CHAPTER III.
DETAILED DESCRIPTION OF THE MOUNTAINS.
MOUNT ELLSWORTH.
It has already been stated that the strata about the bases of the Henry Mountains are nearly level; but the country which is built of them is far from level. The arrangement of the drainage lines has caused the degradation of some parts to greatly exceed that of others, so that while the district at the south, which borders the Colorado River, is paved with the red sandstones of the Jura-Trias series, the adjacent region at the north still carries the yellow sandstones and blue shales of the Cretaceous series. All about Mount Ellsworth are the upper strata of the Jura-Trias. The lower beds of the same series rise upon its flanks and arch over its summit.
A description of the structure of the mountain must include, first, the arch of the strata; second, the faults which modify the arch; third, the system of trachyte dikes and trachyte sheets; and fourth, the sculpture of the mountain. In its general proportions the arch is at once simple and symmetrical. From all sides the strata rise, slowly at first, but with steadily increasing rate, until the angle of 45° is reached. Then the dip as steadily diminishes to the center, where it is nothing. A model to exhibit the form of the dome would resemble a round-topped hat; only the level rim would join the side by a curve instead of an angle, and the sides would not be perpendicular, but would flare rapidly outward (see Figure 11). The base of the arch is not circular, but is slightly oval, the long diameter being one-third greater than the short. The length of the uplift is a little more than four miles; the width a little more than three miles, and the height about 5,000 feet. The curvature fades away so gradually at its outer limit that it is not easy to tell where it ends, and the horizontal dimensions assigned to the dome are no more than rude approximations. But there is another element which can be given more exactly. The line of maximum dip, which separates the convex upper portion of the dome from the concave periphery, is easily traced out in nature, and runs at the foot of the steep part of the mountain. It surrounds an area two miles in width and two and two-thirds miles in length.
Fig. 11a.—View from the west spur of Mount Ellsworth, showing the trachyte dikes of the north spur and revetments of sandstone and trachyte.
Fig. 11.—Stereogram of Mount Ellsworth; an ideal restoration of the form of the overarching strata.
The Ellsworth arch is almost but not completely isolated. The Holmes arch, upon the east side, stands so near that the bases of the two impinge and coalesce, and the same thing happens, though less notably, with the Hillers arch at the north.
The simplicity of the arch is further impaired by faults—not great faults dividing the whole uplift, but a system of small displacements which are themselves subordinate phenomena of the uplift. They are restricted to the central portion, and never occur so low down as the line of maximum dip. The strata of the upper part of the arch are divided into a number of prismoid blocks which stand at slightly different levels but are not sufficiently deranged to destroy the general form of the arch. The greatest throw is only a few hundred feet. All or nearly all of the fault planes are occupied by dikes of trachyte.
The trachyte injections are not confined to the fault planes, nor is their area so restricted as the fault area. Dikes and sheets abound from the crest of the dome down to what might be called its springing line—the line of maximum dip. At the center, dikes are more numerous; near the limit, sheets. The central area is crowded so full of dikes, and the weathering brings them so conspicuously to the surface, that the softer sedimentaries are half concealed, and from some points of view the trachyte appears to make the entire mass. The accompanying plan (Figure 12) shows the arrangement of the dikes in one of the outer amphitheaters of the mountain, where they are less complicated than in the central region. The trends of two spurs (a b and c d) are indicated by the hatchings. They join the main crest of the mountain at e, and inclose between them a deep amphitheater which opens to the west. Upon the steep walls of the amphitheater the dikes outcrop in lines of crags, dividing rough slopes of yellow and purple and brown sandstone. The profile of one of the walls of the amphitheater (from a to b, Figure 12) is drawn in Figure 13 for the sake of exhibiting the relation of the dikes to faulted blocks of sandstone. It will be seen that the throw of the faults is not constantly in one direction.
Fig. 14.—Profile of the Northern Spur of Mount Ellsworth. 1, 2, 3, 4, and 5 are Trachyte Dikes. A, Aubrey Sandstone. S, Shinarump Shale. V, Vermilion Cliff Sandstone. G, Gray Cliff Sandstone.
