A HISTORY OF HERTFORDSHIRE 



Europe, being a very decided physical as well as pateontological one. 

 In both Cretaceous and Eocene strata the most abundant fossils are 

 Mollusca, but the Protozoa, Rhizopoda, Crustacea, and Polyzoa which 

 abounded in the Cretaceous seas were very sparsely represented in Eocene 

 times ; on the other hand there were but few Cretaceous plants and no 

 Cretaceous mammals, while plants, and especially Dicotyledons, are fairly 

 well represented in Eocene strata, and the Tertiary era has been termed 

 the Age of Mammals. The break however is not so much in the classes 

 of plants or animals represented as in the fact that not a single species 

 passes from Secondary to Tertiary rocks, indicating an enormous lapse of 

 time, with perhaps a complete change of conditions. Physically the 

 difference in the strata consists in the fact that hard, distinctly-bedded 

 rocks, and especially those of a calcareous nature, cease with but few 

 exceptions at the close of the Secondary period, giving place in the 

 Tertiary to clays, sands, and gravels. 



This great break is very well marked in Hertfordshire, for we have 

 neither the highest beds of the Chalk nor the lowest of the Eocenes. 

 There are higher beds of the Chalk, though not the highest known, and 

 lower Eocene beds, south of London than we have here. As the 

 Cretaceous sea must have been continuous north and south of London, 

 the inference is that our Chalk must have suffered a greater amount of 

 denudation than that of Surrey, Sussex, Hants, and Kent. That much 

 erosive action has taken place is proved by the great irregularity of the 

 surface of the Chalk in this county and by the enormous quantity of 

 flints and therefore great thickness of strata which must have been re- 

 moved to form the sands and pebble-beds of the Reading Series. The 

 Woolwich and Reading Beds are of two types : in the one, best represented 

 in the Woolwich district, loamy beds with many fossils prevail ; in the 

 other, or Reading type, the beds are more pebbly and sandy, with but 

 few fossils, and it is a significant fact that a greater denudation of the 

 Chalk has taken place where the Reading type is present, as in Hertford- 

 shire, than where the beds are of the Woolwich type, as in Kent. In 

 our area the term ' Woolwich ' is dropped because we have no beds of 

 that type. 



Although such a long interval elapsed of which we have no record, 

 it does not appear that any earth-movements except subsidence then 

 took place within our area, the eroded surface of the Chalk, although 

 uneven, having been approximately horizontal when the earliest Eocene 

 beds were deposited upon it. When the Chalk is covered by a bed of 

 sand through which water can percolate, there is on its surface a layer of 

 unworn green-coated flints usually considered to form the base of the 

 Thanet Sands, but it should rather be regarded as a reconstructed 

 Cretaceous bed, for the formation is not due to the deposition of sedi- 

 ment, the layer of flints being merely the insoluble residue of the Chalk, 

 and its formation being a process probably continuous from or even 

 before the upheaval of the Chalk to the present time. Why this layer 

 seems to form the base of the Thanet Sands is due to the nature rather 



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