 The surface had sandy deposits from an ancient sea, laid over sedimentary clay (this is the blue London Clay).All the erosion down from this higher land surface, and the sorting action by these changes of water flow and direction, formed what is known as the Thames River Gravel Terraces.Since Roman times and perhaps earlier, the isostatic rebound from the weight of previous ice sheets, and its interplay with the eustatic change in sea level, have resulted in the old valley of the River Brent, together with that of the Thames, silting up again.