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of Anglian ice-laid deposits has filled earlier valleys, and new patterns of slopes have
been eroded, completely changing the shape of the earlier scenery. Parts of the plat-
eau margin display incised valleys that have been cut into the ice-laid deposits since
the Anglian. They have been further eroded when their slopes collapsed due to freeze-
thaw mobilisation during subsequent cold spells. Iron-rich concretions in the Lower
Greensand of Cambridge's Western Plateau have given rise to a surprisingly long his-
tory of local iron smelting.
The Greensand ridge behind Sandy ( b9 ) overlooks the River Ivel valley, and the
typical rusty-weathering of the sandstones is visible in a number of quarries. The ridge
extends more widely across Area 13, forming the northwestern edge of the Early Creta-
ceous, though it varies from under a few metres in thickness in the northeast of the Area
to a much more important 70 m in thickness in the southwest, near Leighton Buzzard
(Fig. 240, b8 ). In this thicker development in the southwest, the Lower Greensand has
been quarried extensively as a source of sand, which has been most prized where the
iron minerals that are often present have been removed by solution in the groundwater
to yield a pure quartz sand. In some localities the quarries reveal stratification patterns
typical of sandbars that ranged up to several metres in height, migrating under the ac-
tion of the Early Cretaceous tides (Fig. 238). The variation in thickness of the Lower
Greensand is probably due to episodes of non-deposition and very gentle movement
of the Earth's surface, before and after deposition on the floor of the Early Cretaceous
sea.
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