To the west of Cambridge, between the Ouse Valley to the north and the Cam Valley to the south, lies an undulating plateau known for its ‘boulder clay’ woodlands, giving a clue to the reason for this higher land.
Boulder Clay is a previous name for the Glacial Till deposits left after glaciers have retreated, leaving a variety of material ‘dropped’ as the ice melted. These deposits probably date from the last time that glaciers covered Cambridgeshire – the Anglian glaciation which occurred about 480,000 years ago (during the Pleistocene Period). Influenced by the land that the ice has passed over, the till in this area is mostly a chalky clay but with some sand in places and larger rocks including scattered boulders, the latter sometimes brought from many miles away by the ice.
The till in this area overlies a series of rock units mostly dating from the Cretaceous Period but some older, from the Jurassic. In places the glacial deposits are very thick but in others the older rocks are at the surface. These are mostly clays, although the chalk underlies the south of the plateau and there is a band of sandstone representing the north eastern extent of the Greensand Ridge, better known from Bedfordshire.
As in many places, the current surface hides the interesting landscapes of the past – and research has shown that there are deep, hidden channels under some of the plateau, now infilled with large amounts of material disguising their presence.
Springs sometimes occur when a more impermeable rock layer is reached by water percolating through overlying permeable deposits.
Many of the woods on the ‘boulder clay’ hold populations of the scarce Oxlip, a plant related to primroses that thrives in the seasonally water-logged conditions that the clayey soil provides. The Wildlife Trust has a number of nature reserves on the Plateau forming the Cambridgeshire Hundreds Living Landscape.
Looking south, near Waresley Wood, to the valley of the Dean Brook which lies in the Ampthill Clays and West Walton Formation of Jurassic Age.
The church at Cockayne-Hatley includes local ironstone and cobbles in its walls.
Oxlips growing in Hayley Wood
Sand from the Lower Greensand (Cretaceous Period), easily excavated by badgers!