The Eocene Deposits of Dorset

Abstract

The new survey of the western end of the Hampshire Basin having necessitated certain modifications in the geological map, it may be useful to lay the principal results before the Society. If the alterations were mere matters of detail, this would scarcely be worth while; but the more accurate mapping has led to the discovery of a sudden westerly change in the character of the Lower Bagshot Sands, and of a well-marked overlap at their base. The mapping of the Tertiary strata proves also that there is evidence of other periods of earth-movement in southern England, besides those already known, and that we are dealing with one of those regions where folding affects the same area again and again. When the Eocene strata are followed westward through Sussex and Hampshire into Dorset, one finds constant local changes in the lithological character of the deposits, though these changes seem all to tend in the same direction. Marine beds become less conspicuous, coarser, more estuarine, and estuarine deposits become truly fluviatile. Thus the London Clay, which exceeds 300 feet in thickness to the east, dwindles to less than 100 feet in Dorset, and becomes more sandy and pebbly, though still apparently of marine origin. The Woolwich and Reading Series—fluvio-marine at Newhaven and Portslade, and slightly so at Lancing— becomes more fluviatile westward, lenticular patches of subangular gravel appearing in it west of Wareham. To what extent the Reading Series rests unconformably upon the Chalk is difficult to say, though overlap is clearly