The zone of sheets is just inside the line of maximum dip. Usually only one or two sheets are laid bare by the erosion, but at one point (see Figure 14) four can be counted. Toward the center of the uplift all of these are limited by the erosion and exhibit their broken edges. Downward, or toward the periphery, they dip out of sight. Laterally they can be traced along the mountain side for varying distances, but they soon wedge out and are replaced by others en echelon. In thickness the sheets rarely exceed 50 feet, and never 100. They are always thin as compared to the rock masses which separate them, but, by reason of their superior ability to resist erosion, monopolize a large share of the surface, and mask a still greater amount with their débris.
Fig. 12.—Ground plan of Trachyte Dikes on the western flank of Mount Ellsworth.
Fig. 13.—Profile of a western spur of Mount Ellsworth, showing the arrangement of Dikes and Faults. The dotted bed is the purple band at the top of the Vermilion Cliff Sandstone. The dikes of trachyte are indicated by letters.
The sedimentary rocks are not altered beyond the region of trachyte intrusion. The mere flexure of the strata was not accompanied by a perceptible change of constitution. In the zone of sheets there is little change except along the surfaces of the contact. For a few feet, or perhaps only a few inches, there is a discoloration (usually a decolorization) and a slight induration, without notable alteration of minerals. But in the region of reticulated dikes none of the sedimentaries are unchanged; crystals are developed, colors are modified, and hardness is increased, so that the physical properties of familiar strata no longer serve for their identification. Still there is no crumpling.
The trachyte masses and the altered rocks in contact with them are so much more durable than the unaltered strata about them that they have been left by the erosion in protuberances. The outcrop of every dike and sheet is a crag or a ridge, and the mountain itself survives the general degradation of the country only in virtue of its firmer rock masses. Nevertheless, the mountain, because it was higher than its surroundings, has been exposed to more rapid erosion, and has been deprived of a greater depth of strata. From the base of the arch there have been worn 3,500 feet of Cretaceous, and from 500 to 1,500 feet of the Jura-Trias series, which is here about 3,000 feet thick. From the summit of the arch more than 2,500 feet of the Jura-Trias have been removed.
The strata exposed high up on the mountain being older than those at the base, and the dip being everywhere directed away from the center, it is evident that the mountain is surrounded by concentric outcrops of beds which lift their escarpments toward it. It is usually the case, where the strata which incline against the flank of a mountain are eroded, that the softer are excavated the more rapidly, while the harder are left standing in ridges; and an alternation of beds suitable for the formation of a ridge occurs here. One of the upturned beds is the massive Vermilion Cliff sandstone, and beneath it are the shales of the Shinarump Group. By the yielding of the shales the sandstone is left prominent, and it circles the base of the mountain in a monoclinal ridge. But the ridge is of a peculiar character, and has really no title to the name except in the homology of its structure with that of the typical monoclinal ridge. It lacks the continuity which is implied by the word “ridge”. The drainage of Mount Ellsworth is from the center of the dome outward. A half dozen drainage lines originate in the high crests and pass outward through the zone of upturned strata. Lower down their interspaces are divided by others, and when they reach the circling escarpment of the Vermilion sandstone their number is fifteen. Each of these cuts the ridge to its base, and the effect of the whole is to reduce it to a row of sandstone points circling about the mountain. Each point of sandstone lies against the foot of a mountain spur, as though it had been built for a retaining wall to resist the out-thrust of the spur. Borrowing a name from the analogy, I shall call these elementary ridges revet-crags, and speak of the spurs which bear them as being revetted. The accompanying sketch is designed to illustrate the structure, but is not drawn from nature. In the view of Mount Hillers (Figure 27) the revetments may be seen, and in the bird’s-eye view of the Henry Mountains (Plate V), as well as in the Frontispiece, the revet-crags of Mount Ellsworth also are portrayed. The diagram of the north spur of Mount Ellsworth (Figure 14) shows the revet-crag of that spur at V.
Fig. 15.—Revet-Crags.
The revet-crags of Vermilion sandstone follow, in a general way, the line of maximum dip about the base of Mount Ellsworth; but a few of them rise higher, and one—that which joins the northwest spur—climbs until it is but little lower than the summit of the mountain. Outside the circle of Vermilion Cliff sandstone lies the Gray Cliff sandstone, and in a few places it takes the form of a revetment. Inside the same circle there are many revetments, constituted by trachyte sheets bedded in Shinarump shales (Figure 14). Conforming perfectly with the strata, the sheets yield by erosion forms which are identical with those afforded by hard sedimentary beds, and to the distant eye the impression of the arching structure of Mount Ellsworth is conveyed less by what can be seen of the strata than by ascending revetments of trachyte sheets, which simulate and interpret the strata.
