Or HERTFORDSHIRE. 
139 
The “Drift Series” at Goring Gap. 
Although Goring Gap is some distance from Hertfordshire, it 
plays an important part in the present inquiry. As before stated, 
it is the only means, at present, of fluviatile connection between 
the country north of the escarpment and the Lower Thames basin. 
As a consequence we get in its vicinity numerous superficial 
deposits forming a more or less complete “Drift Series.” The 
absence, also, of boulder-clay renders their interpretation much 
simpler than that of deposits further east. Briefly the deposits 
may be divided into three classes: 
1. A high-level drift consisting mainly of flint-pebbles and other 
debris of the Lower Tertiary strata, with which is mixed 
a variable quantity of non-local material, such as quartz- 
pebbles and other materials derived from a Palaeozoic source. 
2. The drift with Bunter debris which comes in below that just 
described, and reaches down to 
3. The three terraces of modern river-drift, containing Palaeo¬ 
lithic implements, remains of Pleistocene Mammalia, etc. 
It must not be supposed, however, that there is any sharp line 
of demarcation between these, but deposits of an intermediate kind 
are found, and slipping may occur. In fact, they shade off one 
into the other in just such a manner as is required by a fluviatile 
hypothesis of their origin. 
The upper members of the Goring Gap “Drift Series” can be 
traced across South Bucks, Herts, and East Anglia, while the lower 
members follow the direction of the present valley. 
The High-level Superficial Deposits. 
On the higher parts of the Chalk slope the superficial deposits 
are of subaerial origin, and consist of the well-known Clay-with - 
flints and debris from the Lower Tertiary beds, often including 
large sarsen-stones and blocks of Hertfordshire Conglomerate, 
which at one time occupied larger areas' than they do now, and 
protected the soluble Chalk strata beneath them. Pits may be 
seen at Caddington (Beds), Hyde Heath west of Chesliam (Bucks),* 
between Buntingford and Hare Street (East Herts), and in the 
cuttings of the Metropolitan Bailway at Chalfont Boad Station and 
further north. 
The Tertiary outliers which remain are usually capped by 
gravel similar to that found at high levels near Goring Gap. This 
gravel has protected and preserved the strata below them, while 
much larger tracts, not so protected, have succumbed to the 
action of denuding agents. 
These deposits are often disturbed owing to the piping of the 
chalk below and to rootlets and burrowing animals descending 
* Upfield Green and H. B. Woodward, “Excursion to Wendover,” ‘Proc. 
Geol. Assoc.,’ vol. xii, p. 340. 
