Essex occupies the London Basin which has for its framework the great chalk formation. The northern rim of the basin appears in the uplands of Saffron Walden. Above this are deposits of drift, scattered over the uplands in the forms of gravel and boulder clay, and spread along the river valleys as gravel, brickearth and alluvium.
The Middle Chalk, which is exposed at Great Chesterford and Hadstock, includes as its base the Melbourn Rock. Between Heydon and Chishall the chalk is locally disturbed. Glacial action has been potent, as manifest from the deeply excavated trough near Newport.
The Upper Chalk may be seen at Farnham and Clavering in the Stort valley, at Quendon, Newport, Audley End and Saffron Walden in the Cam valley. It consists of soft chalk with layers of flints. There is no doubt that the chalk has been used from time immemorial for chalking the land, and more extensively in old days.
The chalk itself has little direct influence on the agriculture, but open down-like tracts occur near Saffron Walden, and the soil generally on the uplands bordering the Cam valley and extending to Heydon is dry, chalky and gravelly. Along the Cam valley below Newport many springs issue. In very wet weather the valley is liable to be flooded from the drainage off the clay uplands.
The Woolwich and Reading Beds comprise mottled clay, laminated clay and sand, also shelly clays and lignite, and usually at the base greenish-grey sand with flint pebbles. The beds occur along the northern outcrop near Farnham and Stansted Mountfitchet. Near Elsenham and Debden they probably occur further north. They are seen again north of Thaxted.
The London Clay is found more in the south of Essex, but it occurs in a few places at Thaxted, Dunmow and along the Essex borders.
Glacial drift and especially boulder clay occupy a large portion of the surface of Essex and form some of the more fertile agricultural areas. The boulder clay is a tough unstratified clayey deposit of irregular thickness, containing numerous rounded fragments of chalk of all sizes, hence it is known as chalky boulder clay. It contains also numerous fragments of flints, pebbles, fossils and rocks. The soil is a strong loam which forms good land for wheat, barley and beans. A deep gorge filled with drift occurs along the Cam valley, partly in Essex and partly in Cambridgeshire. There is great thickness at Newport (340 feet), Wenden (272 feet), Littlebury (214 feet) and Great Chesterford (156 feet). Boulder clay occurs in the neighbourhood of Chrishall, Debden and Broxted. It forms a continuous sheet at Hatfield Forest, Hatfield Broad Oak, High Easter and the Rodings. Eastwards it is broken up by valleys which divide the high grounds, for instance near Thaxted and Dunmow.
In the ancient hollow along the Cam valley the boulder clay was much mixed with sand and gravel, for example at Elsenham. Minor patches of loam occur on the boulder clay at Hatfield Heath. In former days the boulder clay was much used for marling the ground, hence numerous old pits, now for the most part ponds, may be noticed in the fields.
Sands and gravels were distributed through glaciation and occur along the Cam valley at Quendon, along the Stort valley at Clavering, along the Pant from Radwinter and elsewhere. There are deposits of river gravel along the Cam at Wenden and Great Chesterford.
whole aspect of Essex appears geologically to be
one of somewhat sluggish repose despite the constant waste of the
rain and rivers and sea.
Extracted from the geology chapter by H.B. Woodward in The Victoria History of the County of Essex, vol 1, pp 3-23 (1903).