allowed an undisputed place in the treatment of the lands, however
fully it is accepted as appropriate to the presentation of other
divisions of physical geography. But the manner in which explanation
is extending over a larger and larger part of the subject gives
assurance that the geographers of the coming century will insist upon
a uniformly rational treatment of all divisions of their science. The
active phenomena of the earth’s surface first secured explanation; it
has long been considered essential to explain as well as to describe
such phenomena as the winds of the air and the currents of the ocean;
indeed, this is now so habitual that many geographers who may object
to the explanation of a peculiar kind of a valley as a trespass upon
geology, will nevertheless demand an explanation of rainfall and tides,
although these truly geographical subjects are manifestly shared with
physics and astronomy. Land forms of very elementary character, like
deltas, or of rapid production, like volcanoes, have had to give some
account of themselves all through the century; but it was not for many
years after the announcement of Playfair’s law, that the erosion of
valleys by the rivers that drain them came to be regarded as a subject
appropriate to a geographical treatise. Only in the later years of
the century has the fuller treatment of this beautiful subject been
attempted; even now much of it remains to be developed in the century
to come.
The treatment of physical geography will be much more even, to the
great advantage of its students, when explanatory description is
applied to all its parts. The alluvial fans at the base of arid
mountains should be accounted for as well as the dunes of deserts.
The fault cliffs of broken plateau blocks and the weathered cliffs of
retreating escarpments deserve to be considered as carefully as the
wave-cut cliffs of coasts; the essential differences of these forms are
reached most easily through their explanation. The varied sculpturing
of a mountain slope may, in time, come to be as well understood as is
now the erosion of a simple valley in a low plain.
One of the most notable elements of the century’s progress is the
increasing breadth of view gained as explanatory descriptions are
extended further and further over the geographical field. At first
explanation was given to various individual features, item by item;
now it is recognized that an appropriate place must be provided for
all kinds of land forms in a comprehensive scheme of physiographic
classification. Many instances of the earlier stage might be given,
beginning with examples from the works of Humboldt, the acknowledged
leader of scientific explorers in the opening decades of the century.
His attempts, more or less completely successful, to explain the facts
that he observed, as well as to correlate life with environment, may be
traced all through his writings; but his ‘Cosmos’ (1845) did not reach
a careful discussion of land forms, although it entered so far into an
explanatory treatment as to consider the formation of mountain ranges.
Innumerable examples of isolated facts and special explanations,
unrelated to a comprehensive scheme of physiographic classification,
might be taken from the reports of exploring expeditions and of
geological surveys; from books of travel and from geographical and
geological journals with which the nineteenth century has filled so
many library shelves; but lack of space will prevent mention of all
sources, save a few treatises in which the accumulated knowledge
of their time is summarized. Such a work as Mrs. Somerville’s
‘Physical Geography’ (1848) gives in the early pages a brief general
consideration of land forms, and then enters at once upon the areal
description of the continents; later pages present a short outline of
the features of rivers, and then the rivers of the world are taken
up. This is as if a text-book of botany should pass rapidly over the
structure and classification of plants, and devote most of its pages to
the flora of different regions. Again, Klöden’s compendious geography
includes a volume on ‘Physical Geography,’ in which much material
is gathered (3d ed., 1873); but the treatment is very uneven, as is
natural in the absence of a good scheme of classification. Glaciers
receive much attention, but valleys are rather curtly dismissed;
deltas are elaborately described, but little space is given to other
forms assumed by the waste of the land on the way to the sea. Ansted’s
‘Physical Geography’ (5th ed., 1871) contains abundant fact, but much
of it is a kind that is better presented on a map than in verbal form.
Many pages are devoted to statistical statements, from which no student
can gain inspiration for further study, for example: “The Danube
receives a large number of tributaries, of which the most important
are, on the right, the Isar, Inn, Raab, Drave, Save, Morave, and Isker.
On the left are the Altmühl, Regen, Waag, Gran, Theiss, Temes, Aluta,
Sereth, and Pruth. Many of these are large streams with other important
tributaries. The Danube drains upwards of 300,000 square miles of
country.”
A decided advance over earlier books in the way of rational or
explanatory treatment is found in the works of Peschel and Reclus; it
is to the former that a reaction against the historical treatment of
geography in Germany is largely due; while the latter is to be credited
with an enlarged attention to the detail of land forms; but the books
of neither of these authors recognize the systematic evolution of
land forms. The same may be said of various other treatises which
approach, but do not yet reach, the ideal that seems to be in sight.
