Geology Reference
In-Depth Information
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a
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b
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g
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c
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d
Roman
dams
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e
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f
Younger fill
Older fill
Bedrock (predominantly limestone)
Calcareous crust
Figure 1.4
A reconstruction of the geomorphic history of a wadi in Tripolitania. (a) Original valley. (b) Deposition of
Older Fill. (c) River cut into Older Fill. (d) Roman dams impound silt. (e) Rivers cut further into Older Fill and Roman
alluvium. (f ) Deposition of Younger Fill. (g) Present valley and its alluvial deposits.
Source:
After Vita-Finzi (1969, 10)
and colluvium was laid down during two episodes of
increased aggradation (times when deposition of sed-
iment outstripped erosion). Figure 1.4 is a schematic
reconstruction of the geomorphic history of a valley in
Tripolitania (western Libya). The key to unlocking the
history of the valleys in the area was datable archaeo-
logical material in the fluvial deposits. Vita-Finzi found
three main deposits of differing ages. The oldest contains
Palaeolithic implements and seems to have accumulated
during the Pleistocene. Rivers cut into it between about
9,000 and 3,000 years ago. The second deposit accu-
mulated behind dams built by Romans to store water
and retain sediment. Late in the Empire, floodwaters
breached or found a way around the dams and cut
into the Roman alluvium. Rivers built up the third
deposit, which contained Roman and earlier material
as well as pottery and charcoal placing in the Medieval
Period (
AD
1200-1500), within the down-cut wadis. The
deposition of this Younger Fill was followed by reduced
alluviation and down-cutting through the fill.
Wider examination of alluvia in Mediterranean val-
leys allowed Vita-Finzi to recognize an Older Fill dating
from the Pleistocene and a Younger Fill dating from about
AD
500-1500. The Older Fill was deposited as a substan-
tial body of colluvium (slope wash) under a 'periglacial'
regime during the last glacial stage. The Younger Fill was
