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 