Fig. 16.—Mount Holmes, from the north.
The laccolite of Mount Ellsworth is not exposed to view, but I am nevertheless confident of its existence—that the visible arching strata envelop it, that the visible forest of dikes join it, and that the visible faulted blocks of the upper mountain achieved their displacement while floated by the still liquid lava. The proof, however, is not in the mountain itself, but depends on the association of the phenomena of curvature and dike and sheet with laccolites, in other mountains of the same group. In the sequel these will be described, but it chances that the mountain next to be considered is even less developed by erosion than Mount Ellsworth.
MOUNT HOLMES.
The order of sequence which places Mount Ellsworth before Mount Holmes is the order of complexity. The former contains one laccolite, the latter two. Neither of the two is visible, but the strata which envelop them shadow forth their forms and leave no question of their duality. They are so closely combined that the lesser seems a mere appendage of the greater. From the center of the greater there is a descent of strata in all directions, but from the center of the lesser the rocks incline toward one-half only of the horizon. Where the two convex arches join there is a curved groin—a zone of concave curvature uniting the two convexities. About the compound figure can be obscurely seen a line of maximum dip, and beyond that the fading of the curves. The curves throughout are so gentle that it was found exceedingly difficult to establish their limits. In a general way it may be said that each of the Holmes arches is as broad as the Ellsworth arch, but the vertical displacement is less. In the formation of the greater Holmes arch the amount of uplift was 3,000 feet; for the lesser arch, 1,500 feet.
There is no evidence in the forms of the arches which proves one to be older than the other. Studying the curves in the field, I could not discover that either arch asserted itself more strongly than the other in their common ground. They seem to meet upon equal terms. Still it is probable, a priori, that they were formed successively and not simultaneously. The coincidence in time of two eruptions of lava from neighboring vents is no more unlikely than the coincidence of the two irruptions, and the same principle of least resistance which causes individual laccolitic arches to assume spheroidal forms, would have given to the compound arch of two laccolites, coincident in time, a simple instead of a compound form.
Assuming that the arches were successive in origin I shall in another and more appropriate chapter discuss the problem of their chronological order in the light of their somewhat peculiar drainage system.
The lesser arch betrays no dikes nor sheets. The Vermilion Cliff sandstone covers it to the top. The greater is crowned by a few grand dikes which govern its topography. From the center a long dike runs to the south, a short one to the north, two to the east, and one to the west. The course of each is a mountain spur, and between them are amphitheaters and gorges. Clinging to the dikes are bodies of altered sandstone, but the great sandstone masses of the summit were unaltered and from them have been excavated the gorges. Along the dike-filled fissures there has been some faulting, but there is no reason to believe that the displacement is great in amount. Toward the flanks of the mountain there are a few sheets, the outermost of which is far within the line of maximum flexure. Their escarpments instead of facing upward like the revetting sheets of Mount Ellsworth, face downward; their buried and unknown edges are the edges toward the mountain. Their thinning toward the periphery of the arch is conspicuous to the eye in many instances, as is also the thinning of the dikes.
Another peculiarity of dike form, one which has since been noted in a number of localities, was first detected in Mount Holmes. It consists in a definite upper limit. The dike so marked is often as even upon its upper surface as an artificial stone wall. The upper surface may be level or may incline toward one end of the dike, but in either case it is sure to be found parallel to the bedding of the strata which inclose the dike. This fact led to the suspicion, afterward confirmed by more direct evidence, that the flat top of the dike was molded by an unbroken stratum of rock bridging across the fissure which the lava filled (Figure 20). The converse phenomenon can be observed in the ridge which joins Mounts Ellsworth and Holmes. A great dike there forms the crest of the ridge for half a mile, its base being buried in sandstone; but at the end of the ridge the strata are seen to be continuous beneath it (Figure 21).
Fig. 17.—Stereogram of the Holmes Arches; an ideal restoration of the form of the overarching strata.
Fig. 18.—Ideal cross-section of the Laccolites of Mount Holmes.
Fig. 19.—A flat-topped dike.
Fig. 20.—Ideal Cross-section of Flat-topped Dikes; a, before denudation; b, after denudation.
