348 Exogenic process€s and landforms
modification has been extremely slow, apparently involving
an average of less than l00 m extension over the past 5 Ma.
In the eastem Mojave Desert 60 basalt flows up to 4 Ma old
partly bury pediment sutfaces, and comparison of the relative
positions of pediments covered by dated basalts and the
modem pediment surfaces indicates the degree of modification
that has occurred over this time interval. The conclusion
is the amount of modification has been minimal, with
downwearing predominating over backwearing"
Although the exhumation model of pediment formation,
along with its requirement for a previously humid climate,
seems to have widespread applicability there appear to be
situations where it cannot apply because there is compelling
evidence ofprolonged aridity, One such case is provided
by the pediments flanking the domed inselbergs, or
bornhardts, of the Namib Desert in southern Africa (Fig.
14,3). Here the cold Benguela Current offshore in conjunction
with the dominant sub-tropical high pressure system
has created a hyper-arid coastal desert climate which has
probably prevailed since at least the mid-Miocene. A1though
the present rate of slope retreat seems to be extremely
slow there are no remnants of an earlier phase of significant
rock weathering to support the idea of exhumation. Such
pediment and inselberg forms represent one of the major
enigmas of geomorphology.
14.2 The significance of climatic change
The presence of landforms, such as certain kinds of pediments,
in climatic environments in which they could not have
developed obviously raises the question of climatic change
in landform interpretation. The state of the atmosphere varies
from day to day and even hour to hour, and it is average
weather conditions together with their variability that characlerize
the climate of a region. But climate also fluctuates at
all time scales from decades to tens of millions of years.
Whether a climatic change is of geomorphic significance
depends on its magnitude and duration, and on the properties
of the landform concemed. The larger or more resistant
a landform, the longer, in general, it takes to adjust to a
change in climate. The 'sensitivity' of different landscapes
to climatic fluctuations is an important but complex question
which affects the way we conceive long-term landscape
development and we examine it in detail in Chapter l8 (see
Section 18.2,3).
An important distinction is between 'active' landforms
which seeryto be in 'equilibrium' with the prevailing climatic
environment, and inactive or relict landforms which could
not have been formed under the current climatic regime. As
will have been evident from our discussion of the pediment
problem, this distinction is not always easy to make. Although
we saw in Section 1.5.3 that it is possible to relate
broad associations of geomorphic processes to major climatic
zones, the overlap between categories is significant since
most processes operate in most environments at least to
some degree. In many cases we do not yet know enough
about the development of specific landforms to be abie to
say with any confidence which are being actively formed
in a particular climatic environment. The problem of distinguishing
between active and relict forms can be parlicularly
acute in those environments where even 'active' geomorphic
processes operate sporadically. This is the case in
many arid and semi-arid environments where the irregular
pattems of precipitation mean that fluvial activity is very
limited in duration, but, none the less, often very important
in shaping the landscape. The question here is how long
does a landform have to be inactive for it to be described asl
relict?
The past two decades have seen a quantum leap in our
understanding of climatic change largely as a result of the
exploration of the ocean floor, which contains a relatively
complete and undisturbed climatic record, and of the development
of new dating procedures and techniques of palaeoenvironmental
reconstruction. Our knowledge of climatic
fluctuations on the continent§, nevefiheless, is still severely
constrained by a lack of palaeoclimatic indicators capable
of yielding datable material. In fact landforms themselves
have been extensively used to reconstruct changes in temperature,
precipitation and wind intensity and direction on
the continents. The use of this kind of evidence actually
raises a problem for geomorphologists concemed with interpreting
the landscape in terms of climatic change because
in many cases the climate changes themselves have been
interpreted largely from landform evidence. Clearly there is
a danger of circular arguments here and, if possible, we must
use independent information on climatic change.
