Geoscience Reference
In-Depth Information
4 Ma eruption of cindery tuff
(? from Ida Ale)
Block-faulted Afar
stratoid series lavas
Fanglomerates
Swamp
clays
Ethiopian
escarpment
WEST
EAST
Alternating dark swamp clays (regressions) and
white lacustrine diatomites (transgressions)
Figure 11.2. Early Pliocene lake deposition in the Middle Awash Valley, southern
Afar Rift. (After Williams et al.,
1986
.)
source, its presence tells us more about conditions in the catchment headwaters than
conditions at the site of the lake itself, as hinted at in the quotation from Robert
Frost's poem at the start of this chapter. A good example of this is provided by the
late Pleistocene Willandra Lakes in semi-arid western New SouthWales (Figure11.3),
which were fed by run-off from the Eastern Highlands of Australia via a distributary
channel of the Lachlan River (Bowler,
1998
; Bowler and Price,
1998
). Whether the
demise of these lakes was linked to a reduction in run-off from the headwaters of the
channel flowing into and through the lakes, or whether run-off was abruptly curtailed
as a result of channel avulsion or river capture is still unclear (Williams et al.,
1991
b;
Bowler et al.,
2011
).
The most certain way to establish previous lake levels is to map the former lake
shorelines (Grove and Pullan,
1963
;Servant,
1973
; Cooke and Verstappen,
1984
;
Magee,
1998
; Magee and Miller,
1998
; Drake and Bristow,
2006
; Burrough et al.,
2009a
; Burrough et al.,
2009b
; Barrows et al.,
2014
), assuming that they have not
been distorted by isostatic (Gilbert,
1890
) or other tectonic effects (Flint,
1959a
; Flint,
1959b
; Burrough et al.,
2009b
). For a lake to remain stable at any given level, water
losses from evaporation, seepage and run-off from the lake must balance water inputs
from run-off and precipitation directly onto the lake surface. Thus:
A
c
P
c
k
+
A
w
P
w
=
A
w
E
(11.1)
