The Berwickshire Cementstone Group was deposited on a coastal plain crossed by meandering rivers flowing S into a marine gulf, the Northumberland basin. Sedimentation was mainly by overbank flooding onto an alluvial plain which largely dried out between floods. The resulting sediments include poorly-stratified mudstones and siltstones and crevasse splay sheet sandstones. The depositional environment of the cementstones is not clear. Lacustrine and tidal flat origins are considered. Semi-permanent floodplain lakes were very rare, but in one the uniquely fossiliferous Foulden Fish Bed was deposited. This lake was shallow and filled by repeated influxes of sediment carried from rivers by sheetfloods. On entering the lake, these sheetfloods may have become density underflows from which thin, graded siltstones or sandstones were deposited. These floods carried plant debris into the lake and rapidly buried the remains of the lake fauna. The salinity of the lake cannot be determined, but the input of both fresh and saline waters can be envisaged. There is no evidence that the lake was permanently eutrophic. The mortality of the animals may have been due to changes in water chemistry produced by hot dry spells of weather followed by storms and floods.