GEOLOGY OV HERTFOUDSHIRE. 151 



By the courtesy of the authorities at the Geological Survey Office 

 I have been able to examine M8. maps of the superficial geology of 

 those portions of our county which have not yet been published ; 

 and I have endeavoured to show the main features of these super- 

 ficial beds upon the map forming Plate II. It will be seen that 

 while on the east side of the county the valleys are cut through 

 boulder-clay exposing the Mid-glacial sands and gravels beneath, 

 on the west side the valleys are cut chiefly through the clay-with- 

 flints. In the southern portion of the county we find the Colne 

 valley occupied chiefly by sands and gravels of glacial age, while 

 the London-Clay area is comparatively free from drift deposits 

 except a few isolated patches of pebble- gravel, and a little boulder- 

 clay in the east. In discussing the nature and origin of the soils 

 of our county, it will be convenient to consider in turn the soils 

 overlying each geological formation. 



The Eocene District. — As will be seen from the geological map, 

 the London Clay stretches across the south-eastern borders of the 

 county approximately south of a line drawn from Bishop's Stortford, 

 through ^V'are, Hatfield, St. Albans, and Eickmansworth; but it is 

 only in the extreme southern portion of the county that the soil is 

 found to be of that stiff, tenacious character, which indicates a clay 

 formation. This stiff clay country will be found inclosed within 

 a line drawn through Shenley, Letchmore Heath, and Bushey, to 

 Moor Park. Another strip of heavy yellow clay extends along the 

 flanks of the Lea valley from Bayford and Hoddesdon in the north 

 to Goff's Oak and Northaw in the south. The remainder of the 

 London-Clay district is covered up by su.perficial deposits which 

 completely alter the character of the soil. Of these soils, the most 

 noticeable are the sandy loams formed by the alluvium of the Lea 

 valley, and the loamy clay in the valley of the Stort, derived from 

 the Essex boulder-clay which spreads over the eastern boundaries 

 of our county. The heavy soils in this district are often improved 

 by admixture with the Mid-glacial sands and gravels which crop 

 out in the lower parts of the valleys. 



The Woolwich and Eeading beds, although they have a very 

 thin outcrop, yet contribute materially to the improvement of the 

 heavy soils overlying the London Clay. Probably, also, it is 

 from the breaking up of these beds that much of the drift, 

 which covers the chalk, was originally derived. The lower beds 

 of this series are, in many cases, of pure sand and rolled pebbles, 

 from which the hard conglomerate, known as the Hertfordshire 

 plum-pudding-stone, was derived. The " blue-pebble-and-clay " 

 soil of the district extending from Hatfield to JS'orthaw may 

 possibly be due to the disintegration of some of these lower 

 portions of the Woolwich and Reading series. 



The Chalk District. — The Chalk formation covers very nearly the 

 whole of the remainder of the county, except a few patches of Upper 

 Greensand and Gault on the borders of Buckinghamshire, Bedford- 

 shire, and Cambridgeshire. But, with the exception of a narrow 

 strip of land extending through Lilley, Hitchin, and Baldock, to 



