AND OF THE NEIGHBOTJEHOOD OF WATFOED. 97 



It is these beds on the northern parts of the Thames that are 

 more interesting in relation to the geology of "Watford ; for they 

 are the fii'st of the Tertiary series here overlying the Chalk. If 

 you examine the sections at the Bushey Kiln, at the "Watford 

 Heath Kiln, at Chorley Wood and Woodcock Hill in the neigh- 

 bourhood of Eickmansworth, at Hatfield Park, and near Hertford 

 (Figs. 1, 2, 3, 4), you will find that there are certain strata imme- 

 diately overlying the Chalk, which are referred to the Woolwich 

 and Reading series. They are almost unfossiliferous, or indi- 

 cate that they were not deposited under fresh-water conditions, 

 for the remains found in them are only a few Ostrem. Hence, 

 therefore, you will see the necessity of carefully working out the 

 characters and contents of deposits in order to learn the conditions 

 under which they were formed.* It is not improbable that while 

 to the southern side of the Thames, rivers existed, as before men- 

 tioned, on the northern side there was more or less open sea in 

 which marine sand or mud accumulated. But these beds have 

 equivalents in the western area of the London Basin, and also in 

 the Hampshire Basin, where the red clays which lie against the 

 Chalk in Alum and White Cliff Bays are the representatives of the 

 beds in the neighbourhood of Watford. In the Paris or Belgian 

 area at this period marine conditions partly prevailed, but around 

 Epemay are found similar fresh- water strata to those in the neigh- 

 bourhood of Woolwich. Hence you see a great physical change 

 must have taken place in the geography of the period during the 

 deposition of the Woolwich series.f 



Perhaps it might be interesting to notice that if we compare 

 the fertility of the Chalk hills of Hertfordshire with that of the 

 Downs south of the Thames, it will be found that a great part of 

 the fertility of the Chalk district is due to the spreading over it of 

 more or less of these Woolwich clays or sands, and hence, there- 

 fore, the difference in the wooded and agricultui-al character of the 



* " In Surrey, west of Croydon, and along the north-western outcrop in 

 Buckinghamshire and Hertfordshire, the other or Eeading tj-pe occiu's exclu- 

 sively. It is unfossiliferous (?), and characterized by the presence of soapy 

 mottled plastic clay, of various and often rich colours, some shade of red gene- 

 rally showing ; but sand is mostly present also, and sometimes with pebbles ; 

 at the northern margin of the district, between Aldenham and Shenley, there is 

 a regular pebble-bed, hardened into stone of just the same kind as the blocks 

 of the well-known ' Hertfordshire pudding-stone,' which are found so commonly 

 over the Chalk-ti-act beyond, and were indeed most likely derived from this bed." 

 — Whitaker, 'Guide to the Geology of London,' p. 31. 



t " The reasons for believing that the temperature of the sea at the ' Thanet 

 sands ' period was lower than that which prevailed during the period of the 

 London Clay, apply in some measnre probably less forcibly to this intermediate 

 epoch of the "Woolwich and Eeading series. The general character both of the 

 fauna and flora shows a preponderance of forms such as, on the whole, we might 

 expect to meet with at present in more moderate climates than the one in which 

 the more tropical-seeming vegetation and animals of the London Clay could have 

 flourished." — Prestwich, 'Quart. Journ. Geol. Soc' vol. x. p. 136. See also Dr. 

 Hooker's Note on the Fossil Plants fi'om Reading (vol. x. p. 163), in which he 

 states that they " represent a vegetation diflering in no important respect from 

 that at present inhabiting the north temperate zone." 



