138 
A. E. SALTER-THE GRAVELS 
which have been advanced, and may even he able in some districts 
to show that they are superfluous and unnecessary, or that others 
of a totally different character are more feasible. 
If a contoured map, or, better still, a large raised model of the 
country on the north side of the basin of the Lower Thames, he 
examined, this district will he found to be made up of the following 
parts:— 
1. A long dip-slope down which many streams have formed 
valleys. One valley only,* that of the Thames, extends 
beyond this dip-slope, hut the presence of beheaded valleys 
at or near the sources of some of the smaller streams points 
in such cases to a former extension northwards. 
2. A steep escarpment to the north. This varies in height from 
a little over 800 ft. O.D. in the west to 519 ft, O.D. at Reed 
in the N.E. of Hertfordshire. It is broken at Goring Gap, 
through which the Thames flows at about 190 ft. O.D. ; at 
the Stevenage Gap, the lowest point of which is just above 
300 ft. O.D. ; and further east by the Bishop’s Stortford Gap, 
the lowest point of which at Elsenham is about 300 ft. O.D. 
Neither of the two last-mentioned gaps has fluviatile con¬ 
nection now with the Midlands. The gravels, however, 
which are found associated with them show that such 
a connection formerly existed. 
3. A large extent of comparatively low ground north of the 
escarpment across which a few short anti-dip (obsequent) 
streams flow northward into the larger longitudinal streams 
which flow west into the Thames and east into the Ouse. 
4. A region of higher ground lying south of the main slope. 
This is now much cut up by denudation. It rises to just 
over 500 ft. O.D. on Stanmore Common. The highest points 
are capped by gravel, to which circumstance their preserva¬ 
tion is due.j* Whether this feature may be partly due to 
earth-movement is a question needing investigation. It is 
significant, however, that in the road leading to Mimmshall 
Wood, N.E. of South Mimms, the Woolwich and Heading 
Beds are to he seen resting on the Chalk just below 
400ft. O.D./whereas they are found at much lower heights 
a little to the N.E. This may point to a fault or a gentle 
fold in that neighbourhood. 
5. A belt of lower ground lying between 1 and 4 partly occupied 
by the Colne and Lea Yalleys but continued eastward into 
Essex. 
All these features are well developed in Hertfordshire, and have 
an important hearing upon the interpretation of the mode of origin 
of its superficial deposits. 
* It may be noted that the Lea rises just beyond the Chalk escarpment on 
Lower Cretaceous strata. 
t A. E. Salter, “Excursion to Stanmore,” ‘ Proc. Geol. Assoc.,’ vol. xvii, 
p. 175. C. fleid, in ‘ Record of Progress of the Geological Survey,’ 1899, p. 140. 
