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 A HISTORY OF NORTHAMPTONSHIRE from it almost anywhere along that line ; but some particular places being slightly more favourable than the average for discharging it gradually monopolized the water from an increasingly large area up to a certain limit determined by friction. Thus the bed, although pretty uniformly porous, is drained by distinct widely-separated springs. Springs that have once asserted themselves in this manner never lose their advantage whilst the porous bed lasts, for, although slipping of the wet clay in front may expose a new junction, it is always in the direction in which the water had already been making for itself a trough. So every valley has been elongated in the direction it now has by the gradual cutting backwards of the spring now at its head.' A newly- opened junction does not show widely-separated springs, and a valley that is new, geologically, does not either, but instead swampy ground, or numerous small springs only a little above the level of the stream. Hills, Slopes, Escarpments Whatever tends to produce a valley of course tends to leave a hill, which may be isolated by sufficient denudation on all sides. All hills in Northamptonshire are essentially hills of denudation, notwithstanding what was said about the Northampton Heights. Certain features of hill and valley formation referred to below have been very commonly over- looked or misinterpreted. The Northampton Sand being a water-bearing bed, and the Upper Lias Clay on which it rests impervious, the junction between the two is wet and slippery ; so the upper bed, especially if sandy, tends to slide downwards on a hill-side. It is not uncommon to find Northampton Sand covering the entire slope of a hill through a vertical range of loo to I 20 feet, although the actual thickness of the bed in the district is only 30 feet. Such slips, to distinguish them from others of a different nature, may be called high-level slips. The Northampton Sand thus commonly forms a kind of saddle to the Upper Lias hills, and springs may be met with at various heights.^ Instead of, or in addition to the Northampton Sand sliding over the clay slope, the clay itself may give way at a low level, owing to saturation with water, and denudation having produced a steeper incline than wet clay can maintain, thus great landslips — low-level slips — occur, carrying down Lias Clay, Northampton Sand, and even higher beds, en masse, shattered somewhat, and tilted at a high angle to their original position. Numerous examples occur along the Nene valley and elsewhere. As the general dip of the strata in Northamptonshire is from north- west to south-east, it will be obvious that slipping in general and high- level slipping in particular will most easily take place towards the south- east, along the dip-slope as it is called, and in this direction, or the one most nearly approaching to it, the hill-slope is generally longer than in any other. In the western parts of the county, where conspicuous isolated ' Beeby Thompson, 'The Junction Beds of the Upper Lias and Inferior Oolite in Northamptonshire,' Journ. North. Nat. Hist. Soc, vol. ix. pp. 131-149. '' Ihid. 32