Isles were covered with glacial ice, and the sea level was about lower. The climate later became warmer, and during the Last Glacial Maximum, around 12,000 BCE, Great Britain, as well as much of the North Sea and the English Channel, was an expanse of low-lying tundra. Evidence, including the contours of the present seabed, indicates that after the first main Ice Age the watershed between the North Sea and the English Channel extended east from East Anglia, then southeast to the Hook of Holland, rather than across the Strait of Dover. The Seine, the Thames, the Meuse, the Scheldt and the Rhine joined and flowed west along the English Channel as a wide slow river before eventually reaching the Atlantic Ocean. In about 10,000 BCE the north-facing coastal area of Doggerland had a coastline of lagoons, saltmarshes, mudflats and beaches as well as inland streams, rivers, marshes and lakes. It may have been the richest hunting, fowling and fishing ground in Europe in the Mesolithic period. One large river system found by a 3D seismic survey, undertaken by the Birmingham "North Sea Palaeolandscapes Project", drained the southeastern part of the Dogger Bank hill area into the east end of the Outer Silver Pit lake. It has been named the Shotton River after the Birmingham geologist Frederick William Shotton. Disappearance As ice melted at the end of the last glacial period of the current ice age, sea levels rose and the land began to tilt in an isostatic adjustment as the huge weight of ice lessened. Doggerland eventually became submerged, cutting off what was previously the British peninsula from the European mainland by around 6500 BCE. The Dogger Bank, an upland area of Doggerland, remained an island until at least 5000 BCE. Key stages are now believed to have included the gradual evolution of a large tidal bay between eastern England and Dogger Bank by 9000 BCE and a rapid sea level rise thereafter, leading to Dogger Bank becoming an island and Britain becoming physically disconnected from the continent. A recent hypothesis suggests that around 6200 BCE much of the remaining coastal land was flooded by a tsunami caused by a submarine landslide off the coast of Norway known as the Storegga Slide. This suggests "that the Storegga Slide tsunami would have had a catastrophic impact on the contemporary coastal Mesolithic population ... Britain finally became separated from the continent