along the water-partings of the various catchment-basins, but not always exactly along them, for wherever there is a large abstraction of water from the Chalk, the plane of saturation must be lowered and the sub- terranean basin enlarged, causing the water-parting underground to recede beyond the water-parting above ground. Nearly all our rivers derive most of their supply of water from the Chalk, instead of from surface-drainage as do rivers flowing over clay or other impermeable strata, and therefore they are not dependent upon a continuous rainfall, but throughout almost the whole of the year mainly upon the rain which falls during the winter months. From experiments with percolation-gauges at Nash Mills near Hemel Hempstead, and at Lea Bridge, it has been ascertained that in the six summer months about 6 per cent, of the rain which falls finds its way through three feet of soil or chalk with grass growing on the surface, and in the six winter months 1 The author desires to express his indebtedness to Sir John Evans, Mr. Richard Lydekker and Mr. H. B. Woodward for their kindness in reading the proof of this article. 27