The Ice Age
About 478,000 to 424,000 years ago during the ice age period known as the Anglian Stage, glaciers approached from the North Sea and reached as far south-west as Bricket Wood. Glacial gravels and boulder clays cover a great deal of the whole area to the north east of the county and the Upper Chalk itself has been disturbed at Reed and Barley by glaciation.
Prior to the ice ages the River Thames followed a path through the southern part of Hertfordshire, running from the area of modern Staines up the valley of the Colne to Hatfield and then eastward across Essex originally towards the primeval Rhine but later down the valley of the modern River Lea. This path was blocked by a mass of ice near Hatfield and a lake ponded up to the west of this around St Albans. Waters eventually overflowed near Staines to cut the path of the modern Thames through central London. When the ice retreated about 400,000 years ago the river bed along the new route followed the lower path and so the river remained on its present day course. The flow in the Colne valley reversed, now flowing south as a tributary into the modern Thames. Superficial gravel deposits from the primordial Thames, are found throughout the Vale of St. Albans.
At the retreat of the glaciers, wind blown powdered rock known as loess was deposited over the whole county, forming thin layers under a meter thick. This makes for fine, easily cultivated and fertile soils.
Read more about this topic: Geology Of Hertfordshire
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