OF HERTFORDSHIRE. 
141 
flowing stream, the gravels deposited by which can still be traced 
across the northern part of Central Essex and the south of Suffolk. 
The Gravels containing Triassic Debris occurring at 
INTERMEDIATE HEIGHTS. 
Referring once more to the “Drift Series” connected with the 
Goring Gap, it may be noticed that the middle part of that series 
consists of gravel patches made up chiefly of flint, hut containing 
also a fair proportion of quartzite pebbles, etc., from the Bunter 
pebble-beds, materials derived from the older gravels, and pieces 
of an igneous rock of a rhyolitic character. This is of a greenish 
colour with numerous felspar phenocrysts, and often shows 
spherulitic and flow-structure well. Rarely also pieces of a greenish 
ash occur. Jurassic material, basalt, and granite are absent. 
These gravels may be traced in a direction roughly parallel to, 
and occupying heights lower than the previous set of deposits, 
across Bucks, at the top of the hill leading from Loudwater to 
Beaconsfield, at Gerrard’s Cross, and at Gravel Hill north of 
Chalfont St. Peter, where sections may be seen; and in Herts, on 
the hill west of Chorley Wood Station and in a road-section east 
of the same, at Chenies, at Bedmond, where there is a section 
20 ft. deep to the east of the village near the pond, north and 
south of St. Albans, on Woodcock Hill near Riekmansworth, 
and at Kemp Row west of Radlett. There are also some good 
sections at Twyford south of Bishop’s Stortford, and near Hockerill, 
containing Triassic debris and supra-Cretaceous material only. These 
are opposite the Bishop’s Stortford Gap, but it is quite possible 
that some of the material was derived from the west. They also 
occur in the same relative position across Essex, as at Hatfield 
Heath, Hunsdon, and Stanstead Bury. 
The line of drainage through the Stevenage Gap was still in 
existence, but there does not appear to be any evidence of Con¬ 
nection with the Wealden area. Sections may be seen in Hotter s 
pits just north of Datchworth Church, in Capt. Blake’s and in 
Smart’s pits at Woolmer Green, near Codicote ; and the pipes 
in the Chalk seen in the railway-cutting north of Knebworth 
contain Bunter quartzites. 
These gravels are much more extensive and thicker than those 
described in the previous section ; they are usually well stratified, 
and often show much current-bedded sand. 
The presence of large quantities of partially-rounded flint shows 
that much denudation of the Chalk was going on owing to the 
erosion of the protecting covering of Lower Tertiary beds. 
The Northern Drift Clays and the Gravels and Boulders 
ASSOCIATED WITH THEM. 
It is not proposed to treat of these deposits fully on the present 
occasion, but only in so far as they affect the other deposits herein 
described. 
