Here it entered a substantial freshwater lake in the southern North Sea basin, south of what is called Doggerland. The overspill of this lake caused the formation of the Channel River and later the Dover Strait gap between present-day Britain and France. Subsequent development led to the continuation of the course that the river follows at the present day. Most of the bedrock of the Vale of Aylesbury comprises clay and chalk that formed at the end of the ice age and at one time was under the Proto-Thames. At this time the vast underground reserves of water formed that make the water table higher than average in the Vale of Aylesbury. The last advance of the Scandinavian ice-sheet to have reached this far south covered much of north-west Middlesex and finally forced the Proto-Thames to take roughly its present course. At the height of the last ice age, around 20,000 BC, Britain was connected to mainland Europe by a large expanse of land known as Doggerland in the southern North Sea Basin. At this time, the Thames' course did not continue to Doggerland but flowed southwards from the eastern Essex coast where it met the waters of the proto-Rhine, -Meuse and -Scheldt flowing from what are now the Netherlands and Belgium. These rivers formed a single river – the Channel River ("Fleuve Manche") – that passed through the Dover Strait and drained into the Atlantic Ocean in the western English Channel. The ice sheet, which stopped around present-day Finchley, deposited boulder clay to form Dollis Hill and Hanger Hill. Its torrent of meltwater gushed through the Finchley Gap and south towards the new course of the Thames, and proceeded to carve out the Brent Valley in the process. Upon the valley sides there can be seen other terraces of brickearth, laid over and sometimes interlayered with the clays. These deposits were brought in by the winds during the periglacial periods, suggesting that wide, flat marshes were then part of the landscape, which the new river Brent proceeded to cut down. The steepness of the valley sides indicates very much lower mean sea levels caused by the glaciation locking up so much water upon the land masses, thus causing the river water to flow rapidly seaward and so erode its bed quickly downwards. The original land surface was around above the current sea-level. The surface had sandy deposits from an ancient sea, laid over sedimentary clay (this is the blue London Clay). All the erosion down from this higher land surface, and the sorting action by these changes of water flow and direction, formed what is known as the Thames River Gravel Terraces.