Before taking up the detailed consideration of the landforms in the San Joaquin Basin, it will be desirable to consider first the nature of the rock materials from which these forms were carved. For this report, the subject may be treated briefly, and mainly from the geomorphological point of view, with emphasis on facts related to the development of landforms. Only incidental reference is made, therefore, to structure and to petrologic details.
The rocks of the San Joaquin Basin belong mainly to the three general classes that occur throughout the greater part of the Sierra Nevada (fig. 5); namely, the granitic rocks that make up the vast compound batholith of Late Jurassic or Early Cretaceous age; the metamorphosed sedimentary and volcanic rocks of Paleozoic and Mesozoic age into which the granitic rocks of the batholith were intruded; and the volcanic rocks of Tertiary and Quaternary age that lie unconformably upon these older rocks as a discontinuous, surficial veneer.
Further mention should be made of the glacial de posits of Pleistocene age in the higher parts of the basin (described in the chapter on glaciation); the alluvial deposits of Quaternary age which underlie many upland meadows and form narrow flood plains in the lower stream channels, notably that of the San Joaquin River, below Friant; and, finally, the sediments of Tertiary age, terrace gravels of Quaternary age, and alluvial fans of Quaternary age at. the west border of the range, where the crystalline complex of rocks of Paleozoic and Mesozoic age constituting the Sierra block passes from view beneath the floor of the Great Valley.4
Of the three main classes of rocks in the region, the granitic rocks are by far the most abundant. They occupy the greater part of the basin, their outcrops extending, with only a few important interruptions, from the crestline of the range to the western foothills. Even the outlying island hills that project above the sediments of the Great Valley of California, in front of the range, are composed largely of granitic rocks.
The term "granitic," as used in this report, includes a considerable variety of undifferentiated plutonic rocks. mainly of siliceous types. Besides granite, granite porphyry, and micropegmatitic granite, they include chiefly quartz monzonite, quartz diorite, granodiorite, and gabbro. In this reconnaissance the writer made no attempt systematically to distinguish and map these rocks, but this has since been done by Calkins (1930, p. 120-129) for the nearby Yosemite region and by other geologists for parts of the San Joaquin Basin. The studies by these geologists serve to shed light on the granitic rocks of the basin as a whole.
On his map of the foothills zone between the San Joaquin and Kings Rivers, Macdonald (1941, p. 217, 251-257, 271-273) distinguishes an earlier group of hornblende gabbros and hornblende diorites from a later group of intrusive rocks which includes hornblende biotite, quartz diorite, pyroxene quartz diorite, and muscovite granite. Of these rocks, the quartz diorite occupies by far the greatest area. It is described as being quite uniform in composition throughout the area mapped but variable in appearance because of differences in grain size, structure, and texture.
In the southeastern part of the Mount Lyell quadrangle, Erwin (1934, p. 7-78; 1937, p. 391-413) found that the composite Sierra Nevada batholith includes intrusive igneous bodies of five types: andesine diabase, diorite porphyry, diorite, quartz monzonite-diorite, and micropegmatitic granite. These rocks, intruded in an order of increasing silica content, are thought to have resulted from one general period of igneous intrusion. The quartz monzonite-diorite composes the main mass of the batholith.
Mayo (1937, map p. 172; 1941, map facing p. 1000), in making a northeasterly geologic section through the San Joaquin Basin, found that the granodiorite granite series extends uninterruptedly from the vicinity of Black Mountain (southwest corner of Kaiser quadrangle) almost to the crest of the range, a distance of about 45 miles. He has differentiated on his map a long narrow intrusion of porphyritic quartz monzonite which extends southeastward from the south end of the Ritter Range. This intrusion, distinguished by large phenocrysts of potassium feldspar, he designated as the Cathedral Peak granite, using the name previously given by Calkins (1930, p. 126-127) to the rock of this type in the Yosemite region.
JOINTING AND EXFOLIATION
In the San Joaquin Basin, as in the Yosemite region (Matthes, 1930a, p. 39), the granitic rocks are sparsely jointed, in many places the intervals between fractures measuring tens, hundreds, or even thousands of feet. However, in some places the granitic rocks are closely jointed (that is, have joints spaced one to several feet apart). As in the case of the Yosemite region, the character of the jointing has played a dominant role in governing the manner and rate of weathering processes, and of erosion by streams and glaciers, as well as in determining the character of the resultant land forms. In fact, the concepts of these processes and the analyses of the distinctive forms produced, as set forth in the author's report on the Yosemite Valley, were based on broad regional studies made in many parts of the Sierra Nevada, including the San Joaquin Basin. These concepts and analyses need not be restated here, but the illustrations in the present report include a number of views depicting representative landforms developed in the granitic rocks of the San Joaquin Basin.
