Geological Survey Professional Paper 160 Geologic History of the Yosemite Valley

THE SIERRA BLOCK

GENERAL CHARACTER AND OUTLINES

Most mountain ranges are carved from great wrinkles or wavelike folds in the outer crust of the earth, produced by the buckling of originally flat-lying strata. Both of the ancient mountain systems that formerly occupied the site of the Sierra Nevada were of that folded type, but the present range consists essentially of a single massive block of the earth's crust that has been dislocated and tilted toward the southwest, apparently without suffering any appreciable bending or warping. (See fig. 1.) The Sierra Nevada is therefore properly termed a "block range."

FIGURE 1.—Generalized diagram of part of tilted Sierra block. The great fault fractures that separate the Sierra block from the Owens Valley block, on the east, are shown by a single line, and the relative directions in which the two blocks have sheared past each other are indicated by arrows. The height and slant of the Sierra block are much exaggerated. The streams are shown in their characteristic arrangement, the main rivers flowing down the western slope but many of their tributaries in directions approximately at right angles to them. No specific streams are represented. In front is a strip of the Great Valley of California, whose thick layers of sand and silt, derived from the elevated part of the Sierra block, bury the sunken part. At the back is a strip of Owens Valley, veneered with a thinner layer of sediment

Even to the first geologists who explored the Sierra Nevada and the adjoining regions the real nature of its structure was apparent from its very shape and outlines. The long linear crest suggested the raised eastern edge of the block; the abrupt eastern escarpment, the side of the block exposed to view by the uptilting; and the gentle western slope, down which the main rivers flow, the slanting upper surface. The essential correctness of this early interpretation has been abundantly confirmed by later observers, who have collected considerable evidence, direct and indirect, showing that great master fractures extend along the eastern base of the range and around its curving southern part, and that on these master fractures the earth's crust has been dislocated, the Sierra block rising, and the block adjoining it on the east sinking, relative to each other, in the manner indicated by the arrows in Figure 1.

The shearing of two earth blocks one past the other in this manner is termed "faulting," and the fractures at the eastern base of the range are therefore properly speaking "faults." The magnitude of the displacement—the "throw "—may be inferred from the great height of the eastern escarpment: it reaches a maximum of not less than 8,000 feet in the vicinity of Owens Lake. (See pl. 5, C.)

This great dislocation at the eastern base of the Sierra Nevada is not to be regarded as an isolated phenomenon. Most of the mountain ranges that traverse the Great Basin, that semiarid province that extends from the Sierra Nevada eastward to the Wasatch Range, in Utah, are bounded by somewhat similar dislocations. Instead of buckling and wrinkling, this province apparently was broken into huge blocks, some of which now stand high in the form of mountains, the others having sunk, relatively, so as to form intermediate desert basins. The Sierra Nevada, accordingly, is but one—the westernmost—of a vast assemblage of more or less closely related block ranges. It is, however, far and away the longest and highest of them all; it is one of the greatest block ranges in the world.

It might be inferred from the perhaps too simple diagram in Figure 1 that a single clean-cut master fracture extends continuously all along the eastern margin of the Sierra block, but that is far from being true. Along the great escarpment that faces Owens Valley, in the southern half, there may be actually a single fault, or a set of closely spaced parallel faults, but farther north the successive offsets in the front of the range indicate the existence of discontinuous northward-trending fractures that replace one another at intervals, thereby splintering the northwestward-trending margin of the block on a large scale. From the neighborhood of Lake Tahoe, which itself lies in a trough produced by the subsidence of a great splinter, long lines of faulting diverge in northerly directions, each marked by an escarpment of its own. Northward the eastern margin of the Sierra block becomes progressively more irregular, the displacements being distributed over a belt that broadens gradually to a maximum of 50 miles. Some of the escarpments measure but a few hundred feet in height, and the highest do not exceed 2,000 feet.