Fig. 21.—Ideal Cross-section of a Flat-bottomed Dike.
Fig. 22.—Diagram to illustrate a hypothetical explanation of Flat-edged Dikes.
That a fissure several feet or several scores of feet in width should end thus abruptly, demands explanation, and the phenomena immediately concerned offer none. Nevertheless it is easy to make an assumption which if true renders both cases clear. If we assume that the fissure instead of ending at the crosshead is merely offset, and resumes its course beyond, and that the dike contained in it has two bodies connected by a thin sheet (Figure 22), we shall have no difficulty in conceiving the erosion which will produce either of the natural appearances described.
The rocks which constitute Mount Holmes are the same as those about its base. The Vermilion Cliff and Gray Cliff Sandstones alone appear in the crests. The underlying Shinarump shales are cut by the erosion at a few points only, and those are near the base. For this reason the Vermilion Sandstone is not undermined about the base, and the circle of revet-crags which surrounds Mount Ellsworth finds no counterpart. There are, indeed, a few revetments of Gray Cliff sandstone, but they are scattered and for the most part inconspicuous.
In the general view of Mount Holmes (Figure 16), one of the main dikes crowns the nearest spur, and another the spur leading to the right. At the left are minor dikes, and high up is a trap sheet notched on its lower edge. At the left base of the mountain lies the lesser arch.
Figure 23 gives a section exhibited by one of the northward cañons. It shows one of the faults of the upper part of the arch and illustrates the thinning of the sheets as they descend.
Fig. 23.—Section shown in a northward cañon of Mount Holmes. a, Vestige of Trachyte sheet. b b b, Trachyte sheets. c, Trachyte dike. 1, Gray Cliff Sandstone. 2 2, Purple Sandstone. 3, Vermilion Cliff Sandstone. 4, Shinarump Shale.
MOUNT HILLERS.
Next in order to the north is Mount Hillers. Let it not be supposed, however, that there is discernible system in the geographic arrangement of the mountains or of the laccolites. A chart of the mountain peaks and a chart of the laccolites would alike prove intractable in the hands of those geologists who draw parallel lines through groups of volcanic vents by way of showing their trend. They are as perfectly heterotactous as they could be made by an artificial arrangement.
The diagram (Figure 24) shows the relation of the laccolite groups to each other and to the meridian. The principal mountain summits are indicated by triangles, and the curved lines inclose areas of disturbance.
Mount Hillers and its foot-hills are constituted by a group of no less than eight laccolites, and a ninth, the Howell laccolite, is conveniently classed with them, although not contiguous.
Fig. 25.—Cross-section of Mount Hillers.
Fig. 26.—The same, with ideal representation of the underground structure.
Scale, 1 inch = 4,000 feet. 1. Tununk Sandstone; 2. Henry’s Fork Conglomerate; 3. Gray
Cliff Sandstone. The full black lines represent trachyte sheets, and the broad black area the
Hillers Laccolite.
Fig. 27.—Mount Hillers, from the south.
The Hillers laccolite is the largest in the Henry Mountains. Its depth is about 7,000 feet, and its diameters are four miles and three and three-quarter miles. Its volume is about ten cubic miles. The upper half constitutes the mountain, the lower half the mountain’s deep-laid foundation. Of the portion which is above ground, so to speak, and exposed to atmospheric degradation, less than one-half has been stripped of its cover of overarching strata. The remainder is still mantled and shielded by sedimentary beds and by many interleaved sheets of trachyte. The portion which has been uncovered is not left in its original shape, but is sculptured into alpine forms and scored so deeply that not less than 1,000 feet of its mass are shown in section. All about the eroded (south) face of the mountain the base is revetted by walls of Vermilion and Gray Cliff sandstone, strengthened by trachyte sheets. At the extreme south these stand nearly vertical (80°), and their inclination diminishes gradually in each direction, until at the east and west bases of the mountain it is not more than 60°. On the north side there are no revet-crags, and the inclination is comparatively slight. It would appear that the laccolite was asymmetric, and was so much steeper-sided on the south that that side suffered most rapid degradation.
Fig. 24.—Ground Plan of the Henry Mountains. The curved lines show the limits of the principal displacements; the triangles, the positions of the main peaks. N, Mount Ellen. P, Mount Pennell. H, Mount Hillers. M, Mount Holmes. E, Mount Ellsworth.
