At the end of the last glacial period, about 10,000 years ago, the river valley that would eventually form the Somerset Levels gradually became salt marsh as the melting glaciers caused sea levels to rise. By around 6,500 to 6,000 years ago the marshes had become reed beds interspersed with rhynes (ditches) and open water, and parts of the area were in turn colonised by wet woodland. When the plants died in the oxygen-poor environment they decayed to form peat, and as the layers built up they eventually formed a large raised bog, which may have covered in Roman times. The formation of the bog was aided by the Somerset Levels' large water catchment area, high rainfall in the Mendips and Blackdown Hills and raised marine clay deposits which restricted drainage to the sea; the same impermeable material underlies the peat, which in places is up to thick.