and theie extension in hertfordshire. 169 



The Eocenes of Hertfordshire. 



The extension of the Eocenes in this county will be at once 

 learnt from an inspection of the Geological Survey map of the 

 district, which, not showing the superficial Post-Pliocene deposits, 

 indicates conspicuously the entire extent of the Lower Tertiaries. 

 Disregarding the Post-Pliocene superficial deposits, the County of 

 Hertford is occupied chiefly by the Chalk, and it is only in the 

 southern part of the county that this formation is overlain by 

 Tertiary beds. These arc a portion of the northern margin of the 

 London Tertiary Basin, the boundary of which in Hertfordshire 

 extends from near Rickmanswortli in the west, by Bushey, Shenley, 

 and Hatfield, to the neighbourhood of Bishop's Stortford in the east, 

 thus forming a line trending in a north-easterly direction, and 

 giving the Eocenes to the south and south-east parts of the county. 

 Beyond, however, the line I have described, there are in several 

 places to the north patches of Tertiary deposits lying on the Chalk 

 quite detached fi'om each other and from the main mass. These 

 occur at the following places : Micklefield Hall, Micklefield Green, 

 Sarratt, Abbot's Langley, Bedmont, Bonnet's End, Leverstock Green, 

 and at the St. Peter's side of St. Albans. These outliers clearly 

 prove the former extension of the Eocenes to a considerable distance 

 beyond the present boundary, since they are undoubtedly fragments 

 of once continuous beds, which have more or less escaped de- 

 nudation. To the south of the boundary-line the Tertiaries are 

 continuous, with the exception of a small area near Northaw, where 

 an inlier of Chalk is uncovered by Tertiary beds. Roughly parallel 

 with the edge or escarpment of the Tertiaries, and at a short dis- 

 tance from it, the river Colne runs from Colney-street to Eickmans- 

 worth. 



In Hertfordshire we have only the "Woolwich Beds and the 

 London Clay — the Thanet Sands, the Middle, and the Upper Eocenes, 

 being absent. The lowest foiTaation, therefore — the Thanet Sands — 

 so conspicuous in Kent, must not be looked for here, and although 

 immediately outside the boundary of the county, at Harrow, a 

 patch of the Bagshot Sands occurs on the summit of the hill, these 

 and all the succeeding Eocenes are absent within the boundary. 



The Woolwich and Reading Beds are exposed, as might be ex- 

 pected, along the edge of the Tertiary area, where they are seen 

 at many points covering over the Chalk. At Watford Heath, 

 at Bushey, in the neighbourhood of Hertford, and on the eastern 

 edge of Hatfield Park, good sections are exposed, and at these places 

 the valuable character of the Woolwich Beds, even within short 

 distances, may be observed. One section shows clays, another sands, 

 and another mingled clays and sands, indicating the changing con- 

 ditions of deposition which marked the Woolwich and Reading 

 period. In this county the Woolwich Beds are not of that fossil- 

 iferous character which distinguishes them in the Kentish area ; but 

 still a diligent search may be rewarded by the discovery of a few 

 species. Most of the sections show at their summits a portion of 

 the Basement Bed of the London Clay, with its characteristic 



