Geoscience Reference
In-Depth Information
face to the southeast, where valleys have eroded parallel to the tilt of the Chalk bed-
rock through a cap of several metres of Clay-with-flints and Upper Chalk bedrock. The
Clay-with-flints is a soil-like deposit resulting from solution and weathering of Chalk.
It forms a capping layer rich in the flints that were present as nodules in the original
Chalk. More generally, this special material has Anglian ice-laid material on top of it
in places, and must therefore have formed at least partly before the ice sheet arrived
some 450,000 years ago. The solution and weathering probably took place under the
warmer climatic conditions that existed in the millions of years before the Ice Age.
The upper, southeasterly surface of the Chalk Hills has a cover of Anglian ice-
laid material, except southwest of Luton (
c2
), where the hills do not seem to have been
overwhelmed by the ice. Another area lacking ice-laid material is the intriguing north-
westerly face of the Chalk hills, which seems to have had an important influence on the
location of historic travel and trade routes. It seems most likely that ice-laid deposits
did cover this face when the Anglian ice sheet melted, but that they have been stripped
off by river and slope erosion over the subsequent 400,000 years.
One of the greatest environmental changes in the Chalk hills landscape was the
arrival of the Anglian ice sheet. Southwest of the upper Lea Valley (
c3
) and Luton (
c2
),
the lack of ice-laid deposits is evidence that the elevation of the Chalk topography was
sufficient to prevent the further spread of the ice sheet in that direction. Northeast of
this ice margin, ice-laid material was deposited over the whole of the southeasterly sur-
face of the Chalk Hills and influenced the form of the valleys later eroded into it.
The patterns of southeasterly- and southerly-flowing river valleys eroded into the
Chalk are striking, particularly on the slope maps, where the paired valley margins are
picked out, clearly contrasting with the flatter valley floors (Figs 230 and 237). Most
of these Chalk valleys now lack a permanent stream, because rainfall runoff percolates
through fractures in the Chalk instead of flowing over the ground surface. It is believed
that these valleys were eroded under Ice Age conditions, when rivers flowed vigor-
ously for at least part of the year because their water could not percolate through the
near-surface zone of permanently frozen bedrock.
The Hitchin-Stevenage gap (
c4
) is a funnel-shaped hollow in the face of the Chalk
hills, some 10 km wide in the north and narrowing southwards. It contains the A1 trunk
road and the main East Coast railway line from London (King's Cross) to Scotland.
The development of Hitchin, Baldock, Letchworth and Stevenage is further evidence
of the importance of these transport routes to local settlement.
The floor of the gap is underlain by a complex of Anglian ice-laid material with
sands and gravels deposited by rivers. An unusual feature of these surface-blanket sed-
iments is that they have often been deposited as material plugging a system of branch-
ing valleys up to 100 m deep (Fig. 242). Valleys that have been almost completely
