in the Drift of the Valley of the Avon. 



225 



found, although diligent search has been made at every fresh, 

 opening. 



When we consider that nearly all the drift gravel has been 

 derived from the chalk, and remark the comparatively few bands 

 of flint nodules which it contains, we feel it is impossible to 

 form an adequate notion of the immense bulk of chalk which, 

 must have been denuded and disintegrated, to produce these large 

 accumulations of flint gravel. 



One is often asked how many years old these gravels are ? A 

 question that can at present only be answered relatively. We can 

 with tolerable certainty say one set of gravels is older than another, 

 but no exact data exist by which any given thickness of deposit 

 can at once be reduced to an equivalent in years. Many of the 

 forces in operation would alter materially in the different periods, 

 and what might be true for one would be false for another. For 

 instance, it is probable that during these periods considerable 

 alteration in the relative level of the surrounding country occurred, 

 yet any such change must have been very gradual, as the 

 regular stratification of the chalk hills is undisturbed, and shows 

 no trace of any sudden upheaval or depression. Again, if we 

 examine the composition of the deposits, it is evident that the 

 physical conditions under which the higher level gravels were 

 deposited, differed considerably from those of the subsequent beds. 

 The large angular flints and the blocks of saccharoid sandstone were 

 probably attached to masses of ice, and thus lightened would easily 

 float into their present position ; whilst the absence of anything like 

 stratification and the very mixed character of the deposit, proves 

 that these various materials were roughly jumbled together. 

 Hence we conclude the high level gravels are the result of tor- 

 rential action during a period of great cold. This turbulent period 

 was succeeded by one of comparative tranquillity, when the 

 great mass of the brick-earth at Fisherton was brought down the 

 valley in the shape of mud and sand : even then the temperature 

 was sufficiently cold to float the few large blocks of sandstone that 

 are found irregularly scattered through the clay. The highest 

 level gravels are almost destitute of organic remains ; Palaeontology 



