344 
Superficial Gravels and Clays. [J uly, 
the sea, and compare it with that above it, we discover no 
alteration in the slopes, which blend the one into the other 
and are continuous, so that it is impossible to distinguish 
that horizon amongst the contours of the country. We 
might have expected that there would be some distinctly 
marked line, when, after the boulder-clay had been spread 
out, the river recommenced the excavation of the valley ; 
but there is none. None, I mean, in Nature, though in the 
fanciful diagrams shown in most of our geological manuals 
the boulder-clay is represented capping the hills and pre- 
senting an escarpment to the valleys. If these sections 
were true ones, or even if there were one such in Nature, 
my opposition to Prof. Prestwich’s theory would be greatly 
weakened, but I can find none, excepting in our books. In 
every valley I have examined, including that of the Oase, 
at Bedford, — often appealed to in support of Prof. Prest- 
wich’s views, — the boulder-clay is not confined to the tops 
of the hills, but descends the slopes as it does at Finchley. 
Prof. Ramsay and others have taught us that rivers exca- 
vate their channels from the sea backwards, somewhat in 
the same manner as the recession of the Falls of Niagara 
is effected. This is proved to be the case when, in new 
countries, the forest is cut down and valleys begin to be 
formed : they commence next the rivers and progress back- 
wards, as Sir Charles Lyell witnessed in Georgia.* I have 
seen similar instances in Nicaragua and Southern Missouri. 
But on Prof. Prestwich’s theory the upper parts of the 
valleys must have been excavated first and the lower after- 
wards — the tributaries of the river before the river itself ; 
for it is evident that the valley of the Brent, at Finchley, 
was formed at the time the Upper Boulder-clay and the 
Middle Sands and Gravels were spread out. 
From the bottom of the valley near Finchley, where the 
road to Hendon crosses the brook, to the line of my sedtion 
shown in Fig. 2 (page 317), where it crosses the same stream 
at Hanwell, the distance down the valley — without following 
the minor sinuosities of the brook — is 9 miles. The boulder- 
clay comes down between Finchley and Hendon to 150 feet 
above the Ordnance datum-line ; the valley gravels and clay 
at Hanwell rise to about 120 feet above the same line. The 
stream itself falls more than 100 feet in the 9 miles, or about 
11 feet per mile. At the end of the distribution of the Upper 
Boulder-clay, if the lower part of the valley was not exca- 
vated, the stream would only fall 30 feet between the same 
* Principles of Geology, ninth edition, p. 205. 
