Finally, during the Tertiary period, another layer of London clay and other soft deposits was spread over the top of the chalk, certainly on the strip between the South Downs and the sea, and probably over the whole district between the Channel and the Thames valley: though in this case, later denudation has proceeded so far that very few traces of the Tertiary formations are preserved anywhere except in the greater hollows. Such being the original disposition of the strata which compose Sussex, we have next to ask, What are the causes which have produced its existing configuration? If the whole mass had merely been uplifted straight out of the sea, we ought now to find the whole country a flat and level table-land, covered over its entire surface with a uniform coat of Tertiary deposits. On digging or boring below these, we ought to come upon the chalk, and below the chalk again, with its cretaceous congeners the greensand or the gault, we ought to meet the Weald clay and the Hastings sand. Wherever a seaward cliff exhibited a section for our observation, we ought to find these same strata all exposed in regular order--the sandstone at the bottom, the clay above it, the broad belt of chalk halfway up, and the Tertiary muds and rubbles at the top. But in the county as we actually find it, we get a very different state of things. Here, the surface at sea-level is composed of London clay; there, a great mound of chalk rises into a swelling down; and yonder, once more, a steep escarpment leads us down into a broad lowland of the Weald. The causes which have led to this arrangement of surface and conformation must now be considered with necessary brevity.