The causes of the uptilting of the Sierra block and of the down faulting of the valley blocks adjoining it on the east—indeed, the causes of the comprehensive upwarpings and complex faulting movements that have affected the whole of the Great Basin—are not yet fully understood. It seems probable, however, that they were primarily deep seated and only secondarily attributable to processes active at the surface of the earth, such as the erosion of valleys in some places and the deposition of the eroded materials in other places. The gradual lightening of mountain areas by the removal of rock waste and the concomitant loading of plains and ocean basins with the rock waste have doubtless acted at times as contributory causes, but such transfer of materials, constituting relatively a mere film of slight density at the surface of the globe, could scarcely have sufficed to initiate the upbulging and fracturing into blocks of a province comprising several hundred thousand square miles. In any event, a block such as that of which the Sierra Nevada is composed must have a tremendous depth, and the main fractures that bound it must reach far down into the earth, to depths where the materials have relatively great density and require correspondingly powerful forces to displace them.

FIGURE 2.—Idealized cross section of Sierra block, showing the composition of its interior. The folded beds in the foothill belt (A—B), at different points on the western slope of the range (C—C), and on its crest (D) are the remnants of a formerly continuous roof of mostly sedimentary rocks, under which the granitic materials welled up in a molten state. They are the roots, so to speak, of the mountain systems that occupied the place of the present range in times long past.

INTERNAL STRUCTURE

Of what manner of rock materials is this huge earth block constituting the Sierra Nevada built up? As will be apparent from the cross section in Figure 2, the block is composed largely of granitic rocks. Only on the lower part of its western slope and in some places on its crest are there considerable bodies of other rocks, such as slate, quartzite, limestone, and lava.

The granitic rocks all are of igneous origin; they are materials that have welled up from the depths of the earth in a molten state and have crystallized as they cooled. In the Sierra block there are present many different types, each formed by a separate uprising of molten material and each having a distinctive chemical and mineral composition. Thus there are different granites, monzonites, granodiorites, diorites, and gabbros (see appendix), but in this preliminary sketch those distinctions need not be considered, and it will be convenient, though not wholly accurate, to refer to all the granitic rocks collectively as granite. Together they form a great complex mass—what is termed a "batholith" (a mass that stopped in its rise at a considerable depth below the surface).

The outstanding fact regarding this granitic batholith is that it is now exposed at the surface of the block over large areas, despite its deep-seated origin. One treads on granitic rocks of one kind or another almost everywhere in the Yosemite region and the adjoining parts of the High Sierra. Most of the prominent peaks, domes, and cliffs—indeed, nearly all the noteworthy sculptural features of these regions—are carved from such rocks. Yet it is clear from their crystalline structure that these igneous materials did not flow out upon the surface but cooled very slowly, under the pressure of a confining crust or roof of other rocks. The explanation is, of course, that they have become uncovered—that they now appear at the surface because the roof under which they crystallized has been in large part removed. The slate, quartzite, and limestone mentioned are, in fact, the materials of which the ancient roof was made, as is manifest from the position of the remnants of those formations on the granite, and from the fact that the fissures and cracks in them are deeply penetrated by the granite, which was formerly fluid. The old roof rocks were originally thousands of feet thick, but in the course of ages, through the continued attacks of atmospheric agents and especially of surface streams, they have been gradually worn away.

It is in the remnants of this ancient rock roof that are to be recognized what have been termed the "roots" of the earlier mountain systems. For that reason they are here of more than passing interest. The largest mass extends along the western foothills and the lower slope of the range, as far up as El Portal, attaining there a breadth of 30 miles. Across this belt the Merced River has cut its canyon, thereby revealing to the traveler following the Yosemite Railroad or the State highway a section of its structure several thousand feet in depth. The mass is seen to be composed almost wholly of upturned beds of slate, quartzite, and marble, inclined 80° or more toward the east (pl. 26, B) and trending generally northwestward, roughly parallel to the crest line of the range. The slate, quartzite, and marble are really shale, sandstone, and limestone, or, to go back to their ultimate origin, mud, sand, and lime, that have been indurated and metamorphosed by pressure and heat, as the beds were folded and squeezed together and baked by the neighboring masses of molten granite. Intercalated with them are ancient lavas—surface flows of earlier geologic times—that have been metamorphosed out of all semblance to their former selves. Cutting northwestward through the belt of metamorphic rocks, finally, is a group of gold-bearing quartz veins—the famous Mother Lode, which was the lure of fortune hunters in the "days of '49."

