33 § 
Superficial Gravels and Clays, 
[July, 
III. Mode of Formation of the Superficial Beds. 
From the time that the deposits at Finchley were described 
by Mr. Spencer, in 1835, they have been recognised as of 
Glacial age. On my first visit to Mr. Plowman’s brick-field 
with Mr. Ives, we found pebbles of hard red and white chalk 
and granite, and specimens of Gryphea incurva in the Upper 
Boulder-clay. Various Liassic fossils, and fragments of 
granite, porphyry, micaceous sandstone, mountain limestone, 
coal, and oolite have been recorded from it. Mr. Searles 
Wood, jun., has recognised its identity with the Upper 
Boulder-clay of the eastern counties. He states that, even 
in South and Central Lincolnshire, and to the north of 
Gainsborough, “ the material of the deposit is so identical 
with that on the brow of the Thames Valley that a basket 
of clay taken from either extremity of this area could not be 
distinguished, although these extremities are 140 miles 
apart.” * 
With regard to the origin of the Upper Boulder-clay there 
have been various theories propounded, but the majority of 
geologists seem to have come to the conclusion that it was 
spread out when the country was submerged and floating ice 
brought the stones contained in it from the north. Mr. James 
Geikie has indeed advanced the theory that the boulder-clay 
is the product of land-ice, and he would dispense with the 
action of floating ice altogether.! Even if some general 
arguments that have been urged against the theory of a 
mantle of clay having been spread over a country by the 
aCtion of land-ice could be met, there are special objections 
to its application to the Finchley district. There, the boulder- 
clay caps the ridges between the valleys, and descends their 
slopes, lying like a cloak over loose beds of sand and gravel. 
Not a trace of the passage of Glacial ice is to be seen in 
these deposits, although the slopes are sufficiently great to 
have caused considerable movement of the ice down them 
if it had existed in the form of a glacier. The clay is 
thickest on the crests of the ridges, and contains there the 
most stones. Many of these stones have been transported 
from Yorkshire and Lincolnshire, and the ice that brought 
them, if it was land-ice, must have been sufficiently thick to 
cover all the low lands of Suffolk, Norfolk, and Essex, and 
pour over into the valley of the Thames through passes in 
* Quart. Journ. Geol. Soc., vol. xxiii., p. 395. 
t See Geol. Mag., 1878, page 73, for Mr. Geikie’s latest views on this 
question. 
