Geoscience Reference
In-Depth Information
the gently tilted, 250 m thick Chalk layer. The orientation of the Chalk hills is the res-
ult of their tilt to the southeast, and their present location is due to the level presently
reached by the processes of landscape erosion.
Even though the tilt is so gentle, it still gives rise to a contrast between the north-
western flank of the Chalk hills, where erosion has cut through successive levels within
the Chalk, and the southeastern flank, where well-defined valleys have been eroded a
few tens of metres into the tilted upper surface of the Chalk (Fig. 230).
The greatest elevations in the Chalk hills of Area 13 are in the southwest, near to
Luton and Whipsnade, where elevations of 250 m occur. In this area the Chalk edge
is unusually distinct, making it a very popular launch site for paragliders keen to ex-
plore the lower ground to the north. Elevations tend to decrease southeastwards, fol-
lowing the tilt of the upper surface of the Chalk layer. Further northeast in Area 13,
Chalk hill elevations rarely exceed 100 m and the Chalk edge facing northwestwards is
much less distinct. In this area, the Rhee branch of the Cam is joined by the Cam and
Granta streams just upstream from Cambridge. These drain the Saffron Walden and
Linton valleys in the Chalk Hills, where the Chalk edge is particularly far to the south-
east. These lower elevations may reflect erosion by Anglian ice in this eastern area,
because it appears that the higher edge in the Luton area (
c2
) was never surmounted by
the Anglian ice sheets.
The lowest part of the Chalk is often called the Chalk Marl, because it is richer in
clay than more normal Chalk, which is almost entirely calcite. During the erosion of
the landscape the Chalk Marl has tended to behave in a similar way to the underlying
Gault mudstones: both usually form rather low ground, in contrast to the rest of the
Chalk, and may have been subject to widespread periglacial mobilisation of near-sur-
face material, producing low, rounded hills and hollows that are typical at this level.
Within the main Chalk layer there are a number of hard bands (Fig. 225), each a
few metres thick, which have often resisted general landscape erosion to produce dis-
tinct escarpments in the topography (Figs 237 and 240). The hard bands tend to vary
from place to place, and they rarely all produce slope features in the same area, but the
following number system is used to refer to the most distinctive: (
1
) Tottenhoe or Bur-
well Rock; (
2
) Melbourn Rock; (
3
) Middle Chalk; (
4
) Base of Upper Chalk. The hard-
ness of the bands appears to reflect changes in the environment of the original Chalk
seas, but it may also reflect differences in the precipitation of calcite within small pores
in the Chalk after its burial. Spring lines are also features of the landscape caused by
these hard bands, for example at Ashwell, where the Cam (Rhee) rises.
In the southeastern corner of sub-area II (Fig. 240) the slope map shows two dif-
ferent slope features forming the northwest face of the Chalk hills. The Chalk hard
bands (1, 2 and 4) have produced the steeper slopes as well as the gently sloping sur-
