Geoscience Reference
In-Depth Information
Figure 14.1. Flowstone overlying cave breccia with Middle Stone Age fossils, Porc
Epic Cave, Dire Dawa, Ethiopia.
evaporation as a thin, surficial, soft, spongy, cellular or porous, semifriable incrustation
around the mouth of a hot or cold calcareous spring or seep, or along a stream carrying
calcium carbonate in solution, and exceptionally as a thick, bulbous, concretionary
or compact deposit in a lake or along its shore'. They also note that 'it may also
be precipitated by algae or bacteria' and comment that 'the hard, dense variety is
travertine'. Given that there is no clear genetic distinction between travertine and tufa,
we will use the general term tufa to include both, but will consider speleothems as
distinct entities.
The actual presence of well-developed cave systems in areas that are now arid
indicates that conditions were once wetter when they formed, provided that they had
originated above the level of the regional groundwater-table. Caves formed below the
watertable and subsequently exposed as a result of tectonic uplift do not necessarily
connote wetter conditions at the surface. Because it is not always possible to ascertain
how a cave originated, the depositional contents of caves have been the focus of more
recent paleoclimatic enquiry and not the caves themselves, although the processes
responsible for cave formation remain a major theme of geomorphic research (Ford
and Williams,
1989
; Ford,
2006
; Harmon and Wicks,
2006
).
The aim of this chapter is to show how the presence of speleothems and tufas in
presently arid and semi-arid areas can provide reliably dated quantitative information