The consequences of climatic change for landform genesis
can, very broadly, be divided into those effects related primarily
to temperature changes, and those related primarily
to changes in precipitation. The growth and decay of ice
sheets is clearly influenced signiflcantly by long-term charrges
in temperature, whereas suďace runoff and fluvial processes,
along with the level of aeolian activity, are highly responsive
to precipitation changes. We must be careful, however, in
making broad generalizations. For instance, although frequently
resulting from a temperature fall, glacier growth
can also occur as a result of an increase in precipitation
when temperatures are stable, or even increase. Moreover,
an increase in surface runoff can arise from a decrease in
temperature (and hence rates of evapotranspiration) as well
as an increase in precipitation. Nevertheless, it is convenient
to separate the discussion of the expansion and contraction
of glacial and periglacial morphoclimatic regimes, which
are largely a response to temperature fluctuations, from the
predominant impact of precipitation changes on fluvial and
aeolian systems. Before we do this, however, it is necessary
to review briefly our present understanding ofthe history of
global climatic change.
1-1.3 The record of climatic change
\ ,,vide range of techniques are used to reconstruct past clir-iates
and they are employed on diverse types of evidence.
For long-term climatic change information is acquired from
,he clranging distributions of plants and animals determined
_rom fossils, and from rock types which are attributable to
rarticular climatic regimes. Evaporites and aeolian sand>tones,
íbrinstance, are indicative of arid environments.
,.,,
hereas tillites demonstrate glacial conditions. Approaches
:!r leconstructing more recent climatic change include the
:nall,sis of pollen (palynology) and landforms themselves.
One technique, however, has done more than any other
]\ er the past two decades or so to revolutionize our under.tanding
of climatic change. In the late 1940s it was pointed
_.ut that the ratio of the two stable isotopes of oxygen, 160
:nd 18O, precipitated in carbonate would vary with water
enperature. This idea was applied in 1955 by C. Emiliani
:.r the analysis of calcium carbonate (cacoj) shells secreted
lr various sea creatures. The technique of oxygen isotope
analysis, as it was termed, was most often applied to forarlinif-era,
single-celled organisms mostly between 0.1 and
lt._] mm across. Emiliani found that the ltlo/l60 ratio varied
.n tbraminifera shells recovered from ocean cores recording
>edimentation over the past few hundred thousand years. Both
planktonic (surface and near-surface) and benthic (bottomJtt
elling) forms of foraminifera occur so it was possible to
jstimate temperature changes on both the sea surface and in
lhe ocean depths. The changes in oxygen isotope ratios were
dated using the palaeomagnetic time scale in combination
rrith an assumed constant rate of sedimentation.
It was subsequently appreciated that 1|9 tsQ/toQ ratio of
the sea water itself would also vary as the quantity of land
ice changed with fluctuations in global temperatures. Such
changes would be reflected in the foraminifera and would
arise because the lower atomic mass of 160 means that it is
preferentially evaporated from the oceans. During glacials a
proportion of this loO-enriched water would be locked up
in ice sheets thus leaving the oceans relatively enriched in
sO. It is difficult to separate these two effects * one indicating
changes in ocean temperatures, the other fluctuations
in the volume of ice sheets - and this has led to divergent
views of climatic history, especially for the past 40 Ma.
14.3.1 The Cretaceous to Neogene record
There are probably few areas in the world where landforms
or weathering deposits can be regarded as having survived
more or less intact for more than the past 100 Ma, although
it has been suggested that even more ancient forms exist in
northem Australia. It is appropriate, therefore, to begin our
brief survey of climatic change with the Cretaceous.
The climate of the Late Cretaceous - 100 to 65 Ma np is
generally thought to have been warm and equable with a
very much less marked temperature gradient with latitude
Climate, climatic change and landform development 349
compared with the present. Polar regions probably experienced
a climate rather similar to mid-latitude and subtropical
climates of the present day, although the amount of
solar radiation received and the seasonality of climate
would have been similar to equivalent latitudes today.
However this widely accepted ice-free Cretaceous interpretation
has been challenged by researchers who have interpreted
large exotic blocks found within Early Cretaceous
mudstones in Australia as dropstones, that is, debris
dropped from floating ice. This evidence indicates that ice
was present at sea level in high latitudes during the
Cretaceous. Similar interpretations have been applied to
exotic blocks in strata of a variety of ages and it has been
argued that these indicate, contrary to conventional wisdom,
that the Earth may only rarely have been ice free in the
past. These interpretations have yet to be confirmed by
other evidence, and meanwhile it is perhaps best to adopt
the generally accepted ice-free Cretaceous model. Towards
the end of the Cretaceous there is good evidence for a slight
cooling, but this trend was reversed during the Paleocene
and Eocene when a warm phase saw tropical soil formation
extending to 45" latitude in both hemispheres and tropical
species living in southern England and western Greenland.
