Geoscience Reference
In-Depth Information
ers of the Chalk had been dissolved by chemical erosion. Clay-with-flints gives rich-
er, more acidic soils than the surrounding chalk grassland and is capable of support-
ing large tracts of woodland. An example is Savernake Forest (
c3
), to the southeast of
Marlborough, which is believed to be very ancient.
Another common feature of these Downs are the scattered sarsen stones or
grey-
weathers
(Fig. 195). It has been suggested that the sarsen blocks are relicts of local
patches of Early Tertiary sandstone which developed a hard silica cement and so res-
isted erosion while the surrounding, less-cemented and softer sandstones were worn
away. These isolated blocks of weathered Tertiary sandstone are often found incorpor-
ated into ancient burial mounds or into the foundations of the area's oldest houses. Gla-
cial
solifluction
processes have transported these blocks downhill, and they are often
seen to be concentrated in lines at the bottoms of dry valleys. The sarsens are thought
to have come from the Tertiary Reading Formation, a greyweathering sandstone which
outcrops as bedrock in a narrow band in the east of Area 10. The wide distribution of
sarsen stones across the Chalk Downs suggests that Tertiary sediments used to be much
more widespread than they are today across Area 10, but have since been eroded away.
FIG
195.
Sarsen stones on Fyfield Down (Fig. 190,
c5
). (Copyright Landform Slides -
Ken Gardner)
Area 10 has a long and archeologically important history of human settlement,
often strongly influenced by the bedrock geology. The Marlborough Downs region in-
cludes many of the most spectacular ancient sites (such as Avebury,
c4
) linked together
by the Ridgeway, an ancient travellers' route from Dorset to the Wash running along
the top of the northern Chalk escarpment. This route has been in more or less constant
use since at least Neolithic times, around 5,000 years ago, and was presumably chosen
because the permeable and well-drained Chalk offered a more direct and reliable all-
weather route than the neighbouring clay vales, which were probably densely wooded
and marshy. Today the Ridgeway is maintained as a popular long-distance walking
