A HISTORY OF NORTHAMPTONSHIRE rested upon an irregular surface of Upper Lias Clay, which, where rising into hunimocky masses, was highly contorted. Again, in the stratified gravel and sand beds above were large and small boulders of Upper Lias Clay, some oxidized and carrying vegetable matter, showing them to be fragments of the pre-glacial soil, and to have been carried by fioating ice. With continued refrigeration (fluctuations apart), we may con- sider that each succeeding winter the ground would get frozen to a greater depth, and each summer the proportion of snow and ice melted less, until a sedentary ice sheet formed in situ. A glacier advancing from more northerly parts would firstly override this, then by a process of regelation incorporate it, and so ultimately compel movement of it. The thickness of matter moved would at first, of course, correspond to the depth of previously frozen ground, and the junction of this latter with the unfrozen rock below the lowest plane of shearing. These basal layers, highly charged with local rock constituents, would move very slowly compared with the glacier as a whole, and so the material be only to a small extent, and that very gradually, incorporated in the main mass of ice, carried as englacial drift and deposited as erratics at a con- siderable distance from its source. Hence we see why Boulder Clay or Till is so commonly in the main composed of local rocks. The movement of ice, like that of water, being along lines of least resistance, such as are afforded by river valleys and low-lying ground, an easy passage for a glacier would be afforded in eastern Northamptonshire and the whole area around the Wash ; combine this with a slightly higher latitude, and it is reasonable to suppose that the outfalls of the Welland, Nene and Ouse were stopped before true glacial conditions prevailed in more westerly parts of the county, and so the ordinary and extraordinary drainage of a large area diverted to the west and south. To water from outside the county in a northerly direction, seeking to discharge southwards, the Northampton Heights and a spur of high ground by Hillmorton and south of Rugby offered an almost complete barrier, and so the water was largely diverted into the Avon valley ; the evidences of this are as follows. At Hillmorton drift sand and gravel are heaped up to the thickness of 170 feet against a highly inclined cliff of Lower Lias Clay facing nearly north. Eastward, towards Crick, the deposit to be seen, 50 feet in depth, passes rather rapidly into gravel ; westward, however, towards Rugby, gravel gives place to fine false-bedded sand, with here and there lenticular patches of gravel, or even clay ; and then, in a kind of bay east and south of Rugby, to a contorted brown sandy clay or loam, with a few erratics in patches, showing that a glacier contributed both water and ice, and of this there is further evidence in disturbed and scratched local limestone block.' One opening into the county, and probably the only one in the ' Beeby Thompson, ' Geology of the Great Central Railway, Rugby to Catesby,' Quart. Journ. Geol. Soc.,o.'v.{Ye.h. 1899) ; 'Excursion to Hillmorton and Rugby,' Proc. Geo/. Assoc, vol. XV. pt. 10 (Nov. 1898). 24