In the Geographical Sketch, reference was made to the plateaulike uplands (p. 8) which are a distinctive feature of the central part of the San Joaquin Basin, and also to the broad shallow valleys which hang with respect to the San Joaquin Canyon and its principal branches. The exceptional preservation of the uplands and their hanging valleys is accounted for by the durability of the massive granite which underlies them. Some geologists would account for the relatively abrupt drop from the plateau to the foothills, south and southwest of Shaver Lake, by faulting, but no clear evidence of faults was discovered, and the same topographic effect could readily have been produced by differential erosion in well-fractured bodies of granite in juxtaposition to massive granite. The edge of the plateau south of Shaver Lake, which is scalloped by the heads of branches of Blue Canyon at the head of Big Creek, can hardly be delimited by a fault or zone of faults, but appears to indicate approximately the boundary between the massive rocks to the north and the fractured rocks to the south, in which a normal erosion topography, with dendritic drainage pattern and forking spurs and spurlets, has been developed.
The large-scale exfoliation of massive granitic rocks, so remarkably illustrated in the Sierra Nevada, is evident in many places in the San Joaquin Basin. The cause of this exfoliation is still obscure. Some geologists consider the most probable primary cause to be the liberation of expansive stresses within the granite, as the result of the progressive removal of superincumbent loads of rock by erosion; probably auxiliary causes are diurnal insolation and secular warming (Gilbert, 1905a, 1905b; Matthes, 1930a, p. 114-115; Matthes, 1937b). Exfoliation along plane or convex surfaces is illustrated at various places on the sides of the San Joaquin gorge, for example below the mouth of Big Creek, and on benches around the head of Evolution Creek, and elsewhere. Domes, the striking rounded forms evolved from giant monoliths by long-continued exfoliation, are neither numerous nor, generally, as spectacular in this basin as in the Merced Basin, but nevertheless examples are found.
Balloon Dome (altitude 6,900 feet) is the outstanding dome in the region (figs. 6, 15). It is situated between the Middle and South Forks of the San Joaquin River, immediately above the junction of the two streams. This dome is referred to in J. D. Whitney's report (1865, p. 401) as "a most remarkable dome, more perfect in its form than any before seen in the state," and it is described as having "exactly the appearance of the upper part of a sphere; or, as Professor Brewer says, 'of the top of a gigantic balloon struggling to get up through the rocks.'" As a matter of fact, it is not as regularly shaped as many of the domes of the Yosemite region, but because of its strange form, unique situation, and bare figureall the more conspicuous because the surrounding uplands are forestedit is an imposing landmark.
Two miles to the northwest of Balloon Dome is Squaw Dome (altitude 7,806 feet), also a conspicuous topographic feature (fig. 7); and in the vicinity are Cattle Mountain (fig. 6), the Balls, and several other relatively small subdued domes. Near Cascada station, providing a background for the Big Creek Falls, is Kerckhoff Dome (fig. 19, not shown on topographic map); and on the north boundary of Kings Canyon National Park, occupying a position within the fork formed by Piute Canyon and the valley of the South Fork of the San Joaquin River, a situation very similar to that of Balloon Dome, is Pavilion Dome (altitude 11,721 feet). On Chiquito Ridge, exfoliation is prevalent but not regular and continuous, owing to steepness of the slopes and the presence of interfering master joints. Peaks and spurs of the ridge are imperfect domes, and on the exfoliation surfaces, rainwash flutings are common (fig. 8).
MODES OF WEATHERING
Exfoliation is not a universal phenomenon of the massive granitic rocks. In places, for example on Bald Mountain, east-southeast of Shaver Lake, one may see immense exposures of massive granitic rock that are not exfoliating in the manner so characteristic of the domes of the Yosemite region. The rock on Bald Mountain disintegrates into granules, which are washed away by rainwater and accumulate farther down the slope in large quantities. Exfoliation partings appear here and there, but are not conspicuous.
A striking feature in fresh road cuts on the steep grade above Tollhouse and on other roads leading to Shaver Lake is the massive granite, breaking down by mechanical processes, and without significant chemical decomposition. On the middle and upper parts of the San Joaquin Basin, accumulations of loose granular sand resulting from disintegration of the granitic rocks is one of the most widespread phenomena. the sand covers the lower slopes of bare rocky hills and ridges, and deeply cloaks the interfluves on the uplands. On the moraines of the later glaciation, the sand accumulates in the swales and on the upslope sides, so that the crests become flattened. The moraines of the earlier glaciation are in many places so mantled with sand derived from the disintegration of the glacial boulders that the materials of the moraine itself are exposed only here and there, generally where the uprooting of trees has revealed boulders and cobbles.
