A HISTORY OF NORTHAMPTOiNSHIRE bedded, and by alternations of pebbles and sand indicate variations in the strength of the water currents. In the eastern half of the county gravel beds of local material are to be met with, as might have been anticipated. At Pytchley, for instance, a deposit of nearly pure Great Oolite limestone gravel, some 1 5 feet thick, covers a considerable area. At Brigstock, a deposit of shelly oolitic limestone has only recently been proved to be gravel by the find- ing in it of land and freshwater shells in small patches of clay, and, when carefully looked for, small quartz pebbles, etc' In the western half of the county the gravels reach their greatest development, the beds are thicker and cover a more extended area than in the eastern parts, all of which is consistent with the supposition that the area was nearer to the extreme limits reached by the ice. The Great Chalky Boulder Clay Again glacial conditions set in, ice once more invaded the county, picking up and incorporating in its mass the loose material of the mid- glacial gravels, clay and fragments of local rocks, and so producing, with the new material it brought from a distance, a more complex mixture than any preceding it. The permanent results were so different to those of the former glaciation as to justify the following comparison. The falling and rising of mean temperature was slower, advance and recession of the ice sheet more gradual, antecedent and consequent floods less violent, period of glaciation longer, thickness of the ice greater, advance southwards further than in the previous period, added to which there was a probable depression of the whole area some 150 to 200 feet. The evidences of the last glaciation of the county are to be found in a mass of clay resting indifferently upon any of the older formations of the county, in which boulders of various rocks, and chalk and flint in particular, are abundant, hence the name Chalky Boulder Clay. The clay is mostly blue, but may be brown or yellowish in colour, and calcareous or sandy in constitution, or even approximate to a dirty gravel, depending upon the comparatively local ground rock which furnished the main mass of the material. The order of relative abundance of the argillaceous matter appears to be Oxford Clay, Kimeridge Clay, Upper Lias, Middle Lias, Lower Lias, and this, judged by fossils found in the Mid-glacial gravels, might well have been the relative order of abundance of argillaceous matter in the earlier Boulder Clay. The so-called ' Gryphaa itjcurva,' abundant in both sets of deposits, is not a Lower Lias fossil, as was long supposed, but a Kellaways Clay or Rock fossil (Lower Oxford Clay). The great thickness — 100 feet or more — of unoxidized clay not far removed from its source may be taken to indicate considerable depth of frozen ground previous to actual incorporation in the moving glacier, as
- Beeby Thompson, ' Peculiar Occurrence of Land and Freshwater Shells in the Lincoln-
shire Oolite,' Geol. Mag., decade iv., vol. ii., No. 371, May, 1895 ; see aXso P roc. Geo/. Asioc, vol. xiv. pt. iii. (July, 1895). 26
