Regions of Erosional and Depositional
Landforms
著者
雑誌名
巻
号
ページ
発行年
URL
MURPHY Richard, E
The science reports of the Tohoku University.
7th series, Geography
20
2
213-220
1971-03
http://hdl.handle.net/10097/44936
Regions
of Erosional
Richard
and
Depositional
Landforms
E. MURPHY*
Any basic text on physical geography or geomorphology devotes a considerable
amount of space to the agents of erosion and deposition. The resulting analyses
of the various third order landform features are instructive and help impart
understanding of forms and origins. From a geographical standpoint, however,
the location and spatial extent of the features and of the operation of the agents are
also matters of primary importance. Unfortunately, only limited attention has
been devoted thereto. This paper is an attempt to examine the problem and to
present the rationale for the divisions made by the anther in Map Supplement
Number 9, "Ladforms of the World" published by the Association of American
Geographers in the ANNALS for March, 1968.
One of the geographer's main research tasks is the grouping of phenomena for
regional characterization. The kind of classifications he uses for the purpose of
identifying spatial entities differs from the kind used by the systematic sciences in
identifying objects. A biologist classifies such things as a type of tree or a leaf or
a blade of grass, whereas the biogeographer groups the plants and describes the
assembly or complex of plants within certain selected and defined limits and
thus differentiates vegetative areas. The geographer classifies an area by what is
in it. Unless the area is very small indeed, it will have more plants than one and
more species than one type, and hence there is the need to combine.
In the geomorphology of erosional and depositional features, one is, of course,
dealing with larger objects than individual plants. A single alluvial fan, a
drumlin, a U-shaped valley, or a delta occupy considerable space and appear as
individual objects on a map of even medium scale. Nevertheless, as soon as
larger areas are considered, grouping must occur if geomorphological landform
regions are to be designated. The problem is somewhat parallel to classification
in economic geography, since the individual objects therein can occupy considerable
space — for example, a steel mill or a hydroelectric installation or a suberban
housing development. Here also, however, for the designation of economic
regions, grouping is necessary.
In grouping landforms which result from the processes of erosion and deposition, three bases of grouping might be considered. In the first place, erosion
and deposition themselves, as characterizing certain areas and as differentiated
* University of New Mexico. Fulbright Professorat Tohoku University, 1968-69,
214
R. E. MURPHY
from each other, could be considered as a base. Secondly, the assembly of
individual features could be used as a base. Thus, for example, an area could be
characterized as having drumlins, eskers, kames, and other features of glacial
deposition. Finally, processes, or the agents which produce them, could be
considered from a spatial point of view. For the purpose of regional characterization, however, there are difficulties inherent in all of these, particularly in
small scale mapping.
One might attempt an initial two-fold separation into (1) areas of erosional
action with resultant forms on the one hand, and (2) areas of depositional action
with resultant forms on the other. The trouble is that an area of any but very
small size usually has both. Often too, a single depositional feature may be cut
by erosion, or a single erosional feature may receive deposition. For example a
natural levee can be cut by river erosion at high water leaving an oxbow lake
which is a feature resulting from both deposition and erosion. In any case, few
are the areas in which both are not operative.
As for the assembly of individual features, this could be a more profitable
base. In considering any grouping of features of greatly varying shape, however,
some form of common denominator is needed. This could be provided by using
the common genetic process or agent. The rationale for grouping drumlins,
eskers, and roches moutonnêes together is their common glacial origin; the rationale
for grouping meander scars, natural levees, and stream terraces together is their
common alluvial origin.
Process and agent would seem, then, to be the most feasible of the three
suggested possible bases of differentiation and classification. Nevertheless, the
extent of the impact of a given process of agent would have to be determined by
noting the extent of observable landform features resulting from the action of the
process or agent. Implicit in an area which has been glaciated is the existence,
within the area, of landforms produced in connection with glacial action. These
landforms may run the whole gamut of such types, but in any case, their very
existence is the evidence for the previous glaciation. The association is an intimate
and inseparable one.