One of the chief responsibilities of the geographer--the description of
landscape--can not be fully met by students who accept the principles
set forth in these books as their guides; for in spite of the
increasing attention given to the lands in modern books, and in spite
of the greater number of forms recognized, the combination of all forms
in a well-organized whole is not yet accomplished.
It seems to have been against the empirical method of such books as
Ansted’s that Huxley protested in his ‘Physiography,’ urging its
replacement by a more educative method. He wrote:
“I do not think that a description of the earth, which commences by
telling a child that it is an oblate spheriod, moving around the sun
in an elliptical orbit, and ends without giving him the slightest
hint towards an understanding of the ordnance map of his own country,
or any suggestion as to the meaning of the phenomena offered by the
brook which runs through his village, or of the gravel pit whence the
roads are mended, is calculated either to interest or to instruct....
Physiography has very little to do with this sort of Physical
Geography. My hearers were not troubled with much about latitudes
and longitudes, the heights of mountains, depths of seas, or the
geographical distribution of kangaroos or _Compositae_.... I endeavored
to give them ... a view of the ‘place in nature’ of a particular
district of England--the basin of the Thames--and to leave upon their
minds the impression that the muddy waters of our metropolitan river,
the hills between which it flows, the breezes which blow over it, are
not isolated phenomena, to be taken as understood because they are
familiar. On the contrary, I endeavored to show that the application of
the plainest and simplest processes of reasoning to any one of these
phenomena suffices to show, lying behind it, a cause, which again
suggests another; until, step by step, the conviction dawns upon the
learner that, to attain to even an elementary conception of what goes
on in his own parish, he must know something about the universe; that
the pebble he kicks aside would not be what it is and where it is,
unless a particular chapter of the earth’s history, finished untold
ages ago, had been exactly what it was.... Many highly valuable
compendia of Physical Geography, for the use of scientific students of
that subject, are extant; but in my judgment most of the elementary
works I have seen begin at the wrong end, and too often terminate in an
_ominum gatherum_ of scraps of all sorts of undigested and unconnected
information; thereby entirely destroying the educational value of that
study which Kant justly termed the ‘propædeutic of natural knowledge.’”
(Preface to ‘Physiography,’ 1878).
Here we find clear recognition of the need of introducing a
consideration of causes, just as was urged by Guyot; and furthermore a
recognition of the need of linking together in their natural relations
all the items which together constitute the content of the subject.
It may, however, be contended that the attempt to combine in a single
course of study the elementary principles of chemistry and physics,
of geology and astronomy, along with those of physical geography, is
not practicable from an educational point of view; such a combination
will not secure either the clear knowledge or the strong discipline
that can be derived from systematic courses in two or three of these
subjects, presented separately. Text-books like Hinman’s ‘Eclectic
Physical Geography’ and Mill’s ‘Realm of Nature,’ in both of which a
broad range of other than geographical subjects is covered, do not seem
to-day to be in so much favor as those books which attend more closely
to the true content of our subject. Indeed, with respect to physical
geography, considered from the scientific and educational point of
view, a report on College Entrance Requirements, recently published by
our National Educational Association,[D] presents the best definition
and outline of the subject that has yet appeared. It advises the
omission of irrelevant matter, however interesting such matter may be
in itself. The principles of physics and the succession of geological
formations with their fossils, the classification and distribution of
plants and animals must be taught elsewhere; but much profit may be
had from terrestrial phenomena by which the principles of physics are
illustrated, and from the consequences of past geological changes in
determining present geographical conditions, and especially from the
physiographic controls by which the distribution of organic forms is
determined.
[D] Proceedings, 1899, 780-792; also in the _Journal of School
Geography_, September, 1898.
The general scheme under which all land forms may receive explanatory
description must consider chiefly the movement and erosion of the
earth’s crust. Deformation offers a part of the earth’s crust to be
worked upon. Various destructive processes of erosion work upon the
offered mass, and the streams, with their transported waste, follow
the depressions in the carved surface. So important is the element
of erosion, and so leading is the part played by rivers in erosive
work, that McGee would gather all land forms under a classification
determined by their drainage systems.[E] Others have preferred a
classification based, first on peculiarities of structure as determined
by accumulation and deformation; and, secondly, on the progress of
erosion; but in either scheme, the erosive work of rivers is so
important that a sketch of the progress of the physical geography of
the lands towards a systematic classification of its items may well
follow the order in which valleys have been explained, branching
off, as occasion may require, from the leading theme of rivers that
flow under a normal humid climate to special conditions of erosion
under an arid or a frigid climate. The progress which has made the
physical geography of the lands what it is to-day is more the work of
geologists than of geographers; and the chief reason for this is the
indifference of many geographers to the physical side of their subject;
an indifference that was undoubtedly favored by the cultivation of
historical geography in continental Europe, and by the acceptance of
the traveler or explorer as a full-fledged geographer in Great Britain.