PLATE 26.—A (top), VIEW DOWN LOWER MERCED CANYON FROM EL CAPITAN. The landscape has three distinct sets of elements, indicative of three stages in the development of the Merced Canyon. The level-topped ridges at the sky line and in gentle opposing slopes that outline dimly a very broad, shallow valley of great antiquity. The steeper slopes below and the broad flats to which they descend outline clearly the sides and floor of a deeper valley cut at a less remote epoch. Abruptly incised in the floor of this valley is a narrow, steep-walled gorge of relatively recent origin. The Merced flows at the bottom of this gorge and is still actively engaged in further deepening it. Photograph by F. C. Calkins. B (middle), UPTURNED BEDS OF SLATE AND SCHIST IN LOWER MERCED CANYON. These upturned strata are the remnants of the huge wrinkles in the earth's crust of which the earlier mountains on the site of the present Sierra Nevada were composed. Photograph by F. C. Calkins. C (bottom), CONTORTED LAYERS OF CHERT EXPOSED IN BED OF MERCED RIVER. Their crumpled appearance attests impressively the intense pressure to which the earth's crust was subjected here at the time when theh earlier mountains were made. Photograph by F. C. Calkins.

That these upturned beds of the lower Sierra slope represent the remnants of a series of closely compressed folds, or wrinkles, there can be not the slightest doubt, although the precise number of folds is difficult to determine, as the beds stand nearly parallel to one another and show little curvature over considerable distances; also because the slates, which predominate, are much alike in appearance, and beds of striking individuality, by the repetition of which one might identify opposite sides of a fold, are scarce.

Strongly bent, folded, even closely crumpled strata are not, however, wholly lacking in the Merced Canyon. A short distance above the mouth of Ned Gulch, for instance, the river cuts across a series of thin beds of chert (originally siliceous sea-bottom ooze) and shale, alternating with each other, that are compressed into astonishingly intricate, labyrinthine wrinkles. On the scoured and polished rocks in the river bed these wrinkles form prominent figures. (See pl. 26, C.)

In the granitic areas above the lower Sierra slope small bodies and individual slabs of metamorphic rock occur isolated here and there. A few are to be seen in the Yosemite region. For instance, on the slope east of Sentinel Dome, at an altitude of about 7,650 feet, a large slab of dark-gray schist (metamorphic shale) lies embedded in the granodiorite. The trail that leads from Glacier Point to the dome passes directly over it. Again, at the side of the Glacier Point Short Trail, a few hundred feet below Union Point, there are several slabs of light-gray schist and quartzite, and a quarter of a mile east of the main branch of Indian Creek, at an altitude of 7,900 feet, there is a knob composed entirely of rusty quartzite. On the north spur of Mount Clark is a body of dark mica schist and yellowish quartzite. The quartzite is of peculiar interest in that it is exceptionally resistant to decay, so that boulders of it, transported by ancient glaciers, still remain here and there as surviving witnesses of early glaciation, the other ice-borne débris that was associated with them having long since disintegrated and disappeared. A boulder of this quartzite lies on the summit of Liberty Cap.

Larger masses of metamorphic rocks, including white marble, occur near May Lake, at the southeastern base of Mount Hoffmann, and also in the rugged headwater basin of Yosemite Creek on the north side of that peak. The great depth below the neighboring granitic peaks at which these remnants of sedimentary rock occur is truly astonishing, but it is to be borne in mind that the ancient roof over the granite was strongly corrugated, having deep downfolds as well as high upfolds. Besides, its under surface must have been ragged in places, for doubtless many masses of rock, large and small, were torn loose from it and engulfed in the rising flood of molten granitic "magma," and others were left partly detached, or dangling, so to speak. Bodies of the latter kind have been aptly termed "roof pendants." The enormous size of some of them is strikingly attested by extensive and deep-lying masses of metamorphic rocks in different parts of the Sierra Nevada.

A spectacular and convincing example of a roof pendant that hung down to a great depth in the granitic magma is visible in the craggy spur that projects from the steep west side of Tuolumne Peak. This spur is composed of dark-hued sedimentary rocks, the contorted and broken strata of which dip steeply, immediately contiguous to the granitic mass of the peak itself. They reach down as far as they are in view, a full 2,000 feet, and doubtless continue some distance farther down.

