A HISTORY OF SURREY diles and fish, together with marine shells numerous both in individuals and in species, and plant- remains ; and these, as already mentioned, indicate a warm, almost sub-tropical temperature. Where the outcrop of the London Clay enters the county on the west, near Farnham Park, it is comparatively narrow from a half to three-quarters of a mile in width owing to the steep northerly dip, but it expands as the dip decreases, until near Leatherhead it attains a width of about three miles. East of this locality, owing to the upward shelving of the south-eastern margin of the London Basin the strike of the beds swings northward, and the London Clay, no longer covered by the Bagshot Beds (except by a few small outliers), lies exposed in a wide sheet extending to the banks of the Thames and occupying all the north-eastern part of the county around the suburbs of London, save the little strip previously described where the underlying strata come to the surface. The Clay also stretches along the Thames Valley at the northern border of the county to Chertsey and Egham, though usually covered in this quarter by the Recent Valley-deposits presently to be discussed. Wherever exposed at the surface it forms a heavy cold clay-land, but in many parts of its outcrop this character is modified by the presence of thin ' superficial ' accumulations of gravel, loam and brick-earth of Post-Tertiary or Recent age. BAGSHOT BEDS After the long period of depression indicated by the London Clay, the pendulum of change once more swung slowly back, and a gradual re-elevation set in which brought shallow water conditions again into our area. This change is indicated by the character of the beds by which the London Clay is overspread, which consist of a thick mass of sand and pebbly beds, with a subordinate clayey portion, known collec- tively as the Bagshot Beds. Fossils are extremely rare throughout this series in Surrey, but the few that have been found indicate that the deposits are of marine origin. The equivalent beds in the Hampshire Basin are, however, in their lower portion, partly fluviatile and estuarine, so that we seem, as in case of the Woolwich and Reading Beds, to have evidence of the existence of a river flowing from west to east during the accumulation of the series. Much has been written respecting the subdivision and correlation of the Bagshot Beds, 1 but for our present purpose it is sufficient to note that in Surrey a threefold division, based on the composition of the strata, is possible. 3 Of these, the lowest (Lower Bagshot Beds) consists mainly of fine whitish or yellowish sand, often micaceous, sometimes slightly laminated or intermixed with clay, and occasionally containing 1 See J. S. Gardner, Geol. Mag. 2, dec. vol. vi. (1879) p. 151, and Quart. Journ. Geol. Sec., xxxv. p. 210 ; Rev. A. Irving, Quart. Journ. Geol. Sec., vol. xlviii. p. 485, and several other papers ; H. W. Monckton, Quart. Journ. Geol. Sac., xxxix. p. 352, and xlviii. p. 48, etc. ; Monckton and Herries, ibid. xlii. p. 415 ; H. G. Lyons, ibid. vol. xlv. p. 633 ; etc., etc. It is now generally agreed that the so-called Upper Bagshot Beds of Surrey are not strictly equivalent to the Upper Bagshots of the Hampshire Basin. 16
Page:VCH Surrey 1.djvu/50
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