In the United States, it is only in the latter part of the century that
the physical geography of the lands has gained a scientific standing,
and the advantages that it now enjoys are geographical grafts upon a
geological stock.
[E] _Nat. Geogr. Magazine_, i, 1889, 27-36.
The emancipation of geology from the doctrine of catastrophism was a
necessary step before progress could be made towards an understanding
of the lands. The slow movements of elevation and depression of certain
coasts in historic time were of great importance in this connection.
Studies of geological structures at last overcame the belief in the
sudden and violent upheaval of mountain chains, which, under the
able and authoritative advocacy of Elie de Beaumont, held a place
even into the second half of the century. But even when it came to
be understood that mountains and plateaus have been slowly upheaved,
it still remained to be proved that the valleys and canyons by which
they are drained were produced by erosion, and not by fractures and
unequal movements of elevation. Advance was here made on two lines.
Along one, a better understanding was gained of the forms producible by
deformation alone; along the other, sea currents, floods and earthquake
waves, to which the earlier observers trusted as a means of modifying
the forms of uplift, were gradually replaced by the slow action of
weather and water. Processes of deformation were found to act in a
large way, producing massive forms without detail--broad plains and
plateaus, extensive domes, straight cliffs and rolling corrugations;
and thus it was learned that the varied and detailed forms of lofty
mountain ranges and dissected plateaus must be ascribed almost
entirely to the processes of erosion. But it should be noted that in
exceptional instances land forms initiated by deformation, so recently
as to have suffered as yet only insignificant sculpture, may exhibit
much irregularity. The most striking example of this kind, an example
of the very highest value in the systematic study of land forms, is
that afforded by the diversely tilted lava blocks of Southern Oregon,
as described by Russell.[F]
[F] 4th _Ann. Rep. U. S. Geol. Survey_, 1883.
Turning now to the second line of advance, it is noteworthy that
so keen an observer as Lesley insisted, as late as 1856, that the
peculiar topographical features of Pennsylvania, which he knew and
described so well, could have been produced only by a great flood.
But the principles of the uniformitarians were constantly gaining
ground against these older ideas; and after the appearance in England
of Scrope’s studies in Central France and of Greenwood’s polemic
little work on ‘Rain and Rivers’ (1857), victory may be said to have
been declared for the principles long before announced by Hutton and
Playfair, which, since then, have obtained general acceptance and
application.
Yet even the most ardent uniformitarians would, in the middle of the
century, go no further than to admit that rain and rivers could roughen
a region by carving valleys in it; no consideration was then given to
the possibility that, with longer and longer time, the hills must be
more and more consumed, the valleys must grow wider and wider open,
until, however high and uneven the initial surface may have been, it
must at last be reduced to a lowland of small relief. The surface of
such a lowland would truncate the underground structures indifferently;
but when such truncating surfaces were noticed (usually now at
considerable altitudes above sea level, as if elevated after having
been planed, and therefore more or less consumed by the erosion of a
new system of valleys), they were called plains of marine denudation
by Ramsay (1847), or plains of marine abrasion by Richthofen (1882).
Today it is recognized that both subaërial erosion and marine abrasion
are theoretically competent to produce lowlands of denudation; the
real question here at issue concerns the criteria by which the work of
either agency can be recognized in particular instances. In the middle
of the century, not only every plain of denudation, but every line of
escarpments was held by the marinists to be the work of sea waves; and
it was not till after a sharp debate that the bluffs of the chalk downs
which enclose the Weald of southeastern England were accepted as the
product of ordinary atmospheric weathering, instead of as the work of
the sea. Whitaker’s admirable essay on ‘Subaërial Denudation,’ which
may be regarded as having given the victory in this discussion to the
subaërialists, was considered so heterodox that it was not acceptable
for publication in the _Quarterly Journal_ of the Geological Society,
of London, but had to find a place in the more modest _Geological
Magazine_ (1867), whose pages it now honors. So signal indeed was this
victory that, in later years, the destructive work of the sea has been
not infrequently underrated in the almost exclusive attention given to
land sculpture by subaërial agencies. Truly, the sea does not erode
valleys; it does not wear out narrow lowlands of irregular form between
enclosing uplands, as was maintained by some of the most pronounced
marinists in the middle of the century; but it certainly does attack
continental borders in a most vigorous fashion, and many are the
littoral forms that must be ascribed to its work, as may be learned
from Richthofen’s admirable ‘Führer für Forschungsreisende’ (1886). As
this problem can not be further considered here, the reader may be at
once referred to the most general discussion of the subject that has
yet appeared, in an essay on ‘Shoreline Topography’ recently published
by F. P. Gulliver.[G]
[G] _Proc. Amer. Acad._, Boston, 1899, 152-258.