A convenient way of grouping erosional and depositional landforms for small
scale mapping thus suggests itself. By mapping the extent of the action of an
agent as revealed in the landforms themselves, at least we limit the area within
which certain features may occur. The trouble is, however, that the various
agents differ greatly from one another in their utility for regional differentiation.
Let us consider each agent in turn. The active agents generally are considered to be:
running water, standing water, ice, wind, gravity, and living organisms. Each is
an agent of both erosion and deposition, and each may be described in terms of the
Regionsof Erosional and DepositionalLandforms
215
type of erosional or depositional process which takes place, to wit : running water
— alluvial, standing water — lacustrine or marine, ice — glacial, gravity —
colluvial, and living — biological.
(1) Alluvial erosion and deposition
Running water is an almost universal
agent. It is an obvious agent in well watered areas, but it is also a principal
agent in dry areas. The power of running water after the occasional desert
cloudburst is notorious. Plainly, the delimitation of regions where running
water is operative would have to be in terms of amount or intensity, not in terms
of the existence or lack thereof.
(2) Lacustrine and marine erosion and deposition
The existence of lakes
and seas, either present or past, would, of course, determine the existence of
the features produced by standing water. On a small scale map, such areas
would be difficult to show. Most inland lakes are too small to show except as tiny
dots, and the coastal areas of deposition and erosion would be little larger on the
map than the lines drawn for the coasts. The coastline itself would serve generally
to indicate the location of features of marine erosion and deposition.
(3) Glacial erosion and deposition
Features resulting from glacial erosion
and deposition could seem to be more easily restricted to particular locales than
features produced by other agents. Either an area has been covered by glacial
ice or it has not, and the evidence on the land is strong. At least a three-fold
division would seem to be possible to use: areas covered by glacial ice at present;
areas once covered by the Wisconsin or Wiirm glaciation, but no free of ice; and
areas formerly covered by older Pleistocene glacial messes.
(4) Eolian erosion and deposition
If anything, wind is more omnipresent
than even running water, but features which are produced primarily by wind
deposition or erosion are far less widespread. Some areas, of dunes for example,
are identifiable as areas where the wind has been the only significant agent, but most
areas of wind deposition and erosion are simply areas in which wind has been a
present but minor factor.
(5) Colluvial erosion and deposition
Action by gravity alone or primarily is
restricted to areas of steep slopes and is expected in mountainous areas. For
small scale mapping, topographical information indicates areas of probable
colluvial action, and it would seem futile to try to map, at small scale, areas of
colluvial erosion and deposition as areas in any way distinct from mountainous
areas.
(6) Biological erosion and deposition
The greatest biological eroder and
depositor is man. His works are a reflection of his global distribution. Other
biological organisms are generally more restricted in locale, but their affects are
minor when compared to the actions of other agents.
216
R.
E.
MURPHY
To recapitulate, alluvial and eolian action are virtually omnipresent and
thus fail to indicate a regional breakdown, whereas lacustrine or marine action and
colluvial action are too restricted in particular locale to be useful regional
differentiators. The affects of action by biological agents, except for man, are
generally minor compared with the features produced by other agents. As for
man, he is an almost universal agent and is thus, as in the cases of alluvial and
eolian action, not really useful for regional differentiation. It would seem to be
evident that only glacial action is sufficiently powerful and at the same time neither
too universal on the one hand nor too restricted in locale on the other to be used
as a differetiator for regional types of erosional and depositional landforms. Indeed,
one can get a rather satisfactory subdivision of glaciated regions based on the
indicated three-fold separation into (1) areas of present glacial ice, (2) areas
subjected to Wisconsin or Wiirm glaciation, and (3) areas subjected only to
earlier Pleistocene glaciation. These three categories (coupled with the obvious
assumption that areas which are now ice-free are subject to the actions of other
agents) serve rather well to indicate the general types of eroisonal and depositional
landforms created in northern North America, northern Eurasia, the Arctic
islands, Antarctica, and some scattered areas of high mountains. For some sort
of regionalization of the erosional and depositional features of the rest of the land
surface of the planet, however, the location of the action of individual agents per
se will not suffice. As we have seen, the omnipresent character of the actions of
the wind, running water, and man give no regional delineation unless some
measurement of intensity is introduced, while the actions of other biological
organisms or the actions of standing water, and of gravity, either are too minor, or
are too limited in extent to be useful regional some index of intensity could provide
an answer. It would be tempting to fall back on the distribution of wind velocity
or the amount of precipitation. These, however, would simply put us into the
realm of climatology with no necessary correlation with landform features. In
mapping landforms, the land itself must be examined for evidence lest we fall into
the trap of using a map for the distribution of one phenomenon as the location of
the distribution of another phenomenon. One is reminded of the temptation to
classify as "Mediterranean agriculture" all areas of "Mediterranean climate."