The major point of dispute in the climatic record for the
past 50 Ma is when the Antarctic ice sheet became established.
one interpretation of the oxygen isotope data suggests
that it first appeared about 15 Ma ago, but if |80/160 ratio
variations are caused predominantly by ice volume changes
then a date of around 35 Ma is indicated (Fig. 14.4). This
earlier date is supported by geomorphic and sedimentological
evidence, including glacially abraded grains in offshore
sediments, which indicate the first appearance of sealevel
glaciation in East Antarctica at this time. By about
26Ma ago the East Antarctic ice sheet was apparently more
extensive than it is at the present day, An ice sheet in the
Early Cenozoic also helps to account for the record of sealevel
change during this period which is difíicult to explain
without the changes in ocean water volume generated by
periodic fluctuations in ice volume (see Section 11 .5.4'\.
The causes of the major climatic changes involved in the
establishment of the Antarctic ice sheet are beyond the
scope of this book, but continental drift and the drastic
modifications of ocean currents that it generated were certainly
important factor§. Recall that the supercontinents of
Laurasia and Gondwana broke up progressively throughout
the Cretaceous (see Figure 2.16i).In the northem hemisphere a
temperate climate persisted until the mid-Miocene, but
significant cooling can be traced back to at least 10 Ma ago
and the Arctic Ocean became ice-covered by 5 Ma nr.
Although some uncertainty remains, it appears that ice sheets
became established in the northem hemisphere around
3.2Ma np. This date is indicated by the appearance of icerafted
debris in the North Atlantic and North pacific in
sediments of this age.
350 Exogenic procésses and landforms
EPocH 18oocEAN REcoRD
AG,E
MaFlP lCE SHEETEXTENT MAJOR EVENTS
PLElsT
lncrease
<-
o
1o
20
3o
4o
5o
6o lncrease
4
Large N, hemisphere ice sheets
Mountain glaciers in N, hemisphere
Sealevel glaciation in W. Antarctica
+ MountainglaciationinAntarctica?
sharpcoolingof
southern ocean
Anta rctic-Australian passage opens
PLlocENE
MlocENE
oLlGocENE
EocENE
PALAEOCENE
Relative Change in 180
concentration in oceans of lce sheets
worldwide
Fig. 14.4 History of the development of glaciation during,the Cenozo_ic. Note that the variations in ice sheet extent are speculative due
}o'Iack of data, oia inot the datis at whícň glacial activi§began at dffirent localities i1
1.|1o_subje9.t
to,reYision in the light of new
informaiion. (Based, partly on D. E. Sugdeň Q987) in M. J. Clark, K. J. Gregory and A'. M. Gurnell (eds) Horizons in Physical
Geogr aphy . M acmillan, B as ings to ke, Fig. 3. 2 - 3, p. 2 1 7. )
14.3.2 The Quaternary record
By 3 to 2.5}Áa Bp large-scale oscillations in temperature reflecting
glacial-interglacial cycles had become established,
with a regular 100 ka cycle occurring over the past 700 ka,
Many researchers attribute these regular climatic fluctuations
largely to the effects of slight variations in the position
and orientation of the Earth with respect to the Sun which
affect the receipt of solar radiation; this is commonly known
as the Milankovitch mechanism for glacial-interglacial
cycles. Other research, however, has indicated that other
factors, such as the organic productivity of the oceans and
the concentration of CO2 in the atmosphere, may also be
important. The oxygen isotope data indicate that over the
past 3 Ma the Earth has experienced a glacial climate for
a significant majority of the time, with glacial episodes of
around 8G-100ka duration altemating with 10ka-20ka
interglacial interludes, at least over the past 700ka. Significant
changes in the 180/160 ratio are now indicated by
oxygen isotope stage numbers and these provide a,good
guide io the major changes in ice sheet volume and, in,
directly, global temperatures (Fig. 14.5). Key age markers
for this ocean core oxygen isotope record have been provided
by the palaeomagnetic time scalé and there have also
been attempts to link the ocean record with continental
palaeoclimatic data. The most successful of these correlations
has been achieved with loess sequences (see Section 10.3.7).