At about the time when the subaërial origin of valleys and escarpments
was being established in England, the explorations and surveys of our
western territories were undertaken, and a flood of physiographic
light came from them. One of the earliest and most important of the
many lessons of the West was that Playfair’s law obtained even in the
case of the Grand canyon of the Colorado, which was visited by the
Ives expedition in 1858. Newberry, the geologist of the expedition,
concluded that both the deep and fissure-like canyon and the broader
valleys enclosed by cliff-like walls “belong to a vast system of
erosion, and are wholly due to the action of water.” Although he
bore the possibility of fractures constantly in mind and examined
the structure of the canyons with all possible care, he “everywhere
found evidence of the exclusive action of water in their formation.”
This conclusion has, since then, been amply confirmed by Powell and
Dutton, although these later observers might attribute a significant
share of the recession of cliffs in arid regions to wind action. In a
later decade, Heim demonstrated that the valleys of the Alps were not
explicable as the result of mountain deformation, and that they found
explanation only in river erosion. By such studies as these, of which
many examples could be given, the competence of rivers to carve even
the deepest valleys has been fully established; yet so difficult is it
to dislodge old-fashioned belief that Sir A. Geikie felt it necessary
to devote two chapters in his admirable ‘Scenery of Scotland’ (1887)
to prove that the bens of the Highlands were not so many individual
upheavals, but that the glens were so many separate valleys of erosion;
and as able an observer as Prestwich, a warm advocate of the erosion
of ordinary valleys by their rivers, maintained (1886), with the
results of our western surveys before him, that fissures were probably
responsible for the origin of the deep and narrow canyons of the
Colorado plateau.
The tumultuous forms of lofty mountains ‘tossed up’ as they seem to be
when viewed from some commanding height, are, in by far the greater
number of examples yet studied, undoubtedly the result of the slow
erosion of the valleys between them; but it should not be forgotten
that regions of very recent disturbances--as the earth counts time--may
possess strong inequalities directly due to deformation. The tilted
lava blocks of Oregon have already been mentioned. The bold forms
of the St. Elias Alps, also described by Russell, are regarded by
him as chiefly produced by the tilting of huge crustal blocks on
which erosion has as yet done relatively little work. An altogether
exceptional case is described by Dutton, who says that on the margin of
one of the “high plateaus of Utah a huge block seems to have cracked
off and rolled over, the beds opening with a V and forming a valley
of grand dimensions.” ‘Rift valleys,’ or trough-like depressions
produced by the down-faulting of long, narrow, crustal blocks with
respect to the bordering masses, are occasionally found, as in eastern
Africa, where the ‘Great Rift valley’ has been described by Gregory.
Trough-like depressions of similar origin, but much more affected by
the degradation of their borders and the aggradation of their floors,
are known to European geographers in the valleys of the Saône and of
the middle Rhine. But no rift valley, no depression between the tilted
lava blocks, resembles the branching valleys that are produced by the
erosive action of running water.
Thus far, while much attention had been given to the work of rivers,
little or no attention had been given to the arrangement of their
courses. It seems to have been tacitly assumed that the courses of all
streams were consequent upon the slope of the initial land surface. The
explicit recognition of this origin, indicated by the provision of a
special name, ‘consequent streams,’ was an important step in advance
due to our western geologists. The discovery soon followed that rivers
have held their courses through mountain ridges that slowly rose across
their path; the rivers, concentrating the drainage of a large headwater
region upon a narrow line, cut down their channels as the land was
raised. This idea first came into prominence through Powell’s report
on the Colorado River of the West (1875), in which he gave the name,
‘antecedent,’ to rivers of this class. He believed that the Green
river, in its passage through the Uinta mountains, was to be explained
as an antecedent stream. Much doubt has, however, been thrown upon
this interpretation. Other accounts of antecedent rivers have been
published, and to-day the Green is not so safe a type of antecedence
as the Rhine below Bingen, the Meuse in the Ardennes, or several of
the Himalayan rivers in the gorges that they have cut through the
youngest marginal ridges of the range.