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). 



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