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Fig 4. Till types and macrofabric (directional) data from tills in the Chelmsford area.
(After Whiteman, in Allen et al. 1991)
NG - Newney Green GW - Great Waltham B - Broomfield
One part of northwest Essex, around Saffron Walden (and adjoining parts of Cambridgeshire),
lacks a till cover though the glaciers must have passed over the area. This can be explained by
the inter-relationship of the type of rock substrate and the processes of glacial deposition and
erosion. As a general rule in East Anglia, though there are many exceptions, till is usually found
overlying sands and/or gravels and is thin or absent over chalk and clays. Boulton (1974)
describes a glacial process which can be likened to the action of sandpaper. Rubbing with
sandpaper with a degree of pressure and speed will remove paint or wood (= erosion) but if you
press too hard, the sand grains dig into the wood and can become embedded (= deformation and
deposition). When a glacier passes over chalk or clay, subglacial water cannot escape freely
(chalk quickly breaks down into silt and clay sized grains when under stress). This water
lubricates the base of the glacier and enhances its velocity, but also causes a slight amount of
buoyancy, thus reducing pressure at the points of contact between the ice and the substrate. In
such conditions the ice is more likely to be erosional. Bare chalk is exposed at the surface in the
northwest part of county, hence the erosional processes just described may have operated there,
leaving the chalk exposed. South of that, the till overlies early Thames gravels which would
have allowed water to drain away more readily, reducing the degree of lubrication and so
increasing friction at the base of the ice. The buoyancy effect would also have been reduced.
These conditions would have promoted the ice changing to a depositional mode. The southern
Essex Naturalist (New Series) 16 (1999)