By reference to the section (Figure 25) it will be seen that the sedimentary strata of the north flank stretch quite to the summit of the mountain. The same beds which form the revet-crags on the southern base constitute also some of the highest peaks. Since these rest directly upon the laccolite, it is assumed that the next lower beds of the stratigraphic series form its floor; and the base of the laccolite is drawn in the ideal section on the level which the Shinarump Group holds where it is unaffected by the displacements of the mountain.
It is noteworthy that wherever the sedimentaries appear upon the mountain top they are highly metamorphic. But in the revet-crags there is very little alteration. Massive sandstone, divided by sheets and dikes several hundred feet in thickness, is discolored and indurated at the contact surface only, and ten feet away betrays no change.
The engraving of Mount Hillers (Figure 27) exhibits the south face with its revet-crags and bold spurs of trap. In nature the effect is heightened by the contrast of color, the bright red revetments being strongly relieved against the dark gray of the laccolite. The strata of the summits cannot be discriminated at a distance. They are too near the laccolite in hardness to differ from it in the style of their sculpture.
Of the minor laccolites of the cluster there are three so closely joined to the chief that they merge topographically with the mountain. They are not well exposed for study. The smallest, which overlooks the pass between Mount Hillers and Mount Pennell (A, Figure 28) has probably lost the whole of its cover and with it so much of its substance that the original form and surface cannot be seen. Its floor is probably the Tununk sandstone. East of it and lying a little deeper in the Cretaceous series is a second laccolite (B), broader, lower, and less eroded. The third (C) joins the great one on the northeast, and is so closely united that it was at first supposed to be the same body. Later examinations have shown, however, that its immediate roof is the Henry’s Fork conglomerate, and its horizon is thus established as more than two thousand feet above that of its great companion.
Fig. 30.—The Steward Laccolite.
Fig. 28.—Diagram of the Hillers Cluster of Laccolites; Ground plan.
Fig. 29.—Diagram of the Hillers Cluster of Laccolites; Elevation.
The upper horizontal line marks the base of the Cretaceous; the middle, the base of
the Jura-Trias; the lower, the level of the sea.
The Steward laccolite (Figure 30) is better exposed for study. It was buried in the soft bad-land sandstone of the Flaming Gorge Group, and its matrix has been so far washed away that nearly the whole body of trap is revealed. It is weathered out, like a chert-nodule on the face of a block of limestone. At one end it is bared quite to the base, and the sandstone floor on which it rests is brought in sight. The waste of the sandstone has undermined its edge, and a small portion of the laccolite has fallen away. Near the opposite end a fragment of its cover of arching sandstone survives—just enough to indicate that the sedimentaries were once bent over it, and that the smooth low-arching surface which now crowns it portrays the original form which the molten lava assumed. The laccolite is about two and a half miles long and one and one-half broad. The height of its eastward face, where it is sapped by the erosion of the bad-land rock, is six hundred feet, and the central depth must be more than eight hundred feet.
Fig. 31.—Cross-section of the Pulpit arch, with ideal representation of the Pulpit Laccolite. Scale, 1 inch = 3,500 feet. 1, Vermilion Cliff Sandstone. 2, 2, Gray Cliff Sandstone. 3, 3, Flaming Gorge Shale. 4, Henry’s Fork Conglomerate. 5, Tununk Shale.
Pulpit arch is as high and as broad as the lesser arch of Mount Holmes, but its place is not marked in the topography by an eminence for the reason that the degradation of the land has not yet progressed so far as to unearth its core of trachyte. The drainage from Mount Hillers crosses it from west to east and has given it an oblique truncation, as illustrated in the diagram. At the upper end of the slope the Henry’s Fork conglomerate outcrops; at the lower end the base of the Flaming Gorge series, and in the interval the Gray Cliff sandstone is lifted to the surface. The same streams which planed away the crown of the arch have now cut themselves deeper channels and divide the massive sandstone by picturesque cañons, between which it is grotesquely carved into pinnacles and ridges. A curious salient of the sandstone has given its name to the arch. How deeply the Pulpit laccolite lies buried is not known, no sheet nor dike of trachyte betraying its proximity. The valley of the Colorado may have to be deepened thousands of feet before it will be laid bare.
