Geology Reference
In-Depth Information
solution of a basal weathering surface under a thick sheet
of soil or sediment, the surface of which was subject to
wash processes.
Exhumed erosion surfaces
are quite common. The
geological column is packed with unconformities, which
are marked by surfaces dividing older, often folded rocks
from overlying, often flat-lying rocks. Some unconfor-
mities seem to be old plains, either peneplains formed
by coastal erosion during a marine transgression or by
fluvial erosion, or else etchplains formed by the processes
of etchplanation. The overlying rocks can be marine,
commonly a conglomerate laid down during a transgres-
sion, or terrestrial. The unconformity is revealed as an
exhumed erosion surface when the overlying softer rocks
Exhumed landforms
Exhumed landscapes
and
landforms
are common, pre-
served for long periods beneath sediments and then
uncovered by erosion (Box 15.3). They are common on
all continents (e.g. Lidmar-Bergström 1989, 1993, 1995,
1996; Twidale 1994; Thomas 1995).
Box 15.3
BURIED, EXHUMED, AND RELICT KARST
Karst that formed in the geological past and survives to
the present is surprisingly common. Such old karst is
known as
palaeokarst
, although sometimes the term
'
fossil karst
', which is rather ambiguous, is employed
(see Bosák
et al.
1989). Palaeokarst may be divided into
buried karst and intrastratal karst.
Buried karst
is karst formed at the ground sur-
face and then covered by later sediments.
Intrastratal
karst
is karst formed within bedding planes or uncon-
formities of soluble rocks that are already buried
by younger strata. An important distinction between
buried karst and intrastratal karst is that buried karst is
older than the covering rocks, while intrastratal karst
is younger than the covering rocks.
Subjacent karst
is
the most common form of intrastratal karst and devel-
ops in soluble rocks that lie below less soluble or
insoluble strata. No intrastratal karst feature has ever
belonged to a former karst landscape. A complica-
tion here is that, in many places, intrastratal karst
is forming today. Palaeo-intrastratal karst is inactive
or inert. The oldest known buried karst features are
caves and cave deposits in the Transvaal, South Africa,
which formed 2,200 million years ago (Martini 1981).
In Quebec, Canada, Middle Ordovician dolines,
rounded solution runnels, and solution pans have
been discovered, exposed after survival beneath a
blanket of later rocks (Desrochers and James 1988).
Another group of karst features were formed in the
past when the climate and other environmental factors
were different, but survive today, often in a degraded
state, under conditions that are no longer conducive
to their development. Such karst features are called
relict karst
and occur above and below ground. A
cave system abandoned by the groundwater streams
that carved it, owing to a lowering of the groundwater
table, is an example of subterranean relict karst. Some
caves are Tertiary in age, and some relict cave passages
may even survive from the Mesozoic era (Osborne
and Branagan 1988). Similar processes have operated
over these timescales to produce deposits that can be
investigated to reconstruct changing conditions. Other
processes have operated to leave relict features that have
no modern analogues (Gillieson 1996, 106). An exam-
ple of a surface relict-karst feature is the stream-sink
dolines found in the North Great Wood, Hertfordshire,
England, where the Cuffley Brook cuts down through
London Clay and Reading Beds to reach Chalk. At
the eastern end of the wood, where the Chalk lies just
below alluvium, there are several stream-sink dolines
standing higher than the present stream channel and
were probably formed when the stream occupied a dif-
ferent and higher course. On a larger scale, the Qattara
Depression, Egypt, may have started as a river valley,
but karst processes mainly formed it during the Late
