Page 26 - Fundamentals of Geomorphology

WHAT IS GEOMORPHOLOGY? 9

a
b
() ()
()
g

d
() () Roman
c
dams

() ()
e
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 breached or found a way around the dams and cut
increased aggradation (times when deposition of sed- into the Roman alluvium. Rivers built up the third
iment outstripped erosion). Figure 1.4 is a schematic deposit, which contained Roman and earlier material
reconstruction of the geomorphic history of a valley in as well as pottery and charcoal placing in the Medieval
Tripolitania (western Libya). The key to unlocking the Period (AD 1200–1500), within the down-cut wadis.The
history of the valleys in the area was datable archaeo- deposition of this Younger Fill was followed by reduced
logical material in the fluvial deposits. Vita-Finzi found alluviation and down-cutting through the fill.
three main deposits of differing ages.The oldest contains Wider examination of alluvia in Mediterranean val-
Palaeolithic implements and seems to have accumulated leys allowed Vita-Finzi to recognize an Older Fill dating
during the Pleistocene. Rivers cut into it between about fromthePleistoceneandaYoungerFilldatingfromabout
9,000 and 3,000 years ago. The second deposit accu- AD 500–1500. The Older Fill was deposited as a substan-
mulated behind dams built by Romans to store water tial body of colluvium (slope wash) under a ‘periglacial’
and retain sediment. Late in the Empire, floodwaters regime during the last glacial stage. The Younger Fill was