The Jerry Butte is the most conspicuous adjunct to Mount Hillers and topographically is more important and striking than the features which have just been described, but its structure is less clear. Its crest is formed by a great dike several hundred feet in width and two miles in length, and with an even top like those observed on Mount Holmes. The western end of the dike is the higher and forms the culminating point of the butte, and from it there radiate three other dikes of notable size. The inclosing strata, preserved from erosion only by the shelter of the dikes, are the lower portion of the Cretaceous series, and they are so little lifted above their normal level that there is room for no considerable laccolite beneath them. The inclination of the beds is so complicated by the dips of the Pulpit, Steward, and Hillers arches, all of which are contiguous, that nothing can be made out of the form of the laccolite, if it exists.
The Howell laccolite lies apart from the cluster and is well exposed. It differs from all that have been enumerated in its extreme thinness. With a breadth of more than two thousand feet, it has a depth of only fifty. Seen from the east, it might readily be mistaken for a coulée, for on that side it is the thin, hard, black cap of a table carved out of soft, sandy shale (Flaming Gorge Group) by circumdenudation. But followed westward, the table is found gradually to lose its height by the rising of the adjacent land, and at last the lava-bed runs into the slope and disappears beneath the upper layers of the same sandy shale on which it rests. How far it extends under ground can only be conjectured. How far it originally stretched in the opposite direction cannot be known because it is broken away. The original edge is concealed at one end and has been undermined and destroyed at the other, so that the only place where it can be seen is the point at which it emerges from the shale. At this point, where erosion has bared but has not yet attacked the lava, the form and character of the edge are exhibited. In place of the tapering wedge which usually terminates intrusive sheets, there is a blunt, rounded margin, and the lava scarcely diminishes in depth in approaching it. The underlying strata, locally hardened to sandstones, lie level; the overlying curve downward to join them, and between the curved strata is interleaved a curved lava-sheet. In all these characters the intrusive body is affiliated with the typical laccolites, and distinguished from the typical sheets.
Fig. 32.—The Howell Laccolite, as seen from the north.
Fig. 33.—The Edge of the Howell Laccolite.
Fig. 34.—Mount Pennell, from the west.
The laccolite which is marked “D” on the diagrams of the Hillers cluster is identical in nearly all its characters with the Howell; it is thin and broad. One margin is wasting as its foundation is sapped; the other is hidden from view. Its depth does not diminish toward the edge. Moreover, a few dikes issue from its margin, or from beneath its margin. It was intruded within the same formation as the Howell (the Flaming Gorge shale), but at a horizon several hundred feet higher; and, what is specially noteworthy, the rock of which it is composed is identical in facies with that of the Howell laccolite, and notably different from all others which were observed in the Hillers cluster.
Thus there are grouped in this one cluster laccolites of the most varied character, differing in form, in magnitude, in the stratigraphic depth at which they were intruded, in the extent to which they have been uncovered or demolished in the progress of erosion, and also, but very slightly, in their lithologic characters. The greatest is one thousand times as bulky as the least. The length of the most obese is three times its depth; the length of the most attenuated is more than one hundred times its depth. The one highest in the strata lies a thousand feet above the base of the Cretaceous rock series; the lowest is not higher than the summit of the Carboniferous. The latter has not yet been touched by erosion, others have been completely denuded, and some have been partially demolished and removed.
MOUNT PENNELL.
Mount Pennell and Mount Ellen are distinct mountain masses separated by a low pass, but there is no interval between the clusters of laccolites by which they are constituted.
Whether the site of a laccolite shall be marked by a mountain depends in great measure on the relation of the laccolite to the progress of erosion. In the Henry Mountains the laccolites which have not been reached by the denudation scarcely affect the topography. The arched sedimentaries above them are no harder than the same strata in the surrounding plain, and they are brought substantially to the same level. It is to those which the downward progress of erosion has reached and passed that the mountains are due. In virtue of their hardness they survive the general degradation, and conserve with them broad foundations of more perishable material. Mounts Ellen and Pennell mark the positions of the highest of a great cluster of laccolites, and the pass between them marks a part of the cluster where all the laccolites lie low in the strata.
Fig. 35.—Cross-section of Mount Pennell. 1, Blue Gate Sandstone. 2, 2, Blue Gate Shale. 3, 3, Tununk Sandstone. The full black lines represent Trachyte.
