ART. XLV.-Types of Orographic Structure; by Major J. W. POWELL.

[The following pages on "Types of Orographic Structure," are cited from Major Powell's Report on the Geology of the Uinta Mountains." They are preceded in the volume by a brief topographical account of the region between the Sierra Nevada and the Front Range of the Rocky Mountains, and having for its northern boundary, as at present understood, the North Platte and its proper upper continuation, the Sweetwater River. This area is divided into three provinces, namely-passing from east to west,--the Park Province, the Plateau Province and the Basin Province. The first is described as including the great parks of Southern Wyoming, Colorado and Northern New Mexico, and comprises many ranges of mountains which are "characterized by a great development of metamorphic crystalline schists, mostly Archæan in age, with patches and structural basins of marine and lacustrine sediments of later time, and a complicated series of volcanic formations.

The Plateau Province is that directly drained by the Colorado and its tributaries, through which the streams generally run, not in wide valleys, but in cañons; the vast area is strictly a plateau region embracing many table-shaped elevations bounded by cañon and cliff escarpments, and differing in its succession of sedimentary strata from others in North America, and also in its displacements or uplifts; moreover, Cenozoic and Mesozoic rocks prevail, though some of the important plateaus are of Carboniferous beds, and in places the subjacent rocks even down to the Archæan are exposed to view in consequence of erosion.

The Basin Range system includes the Great Salt Lake region, and other valleys and ridges extending through Western Utah, Nevada, Southeastern California and perhaps across the Colorado in Western Arizona; and embraces Paleozoic beds as well as metamorphic schists, along with eruptive beds, some of which are the principal component parts of the ranges.]

It seems convenient to give a general account of the types of orographic structure in the Colorado region before characterizing each province by its special type.

In this discussion I wish to use certain terms with a restricted or relative meaning; i. e., in treating of anticlinal and synclinal flexures I shall speak of those portions of the sedimentary beds which are adjacent to the anticlinal axes as having been upheaved, and those portions near their synclinal axes as having subsided. Again, in blocks which are bounded by faults and tilted, I shall speak of such portions as are at a higher level as having been uplifted, and portions occupying a lower level as thrown. In such cases I do not wish to commit myself to any

theory of upheaval or collapse in the change of the relation of the several parts of these beds to the center of the earth.

In treating of the structure of the mountains under consideration it is necessary to distinguish two great classes, viz: those composed of sedimentary strata, altered or unaltered, and those composed of extravasated material.

MOUNTAINS COMPOSED OF SEDIMENTARY STRATA.

I. Appalachian Structure.

The structure of the Appalachian Mountains, with closely appressed folds and axial planes tipped back from the sea, the modifications of these folds by faults, and the primary and concomitant forms of the mountains, have been clearly explained by the Messrs. Rogers and later writers, and have formed the basis of many discussions concerning geological dynamics. This Appalachian structure needs no further mention here, as it is a type of structure which so far has not been found in the region described above, and should it be found hereafter it will simply be an exceptional type to those known to prevail.

II. Simple Anticlinal Structure.

Mountains or short ranges carved from simple anticlinals are sometimes found, though this type of structure is not a prevailing one. Usually in such a case the great mountain mass lies in the central zone of the uplift. The fold is, of course, always found truncated by erosion, and the mountains represent but the difference between the amount of upheaval and the amount of such erosion. When not complicated by other types of structure the strata dip on all sides from the center of upheaval, gently or more abruptly, but the sides of the folds are never closely appressed. Such mountains in primary form are gently rounded in general outline, modified by the erosion of the streams running down their sides. Sometimes such mountains are severed by rivers running longitudinally, transversely or obliquely through them; the rivers themselves having their sources in regions far away and passing through the mountains in their courses to the sea. In Northeastern Colorado a short distance above the junction of the Snake River with the Yampa, stands Junction Mountain, which serves as a fine illustration of this type of structure. The mountain is divided into two unequal parts by a cañon, through which the Yampa River runs. The axis of the mountain has a north and south direction.

Figure 1 is a section through this mountain, in a north and south direction, along the axis of upheaval. Figure 2 is a section through it in a transverse direction.

heavals, and sometimes such monoclinal ridges are of such magnitude as to be dignified with the name of mountains. Where two or more series of indurated, inclined beds are separated by extensive series of softer material, two or more monoclinal ridges may be formed.

FIG. 1.-Section through Junction Mountain, north and south.
TT

R.W.

5. MILES.

FIG. 2.-Section through Junction Mountain, east and west.

CONCOMITANT FORMS.

