A contour map of part of the basin of the glacial Lake Pickering was constructed from surface and sub-surface levels. This map (figs 2, 3 and 4) gives a pictorial representation of the microtopography of the area and is amplified by an additional map (fig. 13) in which the margin of the organic deposits is given for the early Mesolithic period, based on evidence outlined below. A stratigraphical survey of the basin deposits shows that the infilling of the lake in the Devensian Late-glacial began largely with the deposition of calcareous muds. In the early Flandrian there followed a natural hydroseral development from open water to reedswamp and fen carr. Towards the end of the Boreal period and during the early Atlantic, a rise in water level was responsible for the replacement of fen carr by reedswamp. The rate of hydroseral development was affected by a progressive rise of water level. This has been determined (estimated) by radio-carbon dating the basal organic muds on the 23, 24 and 25 m OD sub-surface contours at eight sites within the study area. During the early Mesolithic (Godwin's pollen Zone IV, c.9600 BP)the water level in the basin was likely to have been in the range of 23–24.5 m OD. By reference to the dating evidence, including published pollen diagrams (Walker and Godwin 1954) and new data presented in Parts 2 and 3, the stratigraphical evidence is interpreted to show that there was considerable variation in the vegetation of the lake margin during the early Mesolithic period (pollen Zone IV). At Seamer Carr there was a large area of fen carr woodland with reedswamp fringing the open water. Star Carr had a much larger area of reedswamp, with open water close to the dry land in places, but with a narrow fringe of carr. The bed of the ancient River Hertford, which drained the basin, probably in the late Mesolithic, has been identified in two of the sections which traverse the basin.