Page:VCH Essex 1.djvu/35

 GEOLOGY ESSEX occupies a large part of the irregular tract which is known to geologists as the London Basin, and which has for its frame- work the great Chalk formation. The southern rim of this Basin rises in the North Downs of Kent, whence the Chalk slopes gradually to the borders of the Thames at Erith and Gravesend, comes to the surface in south Essex at Purfleet and Grays, and then sinks below ground over the greater part of the county. The northern rim of the Basin appears in the downs of Dunstable, Luton and Royston, and in the uplands of Heydon and Saffron Walden in Essex. The contour of the Chalk Basin is to some extent modified by slight undulations and fractures along the Kentish side of the Thames between Greenwich and Erith, so that while the curvature is for the most part gentle, the inclination becomes somewhat steeper on the southern side of Essex. Hence at Dagenham, Rettenden and Southend the Chalk is about 400 feet below ground, at Chelmsford it is about 360 feet, and thence northward it more gradually approaches the surface. Near Hey- don however there are some striking evidences of local disturbance, to which further reference will be made. Resting in the hollow of the Basin, and conforming generally with its curvature, is a series of Eocene strata, the most important of which is the London Clay. The succeeding deposits include small outlying tracts of Crag, but they consist chiefly of the irregular and variable accumu- lations grouped as ' drift.' These lie scattered over the uplands in the form of gravel and Boulder Clay, and are spread along the river valleys in the form of gravel, brickearth and alluvium. The structure of the ground is thus comparatively simple, but there are difficulties in the determination of the relative ages of some of the scattered superficial deposits. There are grave difficulties also when it is sought to decipher the underground rocky structure beneath the Chalk and other Cretaceous strata. The problem is of the highest interest and practical importance, but it raises questions which deep borings alone can solve. Although the county is essentially a flat one, and especially along the river and sea coasts which are bordered by marshes, yet inland there is an abundant variety of soil, and these regions are very pleasantly diver- sified with arable land and pasture, with woodlands, commons and well- timbered hedgerows. Indeed to quote an old agricultural work, ' Every species of soil, from the most stubborn to the mildest loam, is to be found.' 1 1 General firm of A 'grit. ofEtiex, by Messrs. Griggs, 410 (Lond. 1794), p. 7. I I '