The longest record is provided by the loess sequences of
China which seem to give a continuous climatic record
for the past 2.3 Ma. Enhanced rates of deflation and aeolian
dust transport occurred during periods of semi-arid climate
coinciding with glacial advances in the northem hemisphere.
A correlation has now been-made between the past
500ka of a Chinese loess sequence with the l8O record of a
deep-sea core in the north-west Pacific and this has greatly
improved the dating of the loess sequence (Fig. 1a.6).
A major contribution to the understanding of the nature
and magnitude of Quatemary climatic change has come from
the CLiMAP (ClimateÁong-Range Investigation Mapping
and Prediction) project which attempted to document the
global environment at the last glacial maximum l8ka sp.
AGE
Ma
B.P.
0.0
0.73
0.88
0.94
1.72
1,88
PALAEo
/lAGNETl(
TlME-
scALE
18o MARlNE
cLlMATE
STAGES
\ J5
\ 6
.|1
\
1 l9
-
12
14
5
\
22
s 23
ARAMlLLC
{
4
+- 6180 _r
Low High
:iot=pUVÁ[:
Fig. 14,5 Oxygen isotope recordfor the about the past 2 Ma
from the Pacific deep-sea core V28-239 analyzed by N. J.
Shackleton and N. D. Opdyke correlated with the palaeomagnetic
time scale of Berggren et al. The oxygen isotope stage numbers
are shown, the odd-numbered shaded stages indicating relatively
warm interludes. (Modified from W.B. Harland et aL (l9B2) A
Geologic Time Scale. Cambridge University Press, Cambridge,
Chart 2 .l7 , p. 42 .)
Climate, climatic change and landform development 351
Although concerned with just the most recent major glacial
advance, the environmental changes identified were probably
mirrored by earlier glacials of similar magnitude. Large ice
sheets blanketed a significant proportion of the land area of
the northem hemisphere and some 18 per cent of the Eafth's
surface was ice-covered (Fig. 14.7). In eastem North
America the Laurentide ice sheet extended from the Rocky
Mountains to the Atlantic ocean and from the Arctic ocean
to the latitude of New york. In the ice free area to the south
large lakes formed, such as those of the Great Basin of the
western USA, in response to the supply of meltwater, and to
lower evaporation as a result of lower temperatures.
Until the 1960s it was widely thought that the humid
tropics had been little affected by the glacial-interglacial
fluctuations of the higher latitudes. New data from a range
of sources, including palynological studies of sediment cores
and work on changing lake levels, have now shown that,
during the latter part of the last glacial at least, much of the
humid tropics were probably cooler and drier than at
present (Fig. 1a.8). Although the evidence is sparse in some
regions, during the period around the last glacial maximum
at about 18 000 a Bp the tropical rain forests seem to have
contracted to a smaller area than they presently cover. There
was a compensating increase in the extent of savanna vegetation,
indicating a significant reduction in precipitation.
According to some interpretations of the sedimentological
evidence up to half of the land area within 30o of the
equator was covered by active ergs at this time, and lake
levels, especially in the northern hemisphere, were generally
very low in the period 14000-21000 a sp. The evidence
for the magnitude of temperature depression is less
certain with a greater temperature decrease being inferred
from the lowering of the altitude of vegetation and glacier
limits than is indicated by information from deep-sea cores.
It is in fact not a surprise to find evidence of tropical
aridity coinciding with glacial episodes. We would expect a
reduction in precipitation to occur as a result of reduced
evaporation in response to lower ocean suďace temperatures
and a decíease in the area ofthe oceans as consequence of a
global fall in sea level. Exceptions to this glacial aridity
trend did occur though, with some regions such as North
Africa being more humid (Fig. l4.8). This was due to the
equatorward deflection of rain-bearing westerly weather
systems by the large high pressure cells established over
the extensive northem hemisphere ice sheets.