1. Monoclinal Ridges on the Flanks.-Under conditions which are so well known as to need no further explanation here, monoclinal ridges or bogbacks are formed on the flanks of such up

East

B. P., Brown's Park; S. C., Sulphur Creek; J. T., Jura Trias; U. A., Upper Aubrey; L. A., Lower Aubrey;
R. W., Red Wall; U., Uinta.

2. Monoclinal Ridges only.-Sometimes we find that an anticlinal upheaval has been eroded in intaglio, so that there is no great central mountain mass, but the axis of upheaval is the site of a valley or low plain, but the monoclinal ridges on the flanks remain.

3. Inclined Plateaus.-Where the anticlinal upheaval has a great amplitude, as compared with the vertical uplift, the beds incline but slightly. Under such conditions inclined plateaus or mesas are found instead of monoclinal ridges, usually having steep escarpments facing the axis of the flexure.

III. Uinta Structure.

In the Uinta Mountains we have a great range carved from an anticlinal upheaval, the axis of which has an easterly and westerly trend, and is more than one hundred and fifty miles in length. It terminates abruptly against the Wasatch Mountains on the west and is cut off by the short, abrupt anticlinal of Junction Mountain on the east, the latter having its axis in a north and south direction. There are several important facts observed in the study of this great flexure. Its axis has been lifted above the level of the sea about thirty thousand feet, and above the level of the adjacent country about twenty-five thousand feet. From flank to flank the flexure is about fifty miles, but varies much in width. We find on either flank, many miles from the axis, a line of maximum flexure, which line presents a subparallelism with the meandering axis. These lines have the effect of two monoclinal flexures in opposite directions, separated by the broad table, diversified by elevated valleys and peaks of which the great mass of the Uinta Mountains is composed. But the portion between these monoclinal flexures or lines of greatest flexure is itself gently flexed. In many places that which I have called the line of greatest flexure is indeed a fault, in one place on the north side of the Uinta Mountains having a throw of twenty thousand feet. On the south side the line of greatest flexure is very irregular, being complicated in some places by faults having uplifts opposed to the downthrow of the flexure. On either side the great displacement is partly by faulting, partly by flexing, and either flank is a zone of diverse displacement where the strata are faulted, flexed, twisted and contorted in many ways.

The character of these displacements in the Uinta Mountains is illustrated in Plates, 1, 2 and 3 of the Atlas, and in a subsequent chapter the subject will be more fully discussed.

The simplest topographic forms, produced by such displacements under conditions of erosion in general outline, are plateaus with gently rounded summits and abrupt shoulders on the flanks; but such general outline is often modified by the

corrasion due to antecedent or superimposed drainage; that is, by the corrasion of streams that head in remote regions and pass through these uplifts either longitudinally, transversely or obliquely, as in the case of Simple Anticlinals.*

There are other modifications which sometimes greatly obscure the general topographic outline due to consequent drainage, i. e., the local drainage which is due to the upheaval itself and which produces interesting

CONCOMITANT FORMS.

1. Subsidiary Plateaus.-Sometimes the streams which head near the axis of such an upheaval, as they meander to the flanks, excavate valleys and divide the great block, which is a plateau in general outline, into minor plateaus which are separated by intervening but elevated valleys. This is especially the case where the streams in their upper courses follow for some distance the strike of the beds before turning to cross the more or less abrupt lines of maximum flexure. Sometimes these streams run in deep gorges; in such cases the plateaus are bounded by cañons.

2. Projecting Ridges.-When these consequent streams starting near the axis of upheaval take a somewhat direct course across the strike, the general plateau is cut into a series of sharp, abrupt ridges having a trend at right angles to the strike or general axis of upheaval. Thus the points of the ridges face the plain below and are separated by deep gulches and cañons, and the observer on the plain below sees before him what appears to be a line of peaks separated by intervening gulches and valleys, and is apt to misunderstand the topographic character of the great mass which is before him.

In

3. Axial Peaks. At some stages in the progress of erosion the channels of consequent drainage inosculate, and about their heads gorges are formed, with towering amphitheaters. such cases an irregular line of crags and peaks will be found along the axis of upheaval. These I call axial peaks.

4. Flanking Peaks.-Sometimes we find a very hard bed or group of beds underlaid by more friable strata on a flank of the upheaval, which harder beds have been carried away by erosion from those portions of the upheaved mass nearer the axis. In such cases each projecting ridge is crowned with a true peak. I call these flanking peaks.

5. Interrupted Monoclinul Ridges.-On the flanks of these upheavals, but farther from the axis than the flanking peaks, monoclinal ridges are often found sometimes broken by gaps

*For an explanation of what is meant by antecedent and superimposed drainage, the reader is referred to the Report on the Exploration of the Colorado River and its Tributaries, page 160, et seq.