Geoscience Reference
In-Depth Information
(Gilbert,
1877
) deals in detail with the processes involved in fluvial erosion, notably
weathering, transportation and corrasion. He noted perceptively that 'in regions of
small rainfall, surface degradation is usually limited by the slow rate of disintegration;
while in regions of great rainfall it is limited by the rate of transportation' (Gilbert,
1877
, p. 105; see
Chapter 10
). Gilbert returned to this theme in his paper on
The
transportation of debris by running water
(Gilbert,
1914
), in which he introduced
quantitative measures of the relationship between river velocity and the amount and
calibre of debris transported. In this sense, he can be seen as the founder of quantitative
geomorphology, and his successors built on his early insights (Leopold et al.,
1964
).
One aspect of this work on processes of erosion concerns the processes of gully
erosion and the causes of arroyo incision and sedimentation (Bull,
1964a
; Bull,
1964b
;
Tuan,
1966
; Cooke and Reeves,
1976
; Graf,
1982
; Graf,
1983a
; Graf,
1983b
). Another
aspect of accelerated soil erosion by wind rather than water was demonstrated most
forcefully by the suffering caused by the great drought of 1932 in the Great Plains,
vividly described in John Steinbeck's powerful novel
The Grapes of Wrath
(1939)
about the exodus from the Oklahoma Dust Bowl. The root cause was the ploughing up
of fertile prairie soils to grow wheat without any attempt at soil conservation. Recog-
nition that piecemeal conservation measures were inadequate led to the establishment
of the Tennessee Valley Authority, the first attempt at large-scale, integrated catchment
management anywhere in the world and a model soon to be followed elsewhere.
Another of Gilbert's great contributions was his careful mapping of Pleistocene
Lake Bonneville, a vast pluvial lake in the Great Basin (Gilbert,
1890
). He identified
three main shorelines and noted that the highest of these appeared to be contemporary
with the last major glacial advance in this region. From this arose the notion of the
Pleistocene glacial pluvial climates discussed in
Chapter 12
. Gilbert also recognised
that the weight of water in the lake basin and its subsequent release had contributed to
isostatic deformation of the shorelines, although he was not the first to define isostasy -
that distinction belongs to Clarence E. Dutton (
1889
), another of the illustrious pion-
eering geologists of the arid west (Chorley et al.,
1964
; Mayo,
1985
).
As scientifically trained observers penetrated south and west into the drier parts of
North America, their curiosity was aroused by evidence of former human inhabitation
in areas now barren and devoid of surface water. The Anasazi desert farmers of Chaco
and Mesa Verde in the arid south-west are a case in point. Did they bring about
their own demise as a result of the removal of trees and accelerated soil loss, as
Jared Diamond (
2005
) has suggested, or was their demise a result of severe regional
droughts at intervals between about 1200 and 1400, as Bryson and Murray (
1977
)
had argued much earlier? Certainly, the evidence from tree rings and from packrat
middens is consistent with initially wetter conditions followed by periodic severe
droughts.
Another puzzle confronting travellers in the arid south-west arose from the sporadic
fossil remains of now extinct large animals (see
Chapter 17
for details). Once again,