Climatic changes have continued since the end of the last
glacial around l0000a Bp but their magnitude has been
much attenuated. Although mean global temperature does
not appear to have varied by more than about 2 oC over the
past 8000 a or so, evidence from fluctuating lake levels indicates
that the tropics were generally much wetter in the
period 12 000 to 5000 a Bp than they were during the preceding
10000a, or have been over the past 5000a. This
humid episode was, however, punctuated by much drier
352 Exogenic processes and landforms
Xifeng loss V21-146
I I Aeolian ílux
b c (mg cm-2 kyr-l) d
.{GE
(lyr)
0l00
-
200 -
300 -
400 ,00
-
600 -
KEy
|-.-] Loess
D weathered loess
§l§l P213"o"o1 inter-layer
6'' o (gg")
Specmap
6't o
2oo 400 600 800
s3
L
s4
1-5
§fiE Loess inter-layer
I Palaeosol
I Black loam
Fig. 14.6 Correlation of the loess sequence at Xifeng, China, with the orygen isotope record from west Pacific deep-sea core
V21-]4ó. Dating of the loess sequence in column (a) is based on a technique known as magnetic susceptibility and in (b) on correlation
with the aeolian dust flux. Column (c) shows the aeolian flux record of the deep-sea core while column (d) illustrates the orygen isotope
record. Column (e) shows the global standard chronology for the oxygen isotope record (Specmap), (From S. A. Hovan et al. (1989)
Nature 340, Fig.3, p.298.)
periods in particular regions. In Africa, for instance, there
was a marked increase in aridity during the periods l l 000-
10000a Bp, 850M5O0a Bp and 6200-5800a sp. It has
been the subhumid and semi-arid tropical regions that have
experienced the most significant post-glacial fluctuations in
precipitation, so it is not surprising that these aíeas pose
particular problems of landscape interpretation.
Fig,14.7 Probable extent oíthe northern hemisphere ice sheets
at the last glacial maximum ]B 000 a tr (Based largely on G. H.
Denton andT.J. Hughes, (198l)The Last Great Ice Sheets.
Wiley, New York, p.viii.)
BARENTs
o 2ooo
km
Climate, climatic change and landform development 353
ioi#
íoumí--ln, l l
Fig. I4,8 World climate about 18 000 a BP. Note that the palaeoclimatic reconstructions for some areas are uncertain. (Based partly
onT.H.VanAndel(1985)NewView§onanOldPlanet. CambridgeUniversityPress,Fig.4.4,p.6landvariousothersources.)
14.4 Effects arising predominantly from
temperature changes
14.4.1 Landscapes of deglaciation
Virtually all glacial landforms are in a sense relict since
both erosional and depositional glacial landforms are only
fully exposed once the covering ice has retreated. Similarly,
the onset of a glacial morphoclimatic regime requires a
sustained cooling across the temperature threshold at which
snow can accumulate from one yeaí to the next. In Chapter
11 we looked at the development of specific glacial landforms,
but here we will confine our attention to the broad
nature of landscape modification that resulted from the
expansion of the Late Cenozoic ice sheets into the midlatitudes
(Fig. 14.7) and the extension of upland ice caps
and mountain glaciers to lower elevations.
14,4.1 ,1 Relict landscapes of glacial erosion
Mountain, or alpine, glaciation is characterized by a deepening
of valleys by glacial erosion and active physical weathering
on valley sides. Glacial troughs develop generally
along the course of pre-existing river valleys (Fig. 11.16);
1ooo o 10oo
km
bedrock resistance influences glacial trough form with deeper
and narrower valleys occurring in resistant lithologies. In
regions of modest elevation, landscape modification during
glacials may be confined to the formation of cirques. In the
midJatitudes of the northem hemisphere cirques are preferentially
developed on north-east slopes. Snow accumulates
here because of the combination of shade from summer
insolation with protection from the predominant westerly
winds. In the southem hemisphere south-east slopes are
favoured for similar reasons.
The nature of landscape modification by ice sheets is
controlled by ice sheet behaviour, the bedrock geology and
the pre-existing form of the landscape (Table 14.2 anďFig.
14.9). The crucial factor seems to be the state of the basal
ice. Active erosion can occur below warm-based ice and
extensive areal scouring is accomplished where the pressure
melting point is attained, normally beneath the centre
of ice sheets and around the margins in lower latitudes.
This broad pattem can be modified by the subglacial topography
since upland regions will be more likely to be
covered by thinner, cold-based ice. Only very limited erosion
is achieved beneath the cold-based polar flanks of continental
ice sheets.