Correlation there may be, but exact duplication is an unwarranted assumption.
The location and distribution of agricultural types must be determined on the
basis of the mapping of agriculture, not on the basis of mapping climate, even
though we may know or stress a high degree of association. So also with
landforms, we must map the features of the land to determine their extent
regardless of how much they may be associated with certain aspects of temperature
and precipitation. The physical evidence in the land itself of the effects of runoff
Regionsof Erosionaland DepositionalLandforms
217
and wind are the features we need to examine in order to arrive at a measurement of
intensity, even though we know that an arid landscape will produce certain
features in contract to a humid landscape.
Frequently, in geomorphological study and teaching, this very kind of
separation is made, and the terms "arid cycle of erosion" and "humid cycle of
erosion" are common and imply considerable difference between the two in the
third order features produced largely by the action of wind and running water.
There is general recognition that a major difference between dry and humid
erosional and depositional landform regions is the amount of precipitation.
Presumably it would be useful to separate the two, but for landform classification
the separation must be made on the basis of evidence on the land itself.
Fortunately, a very convenient and widely distributed type of feature is
available for making the distinction, and that is simply streams. In general,
the more precipitation there is, the more streams there are. Permanent streams
are faund in general in well watered places, whereas intermittent streams are found
in general in drier areas.
An examination of semi-arid areas of transition from humid to dry landscapes
will reveal a thinning out of permanent streams as the dry region is approached.
Just where to draw a separation line is a somewhat arbitrary matter, but generally
satisfactory results are obtained by choosing a figure of about ten English miles or
sixteen kilometers as the maximum traverse distance between permanent streams
for an area to be considered as a humid landform area. Less than this results in a
dry landform area with the associated features of the arid cycle of erosion.
Such was the line of reasoning through which the writer went in determining
his criteria for the map of the world distribution of erosional and depositional
landforms which formed one of the three maps for his composite map of the
"Landforms of the World" which appeared with the March , 1968, issue of the
ANNALS of the Association of American Geographers. The map of erosional
and depositional landforms is reproduced here, and the definitions for the individual
categories are given below:
h. Humid landform areas
Areas in which the pattern of permanent streams has a density of at least one
stream in every ten miles (16 kilometers) traverse distance, and which have
not been subject to glaciation since the beginning of the Pleistocene epoch.
d. Dry landform areas
Areas in which the pattern of stream density is more sparse than one stream
in ten miles (16 kilometers) traverse distance, and which have not been
subject to glaciation since the beginning of the Pleistocene epoch. Some
karst as well as arid areas appear in this category.
-r
AND
1M Humid landform areas
EROSIONAL
1111
Dry
1•11
•••
Of arid
DEPOSITIONAL
40
landform
40
AO
areas
REGIONS
,
0
xm
:36,-,Nd I e e cops
MC
at
present
44.
EM1
•••
glaciated
Wisconsin
Man
areas
and
40
r
111
cene
glaciated
areas
pre-WOrm
Pleisto-
MURPHY
c •
E.
Pre-Wisconsin
and
and
undifferentiated
N
io
hi.4
•
RICHARD
OD
Regionsof Erosional and DepositionalLandforrns
219
g.
Glaciated areas
Areas covered by glacial ice at some time since the beginning of the
Pleistocene epoch, but earlier than the Wisconsin and Wiirm glaciations.
