The drift differs considerably in its power of interference with the passage of the rain-water into the strata beneath. The ochreous sandy flint gravel, forming so generally the subsoil of London, admits of the passage of water. All the shallow surface springs, from 10 to 20 feet deep, are produced by water which has fallen on, and passed through, this gravel, _g_, Fig. 15, down to the top of the London clay, _a_, on the irregular surface of which it is held up. [Illustration: Section through London Subsoil. Fig. 15.] When the London clay is wanting, this gravel lies immediately upon the Lower Tertiary strata, as in the valley between Windsor and Maidenhead, and in that of the Kennet between Newbury and Thatcham, transmitting to the underlying strata part of the surface water. Where beds of brick earth occur in the drift, as between West Drayton and Uxbridge, the passage of the surface water into the underlying strata is intercepted. Sometimes the drift is composed of gravel mixed very irregularly with broken up London clay, and although commonly not more than 3 to 8 feet thick, it is generally impermeable. Over a considerable portion of Suffolk and part of Essex, a drift, composed of coarse and usually light-coloured sand with fine gravel, occurs. Water percolates through it with extreme facility, but it is generally covered by a thick mass of stiff tenacious bluish grey clay, perfectly impervious. This clay drift, or boulder clay, caps, to a depth of from 10 to 50 feet or more, almost all the hills in the northern division of Essex, and a large portion of Suffolk and Norfolk. It so conceals the underlying strata that it is difficult to trace the course of the outcrop of the Lower Tertiary sands between Ware and Ipswich; and often, as in Fig. 16, notwithstanding the breadth, apart from this cause of the outcrop of the Tertiary sands, _b_, and of the drift of sand and gravel, 2, they are both so covered by the boulder clay, 1, that the small surface exposed can be of comparatively little value. [Illustration: Outcrop covered by Boulder Clay. Fig. 16.] There are also, in some valleys, river deposits of silt, mud, and gravel. These are, however, of little importance to the subject before us. Under ordinary conditions they are generally sufficiently impervious to prevent the water from passing through the beds beneath. HEIGHT OF WATER-BEARING STRATA ABOVE SURFACE OF COUNTRY. The height of the districts, wherein the water-bearing strata crop out, above that of the surface of the country in which the wells are placed, should be made the subject of careful consideration, as upon this point depends the level to which the water in Artesian wells may ascend.