Page 42 - Fundamentals of Geomorphology

WHAT IS GEOMORPHOLOGY? 25

palaeosols, sedimentary rocks, and historical records Charles Darwin used the chronosequence method to
(see Huggett 1997b, 8–21). Sedimentary deposits are an test his ideas on coral-reef formation. He thought that
especially valuable source of information about past land- barrier reefs, fringing reefs, and atolls occurring at dif-
scapes. In some cases, geomorphologists may apply the ferent places represented different evolutionary stages of
principles of stratigraphy to the deposits to establish island development applicable to any subsiding volcanic
a relative sequence of events. Colluvium for example, peak in tropical waters. William Morris Davis applied
which builds up towards a hillslope base, is commonly this evolutionary schema to landforms in different places
deposited episodically. The result is that distinct layers and derived what he deemed was a time sequence of land-
are evident in a section, the upper layers being progres- form development – the geographical cycle – running
sively younger than the lower layers. If such techniques from youth, through maturity, to senility. This seduc-
as radiocarbon dating or dendrochronology can date tively simple approach is open to misuse.The temptation
these sediments, then they may provide an absolute is to fit the landforms into some preconceived view of
timescale for the past activities on the hillslope, or at least landscape change, even though other sequences might
the past activities that have left traces in the sedimentary be constructed. A study of south-west African land-
record. Recognizing the origin of the deposits may also forms since Mesozoic times highlights the significance
be possible – glacial, periglacial, colluvial, or whatever. of this problem, where several styles of landscape evo-
Andsometimesgeomorphologistsusetechniquesofenvi- lution were consistent with the observed history of the
ronmental reconstruction to establish the climatic and region (Gilchrist et al. 1994). Users of the method must
other environmental conditions at the time of sediment also be warned that not all spatial differences are tempo-
deposition. ral differences – factors other than time exert a strong
The recent global environmental change agenda has influence on the form of the land surface, and land-
given environmental reconstruction techniques a fillip. forms of the same age might differ through historical
A core project of the IGBP (International Geosphere– accidents. Moreover, it pays to be aware of equifinality,
Biosphere Programme) is called Past Global Changes the idea that different sets of processes may produce the
(PAGES). It concentrates on two slices of time: same landform. The converse of this idea is that land
(1) the last 2,000 years of Earth history, with a tem- form is an unreliable guide to process. Given these con-
poral resolution of decades, years, and even months; sequential difficulties, it is best to treat chronosequences
and (2) the last several hundred thousand years, cov- circumspectly.
ering glacial–interglacial cycles, in the hope of providing Trustworthy topographic chronosequences are rare.
insights into the processes that induce global change The best examples normally come from artificial land-
(IGBP 1990). Examples of geomorphological contri- scapes, though there are some landscapes in which, by
butions to environmental change over these timescales quirks of history, spatial differences are translatable into
may be found in the book Geomorphology, Human time sequences. Occasionally, field conditions lead to
Activity and Global Environmental Change edited by adjacent hillslopes being progressively removed from
Olav Slaymaker (2000a). the action of a fluvial or marine process at their bases.
This has happened along a segment of the South Wales
Landform chronosequences coast, in the British Isles, where cliffs are formed in Old
Red Sandstone (Savigear 1952, 1956). Originally, the
Another option open to the historical geomorphologist coast between Gilman Point and the Taff estuary was
is to find a site where a set of landforms differ from place exposed to wave action. A sand spit started to grow.
to place and where that spatial sequence of landforms Wind-blown and marsh deposits accumulated between
may be interpreted as a time sequence. Such sequences the spit and the original shoreline, causing the sea pro-
arecalledtopographic chronosequences,andtheproce- gressively to abandon the cliff base from west to east.The
dure is sometimes referred to as space–time substitution present cliffs are thus a topographic chronosequence:
or, using a term borrowed from physics, ergodicity. the cliffs furthest west have been subject to subaerial