(Undifferentiated glaciated areas are also included in this category.)
w. Wisconsin and Wiirm glaciated areas
Areas covered by glacial ice during or since the Wisconsin and Warm glaciations but now free of glacial ice.
i. Icecaps
Areas covered by glacial ice at present.
For classification on the small scale needed for a world view of erosional and
depositional landforms, this five-fold classification permits large regional grouping
of individual features in mutually exclusive categories. Both genesis of types and
the distinction between the humid and dry cycles of erosion are given basic
regional placement.
One of the most fundamental distinctions between the third order features of,
for example, the Sahara and the Congo, or of the Sonora and the Shantung Peninsula,
is the difference between the forms produced within an area wherein the arid
cycle of erosion is operative and those produced within an area where the humid
cycle of erosion is operative. If, in turn, we contrast these areas with the peninsula
of Quebec or the Alps of Switzerland or the Baltic periphery, the factor of glaciation looms to the fore. Here is a three-way distinction —the arid landscape,
the humid but non-glaciated landscape, and the glaciated landscape. The ice
itself still cloaks large areas and forms a type in itself, while the glaciated terrain
divides rather well into the areas of more recent glaciation and the areas of older
Pleistocene glaciation.
If the factor of empirically measured topography is considered along with these
distinctions, then the whole image of individual third order features is conceived.
The flat areas bespeak one set of features in a dry area, another in a humid but
non-glaciated area, and another in a glaciated area. A mountainous terrain
likewise suggests one aspect in a dry landscape, another in a humid but nonglaciated landscape and yet another in a glaciated, mountainous landscape. The
writer's classification of world landforms gives a distinct category to the empirical
topographical factor. Thus the erosional-depositional factor can be treated
as a set of criteria in itself, yet dovetailing with topography to give a more
complete picture. As Professor Arthur Strahler has put it, "A particularly
valuable attribute [of the sytem]... is that [a person].... can use his background
of knowledge to predict or anticipate many characteristic details of the landforms
220
R. E. MURPHY
that
are not implicity
The
correlations
other
phenomena,
high
degree
and
precipitation
the
semi-arid
lands.
example,
Another
in either
glaciation
correlations
of mixed
As
farming
a significant
topographical
form.
typical
but
one hand
aspect
has
and
designation.
problem
Requisite
within the framework
for such a purpose
distinct
and
presents
for our
an overview
examination
The
mutually
total
to
be
between
with
with
ice caps
between
and
For
Wisconsin
other
continent
of
subtropics.
correlations.
on the
of either
landform
may
and
characterization,
included
along
of generalizing
landforms,
system
exclusive
glaciation
striking
and
as compared
subtle
a
location
or
be seen
their
areas
agriculture.
problem
regional
correlation
also more
areas
more
to arid
with
is, of course,
the
tropics
in the division
older
dairying
of
order
are
or Eurasia,
in the
response
is the
There
and
is even
factors
landforms
There
areas
correlation
example
America
The
the
of the human
obvious
of third
dry
to temperature
or grain-livestock
landscape
as are
the
classes. "1
or depositional
landform
the primary
depositional
of erosional
between
habitation.
on the
of the various
are manifold.
aware
North
between
classes
climates,
of human
definitions
to a geographer,
related
we are constantly
limits
Wiirm
of these
of correlation
annual
In turn,
in the
so useful
desert
humid
stated
such
however,
explained
of an over-all
is simplicity,
categories.
with
small
makes
herein
is an
the
basic
and
erosional-
structure
varied
a difficult
attempt
and
features
problem
to
solve
of
the
classification
of world
landforms.
broad
regional
inclusiveness,
yet
The
author
feels
that
this
which is valuable as an initial or fundamental
starting
of the world distribution
of third order features.
system
point
References
Murphy,
R.E. (1968): Landforms of the World, Map Supplement No. 9 Annals of the
Association of American Geographers Vol. 58, pp. 198-200
Strahler,
A.N. (1969): Physical Geography, 3rd ed. New York: Wiley
1.)Arthur
p. 388
N . Strahler,
PHYSICAL
GEOGRAPHY,
3rd ed. (New York:
Wiley , 1969),