Geology Reference
In-Depth Information
a product of phases of erosion during the later Roman
Imperial times, through the Dark Ages, and to the Middle
Ages. Vita-Finzi believed it to be the result of increased
erosion associated with the climate of the Medieval Warm
Period or the Little Ice Age, a view supported by John
Bintliff (1976, 2002). Other geomorphologists, includ-
ing Karl Butzer (1980, 2005) and Tjierd van Andel and
his co-workers (1986), favoured human activity as the
chief cause, pointing to post-medieval deforestation and
agricultural expansion into marginal environments. The
matter is still open to debate (see p. 363).
(e.g. Leopold
et al
. 1964). Stanley A. Schumm, another
fluvial geomorphologist, refined notions of landscape sta-
bility to include
thresholds
and dynamically
metastable
states
and made an important contribution to the under-
standing of timescales (p. 27). Stanley W. Trimble worked
on historical and modern
sediment budgets
in small
catchments (e.g. Trimble 1983). Richard J. Chorley
brought process geomorphology to the UK and demon-
strated the power of a
systems approach
to the subject.
Process geomorphologists have done their subject at
least three great services. First, they have built up a
database of process rates in various parts of the globe.
Second, they have built increasingly refined models for
predicting the short-term (and in some cases long-term)
changes in landforms. Third, they have generated some
enormously powerful ideas about stability and instability
in geomorphic systems (see pp. 19-21).
Process geomorphology
Process geomorphology is the study of the processes
responsible for landform development. In the modern
era, the first process geomorphologist, carrying on the
tradition started by Leonardo da Vinci (p. 5), was Grove
Karl Gilbert. In his treatise on the Henry Mountains
of Utah, USA, Gilbert discussed the mechanics of flu-
vial processes (Gilbert 1877), and later he investigated
the transport of debris by running water (Gilbert 1914).
Up to about 1950, when the subject grew apace, impor-
tant contributors to process geomorphology included
Ralph Alger Bagnold (p. 85), who considered the physics
of blown sand and desert dunes, and Filip Hjulstrøm
(p. 73), who investigated fluvial processes. After 1950,
several 'big players' emerged that set process geomorphol-
ogy moving apace. Arthur N. Strahler was instrumental
in establishing process geomorphology, his 1952 paper
called 'Dynamic basis of geomorphology' being a land-
mark publication. John T. Hack, developing Gilbert's
ideas, prosecuted the notions of
dynamic equilibrium
and
steady state
, arguing that a landscape should attain
a steady state, a condition in which land-surface form
does not change despite material being added by tec-
tonic uplift and removed by a constant set of geomorphic
processes. And he contended that, in an erosional land-
scape, dynamic equilibrium prevails where all slopes,
both hillslopes and river slopes, are adjusted to each other
(cf. Gilbert 1877, 123-4; Hack 1960, 81), and 'the forms
and processes are in a steady state of balance and may
be considered as time independent' (Hack 1960, 85).
Luna B. Leopold and M. Gordon Wolman made notable
contributions to the field of fluvial geomorphology
Measuring geomorphic processes
Some geomorphic processes have a long record of mea-
surement. The oldest year-by-year record is the flood
levels of the River Nile in lower Egypt. Yearly readings
at Cairo are available from the time of Muhammad,
and some stone-inscribed records date from the first
dynasty of the pharaohs, around 3100
BC
. The amount
of sediment annually carried down the Mississippi River
was gauged during the 1840s, and the rates of modern
denudation in some of the world's major rivers were
estimated in the 1860s. The first efforts to measure
weathering rates were made in the late nineteenth cen-
tury. Measurements of the dissolved load of rivers enabled
estimates of chemical denudation rates to be made in
the first half of the twentieth century, and patchy efforts
were made to widen the range of processes measured
in the field. But it was the quantitative revolution in
geomorphology, started in the 1940s, that was largely
responsible for the measuring of process rates in differ-
ent environments. Since about 1950, the attempts to
quantify geomorphic processes in the field have grown
fast. An early example is the work of Anders Rapp
(1960), who tried to quantify all the processes active
in a subarctic environment and assess their compara-
tive significance. His studies enabled him to conclude
that the most powerful agent of removal from the
