104 PROF. MORRIS GEOLOGY OF THE LONDON BASIN 



constitute the fine and extensive lieaths of Pirbright, Frimley, 

 and Jki^^sliot; these may formerly have extended over the area 

 of Hertfordshire, but have been subsequently washed away by 

 the denuding agencies to which the county has been subjected. 



Between the London Clay period and the next overlying forma- 

 tions in this district, there is a great hiatus — a great geological 

 break— during which extensive and different accumulations of 

 strata took place {i.e. the Middle and Upper Eocene, Miocene, and 

 Pliocene), not only in England, but also on the Continent, such as 

 those which now constitute a considerable portion of the rocks 

 forming some of the higher parts of the Pyrenees, Alps, and 

 Himalaya ; for those mountains, high as they are, have been 

 considerably elevated since the London Clay period. Most of 

 the chief cities of Europe are built either upon the Eocene or 

 Miocene strata. 



In this neighbourhood the next scries of strata are those which 

 I alluded to in the former part of the lecture as belonging to the 

 Pleistocene period, and the agencies concerned in their formation 

 have partly tended to modify the features of the district. In 

 descending order they have been divided as follows: — ^'•' 



Alluvium (recent river deposits). 

 Old m,er Drift,P„st.glacial... { S™^»il£">- 



Plateau gravel (of doubtful age) . 



Surface deposits on the Chalk- I Brick-earth (and pebbly loam). 



tract ( Clay with flints (of doubtful age). 



r,, ■ ^ ts -i, ( Boulder-clay. 



Glacial Drift J j^^^,^^^^ „^^^^;^j^ ^^^ ^^^^^ 



? Tre-glacial Pebble gravel. 



Those members of the Society who have attended the field 

 meetings which have been held in the neighbourhood of "Watford, 

 have been shown that there are certain accumulations of gravel 

 and clay spread over the district, which do not present any 

 regular stratified appearance. These are referred to what geo- 

 logists call the Glacial Period, and they are of dilferent kinds and 

 ages. 



Firstly, there are the gravels which are found at Stanmore Heath. 

 If we examine a section from the neighbourhood of Pushey 

 Grove to Stanmore, we observe a little escarpment of the Clialk, 

 covered, as we ascend, by the "Woolwich beds, and these again by 

 the London Clay, until we reach the gravel of Stanmore Heath, as 

 well shown in Mr. "Wliitakcr's diagram (fig. G). These gravels 

 are the first of the series referable to tliis period. They are appa- 

 rently due to the spreading by the wat(>r at the first commencement 

 of, or previously to, the Glacial Period. Py ]\lr. AVhitaker they 



* Whitakcr, Geology of l.ondon, p. 3. 



