| Summary: | A diapiric mass of chalk rises some 35 m through Palaeogene strata beneath the floodplain of the Baughurst Stream, a tributary of the River Kennet. It is argued that this chalk anomaly was initially formed by the rise of Upper Chalk during a period (s) of permafrost degradation. This ultimately pierced the valley floor. Shattering and mobility of chalk at depth apparently arose from repeated permafrost formation to a minimum depth of 75 m. Once present, upflow of groundwater in succeeding cold stages promoted the development of ground ice bodies possibly as pingos. Subsequently ground ice degradation resulted in rockhead depressions within which a tripartite Quaternary succession is recognised. This consists of: (il "Lower Silts" - deposited in an isolated deep lake (s), "Middle Gravels" - deposited in a subsiding basin resulting from thermal degradation of ground ice and, (iii) "Upper Silts" - associated with a subsequent lacustrine phase. The latter implies continued subsidence after the cessation of gravel transport. A biogenic-rich bed at the base of the "Upper Silts" contains early Flandrian pollen and freshwater Mollusca including the first post glacial record of Gyraulus laevis in the Kennet catchment. Infilling of the lake and a99radation of the flood plain commenced after c. 9 ka B. P., the silts probably being derived from anthropogenic disturbance. An alternative mechanism for valley bulgingand hillslope cambering, without invoking prior incision, is proposed. This is allied to periglacia1 mass movement over a low-strength zone resulting from transient high pore-water pressures during permafrost degradation. Comparison with other sites in the Thames Valley, and with pingos and pingo remnants, reveals parallels which could help locate similar features. A new discovery at Colnbrook, Buckinghamshire, is described.
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