An attempt has been made in Figure 2 to represent the roof pendants and other irregularities of the under side of the roof that once extended over the granitic rocks. Possibly their raggedness will seem exaggerated, yet the observed facts fully justify the representation.

Next to the broad belt on the lower slope of the Sierra Nevada, already described, the masses of metamorphic rocks situated near the crest of the range are the most extensive. They make up the bulk of Mount Dana, Mount Gibbs, and Parker Peak, as well as of that jumble of mountains north of Tioga Pass, whose central summit is Mount Warren. These mountains, in consequence of their composition, are variously tinted in subdued yellows, browns, reds, and purples and by contrast with the pale-gray peaks of granite near by appear somber, as if overcast by perpetual shadows. The metamorphic rocks in this crest region, however, differ appreciably in character as well as in structure from those in the lower belt. There is but little slate among them, schist, quartzite, and volcanic rocks being predominant; and the folding is less deep and less complex. These facts are readily observed on and about Mount Dana, which is capped by gently flexed beds of volcanic origin.

The age of the metamorphic rocks of the Sierra Nevada is not easily determined, owing to the dearth of fossil remains in them. However, a few fossils have been found in different parts of the range, notably in the limestone and sandstone of the lower belt, and from these it has been established that the strata composing that belt, though much alike in general aspect, fall into two distinct series, one of which is vastly older than the other. The older, known as the Calaveras formation, is of Paleozoic (Carboniferous) age; the younger, known as the Mariposa formation, is of Mesozoic (Jurassic) age. The older rocks make up approximately the eastern half of the belt, extending along the Merced Canyon from El Portal down to Bagby. The younger rocks make up most of the western half, extending from Bagby down to the foothills.

In the strongly deformed structure of these two distinct series of strata there is thus clear evidence of the former existence of two successive mountain systems. The general character of those ancient mountain systems, moreover, is indicated by the forms, mode of arrangement, and trend of the folds. Especially full are the data available concerning the younger (Cretaceous) system. Indeed, the similarity of its folded structure to that of the present Appalachian mountain system in the eastern United States enables the geologist to reconstruct its features with considerable confidence. Doubtless it was composed, like the Appalachian system, of long, roughly parallel ridges, separated by equally long, roughly parallel valley troughs. The trend was in general northwestward, but it varied locally.

As in the Appalachian Mountains, so probably also in the Sierra of Cretaceous time, the individual ridges were made up not of the rounded tops of entire upfolds but mostly of the stumps of resistant upturned strata, and the longitudinal valleys were etched out, so to speak, in the intermediate weaker strata, in the manner shown diagrammatically in Figure 3. The tops of the upfolds, being from the start fractured by tensile stresses and therefore particularly vulnerable to erosion, doubtless were worn away at an early stage during the slow, progressive upheaval.

FIGURE 3.—Block diagram illustrating parallel mountain ranges carved from strongly folded strata. The ancient mountain ranges that occupied the place of the Sierra Nevada during the Cretaceous period were of this general type.

From the steep inclination of the strata and the tightly compressed character of the folds in the belt traversed by the lower Merced Canyon, furthermore, it is to be inferred that the ridges in that part of the Cretaceous system were prevailingly linear, sharp crested and closely spaced, whereas in the region of gentle flexures near the summit of the present range doubtless the ancient mountains were relatively full bodied, round topped, and spaced far apart.

The almost complete removal of the metamorphic rocks from the middle part of the Sierra slope, finally, would seem to show that there the deformed strata were broadly arched up to considerable height, so that the axis of the mountain system, and presumably its culminating heights and central divide, for a long time were situated in that region. Thus even its drainage system can be outlined in a general way. Although the streams flowed for the most part northwestward or southeastward, in the valley troughs between the parallel ridges, some of them doubtless drained westward, through gaps in the ridges, into the sea and others drained eastward, into adjoining lower lands.

These details of a mountain system that has long since vanished may possibly seem a needless digression, but they will help the reader to understand the significance of many topographic features of the present range; for, in spite of the profound stripping which the Sierra region has suffered since Jurassic time—a depth of stripping estimated at not less than 5,000 feet—the present range still possesses features that were inherited from the ancient mountains just described, as will become apparent in the following pages.