Mount Pennell is not so easily studied as the lower mountains at the south. Its summits are timbered and are carved into alpine forms which do not portray the structure but rather mask it. Upon its flanks however the topography and the structure are in so close sympathy that the latter is easily read; and by their study some of the general features of the mountain have been made out. From the east and south and west the strata can be seen to rise toward it. The uprising strata on the west are the Tununk sandstone and the shales above and below it. The sandstone forms no revetments, but accords so closely in its dips with the slopes of the mountain that it is the surface rock over a broad area, outcropping wherever there is rock exposure. At the head of the foot-slope trachyte sheets are associated with it, some overlying and others underlying it; and at one point a gorge reveals a laccolite not far below it—a laccolite that may or may not be the great nucleus of the mountain. On the west and south flanks the uprising strata are the Blue Gate and Tununk sandstones, with their shales. The Gate sandstone has been worn away nearly to the foot of the slope, and forms a monoclinal ridge circling about the base. The ridge is interrupted by a number of waterways, and it sends salients well up upon the flank, but it is too continuous to be regarded as a mere line of revetments. The Tununk sandstone is not to be seen without search, being covered by heavy trachyte sheets. Trachyte sheets also underlie it, and the whole are carved into a conspicuous series of revet-crags.
Fig. 36.—Mount Pennell, from the north.
The association of overarching strata with sheets of trachyte leaves no doubt in my mind that the core of Mount Pennell is laccolitic, but whether it is simple or compound is not so clear. The collation of all the observed dips shows that if there be but one laccolite it has not the simplicity of form which usually characterizes them.
There are low arches of the strata at the southern, northern, and northeastern bases, which reveal no trachyte and give the impression that there may be a foundation of low-lying laccolites upon which the main trachyte mass or masses of the mountain are based. One of these low arches, that at the northeast, is shown in the foreground of Figure 36. The strata which portray it are of the Henry’s Fork conglomerate, and the laccolite which they cover is of necessity distinct from that which is revealed on the east flank of the mountain.
Fig. 37.—Sentinel Butte.
In addition to the laccolites of the foundation and of the main body, there is a series which jut forth from the northern flank like so many dormer windows. They are comparatively small but are rendered conspicuous by the removal of the soft rocks which originally inclosed them. They are higher in the strata than any other observed laccolites, their position being above the Tununk sandstone and in the Blue Gate shale. The core of the main body of the mountain is probably inclosed in the Tununk shale, and the laccolites under the low arches (at the north at least) are entirely below the Cretaceous. The higher series stand on top of the low arches, and are just outside of the sheets which inclose the central body. The largest of them constitutes Sentinel Butte, and stands guard over Penellen Pass. Sapped by the yielding of its soft foundation it is rapidly wasting, and on three sides its faces are precipitous. The huge blocks which cleave from it as they are undermined strew the surrounding slopes for a great distance. Its depth of 400 feet is made up of two layers, of which the lower is the deeper, and between which there is a slight lithologic difference. The upper surface of the butte is smooth and plane with an inclination to the south. It is probably a portion of the original surface of the laccolite.
Thus we have in Mount Pennell a great central body consisting of a single laccolite or of a number closely massed together; an inferior group of three or more, evidenced by low broad arches; and a superior group of not less than four, all of which are partially destroyed.
MOUNT ELLEN.
The crest of Mount Ellen is as lofty as that of Mount Pennell and it is more extended. It stretches for two miles from north to south and is buttressed by many spurs.
The sculpture of the crest is alpine, and the structure is consequently obscured. There are dikes of trachyte and perhaps the remnants of laccolites; there are Cretaceous sandstones greatly indurated; and there are Cretaceous shales baked to clinking slate; but they are all carved into smooth, pyramidal forms, and each is half hidden by the débris from the rest so that their order and meaning cannot be seen.
The flanks however are full of interest to the geologist. The Ellen cluster of laccolites is a broad one, and all but the central portion is well exposed for study. In the spurs and foot-slopes and marginal buttes no less than sixteen individual laccolites have been discriminated, a number of them most beautifully displayed. They will be enumerated in the order of their position, beginning at the west of Penellen Pass and passing along the western, northern, and eastern flanks to the Scrope Butte, east of the pass.
Fig. 38.—Ground Plan of the Ellen Cluster of Laccolites.
In the chart of the Ellen cluster, Figure 38, an attempt is made to show the horizontal grouping of the laccolites and the order of their superposition wherever they overlap. It will be observed that where the limits are imperfectly known the outlines are left incomplete, and that the central area remains blank, not because it contains no trachyte masses, but because its alpine sculpture has prevented their study. Where it is not evident which of two encroaching laccolites is the superior, they are separated by a straight line.
Fig. 40.—Section of the Lewis Creek Cañon through the Newberry Arch. 1, Henry’s Fork Conglomerate. 2, Upper portion of Flaming Gorge Shale. 3, Newberry Laccolite.
The line a a in the chart is the springing line of a broad, flat arch which underlies all the other arches of the western flank. If it covers but one laccolite, that one is the rival in magnitude of the Hillers nucleus, although widely different in proportions; but it is more probable that it contains a greater number. The upper surface is rolling and uneven, and has not the degree of symmetry which laccolites usually display. At the points b, c, d, and e the altitude of the arch is 3,800, 3,500, 2,000 (estimated) and 2,500 feet. Nevertheless there is nothing in its simple outline to indicate a compound structure; the monoclinal ridges by which it is margined do not exhibit the flexuous curves which are commonly seen about the bases of confluent arches. Whether the nucleus is simple or compound it sends no branches to the surface; the only outcrops of trachyte belong to the overlying arches. The upper parts of the arch have been so carried away that the steepness of the mountain flank is not increased by it, and inferior strata are brought to the surface. At the north the crown of the arch bears the Henry’s Fork conglomerate, while beyond its base the plateau is built of Blue Gate sandstone. At the south the arch bears the Tununk sandstone, and the Masuk lies outside.
Fig. 39.—The Newberry Arch and Laccolite.
The laccolite marked “E” on the chart rests upon the Tununk sandstone. The Blue Gate shale which once buried it has been all washed away except some metamorphosed remnants upon the top, and the trachyte itself has wasted to such an extent that its original form cannot be traced. The laccolite has no near neighbor, and the erosion has left it prominent upon the mountain flank. A continuous and solitary spur joins it to the central ridge.
The Newberry laccolite makes a knob 1,700 feet high, and stands by itself. Its cover of Henry’s Fork conglomerate is re-enforced by a number of trachyte sheets, and is broken through at one point only. At that point Lewis Creek cuts with a straight course across a flank of the arch, and exposes a portion of the nucleus in section (Figures 39 and 40). The conglomerate does not rest directly upon the trachyte, but is separated by one or two hundred feet of shale of the Flaming Gorge Group.
Fig. 41.—The Geikie Laccolite (G), overlapped by the Henry’s Fork Conglomerate (H H), and the Shoulder Laccolite (S).
Two miles to the northward is the Geikie laccolite, smaller than the rest but similar in character. It lies close to the top of the Flaming Gorge shale, and is enwrapped by the Henry’s Fork conglomerate. Upon three sides the conglomerate can be seen to curve over its margin from top to bottom, and upon two of these sides the curves are so broken through by erosion that the trachyte is visible within. On the north two cañons cut down to the nucleus, and on the south there is a broad face of trachyte framed all about by the cut edges of the conglomerate beds.
The Shoulder laccolite overlaps the Geikie, and the two are exposed in such manner as to show their relation in section. The conglomerate runs under the one and over the other and separates them. The upper laccolite is a broad and deep one, and takes its name from the fact that it makes a great shoulder or terrace on the mountain side. Toward the mountain it is buried; toward the valley it is uncovered, and in part bounded by a cliff. It is deeply cleft by cañons. It has not been subjected to measurement, but its depth is not overestimated at fifteen hundred feet nor its area at five square miles.
In the sketch (Figure 42) many of these features can be traced—the Geikie laccolite at the left and the Shoulder in the center, with an outcrop of the conglomerate curving down from the roof of the one to the floor of the other, and the Newberry arch in the foreground with its cleft side. At the rear is the pyramidal Ellen Peak and the F laccolite, overlooking the Shoulder. In the distance at the left is the Marvine laccolite.
Little more can be said of the F laccolite than that it exists and is the nucleus of a lofty spur. Its summit rises too nearly to the crest of the mountain to be well defined, and at its base the sedimentaries are hidden by talus.