As was explained in the author's report on the Yosemite Valley (1930a, p. 107-108), the disintegration of the granitic rock in these situations appears to be in the main a phenomenon of mechanical weathering, due largely to the disruptive effect of solar heat. Frost presumably plays a part in this disruption, but if so, only a very subordinate part, for typical frost cracks are absent in the rock masses wherein it occurs. The loose grains show scarcely any effects of chemical decomposition, even when examined under the microscope. The crystals of feldspar are but slightly cloudy at the edges, and the flakes of biotite and rods of hornblende as a rule show no alteration. However, this crumbling of the granite into undecomposed grains takes place only on the domes, cliffs, and other conspicuously bare rock masses that are subjected to intense insolation. In the densely forested areas on the uplands, where the heat of the sun is partly excluded by the foliage of the trees, and where the granitic rocks are covered by a layer of moisture-conserving, acid-producing humus, the chemical processes reduce the granite in much the same way as in a humid region.
The massive granitic rocks also disintegrate by flaking off in thin scales, the thickness of these as a rule being only 1/4 inch but ranging from 1/8 inch to 1 inch. The scales vary in size but commonly break off in patches a few inches in diameter. Evidences of decomposition under the flakes, promoted by moisture and by the presence of green algae and lichens, are found in some places.
The granitic rocks are interrupted at many places by small to moderately large masses of metamorphic rocks of Paleozoic and Mesozoic age. The largest of these masses are found in the foothills region, in the Mount Ritter region at the northern end of the basin, and in the Goddard Canyon region in the southeastern part of the basin.
The metamorphic rocks in the foothills region are a part of the broad belt of such rocks extending along the lower west slope of the Sierra Nevada. The belt is traversed, farther north, by the gold-bearing quartz veins of the Mother Lode system (Jenkins, 1938). Remarkably continuous as far southeast as the Merced Basin, this belt narrows notably near Mariposa, but tongues continue across the Chowchilla Basin and into the Fresno Basin (fig. 5). In the San Joaquin Basin the metamorphic rocks are represented by a series of relatively small disconnected bodies. Still farther to the southeast, in the basins of the Kings, Kaweah, and Kern Rivers, they again occur in larger masses along the lower west slope. Accordingly it would appear that the San Joaquin Basin, of all the drainage basins in the central and south-central part of the Sierra Nevada, is the one in whichso far as the lower west slope is concernedthe metamorphic rocks comprise the least, and the batholithic rocks the greatest proportion of the area.
In the foothills section between the San Joaquin and the Kings Rivers, the metamorphic rocks of Paleozoic and Mesozoic age occur in tightly folded anticlines and synclines which strike northwestward (Macdonald, 1941, p. 217, 270-2 2). The oldest division of the metamorphic sequence is a metasedimentary series between 20,000 and 30,000 feet thick, consisting chiefly of mica schist. Above this is a metavolcanic series at least 10,000 feet thick containing minor amounts of interbedded metasediments. The volcanic rocks, which are intermediate to basic, have been converted into plagioclase amphibolite and amphibole schist. The youngest division of the metamorphic sequence is another metasedimentary series close to 10,000 feet thick, consisting mainly of mica schist. Sills of serpentine and a few bodies of olivine gabbro have been metamorphosed to a degree comparable with the surrounding rocks.
The body of metamorphic rocks in the Mount Ritter region is the largest in the basin. This body, in its broader relations, is the expanded southern part of the narrow belt of metamorphic rocks, of Mesozoic age, which extends for about 40 miles along the crest of the range, at the heads of the Tuolumne, Merced, and San Joaquin Basins (Jenkins, 1938). The body is irregular and measures about 12 miles across in its larger dimensions. It forms the bulk of the Ritter Range, and extends across both the Middle Fork and the North Fork Canyons. At the northeast it passes beneath Tertiary lavas capping the main divide.
The rocks of the Mount Ritter region have been described by Erwin (1934, 1937) and also considered in the regional studies by Mayo (1935, 1937, and 1941). They consist mainly of intensely folded metavolcanic rocks which form an almost isoclinal series striking in general N. 30° 40° W. The metavolcanic rocks, predominantly pyroclastic rocks, include diverse rock types altered into schists and, particularly, mylonites. There are minor intercalations of metasediments, and Erwin has mapped these separately in the upper Middle Fork basin.
In the Middle Fork basin, the metavolcanic rocks, being of various lines, give the landscape a distinctively banded appearance which contrasts with the uniformly dark or light aspect of nearby areas. The Ritter Range looms almost black above the light-colored granitic rocks to the south and southwest. Its rocks, mostly metatuffs and breccias, are generally more massive than those in the Middle Fork basin, and apparently less severely metamorphosed. In the North Fork basin and to the west of it, the metamorphic rocks again are more completely altered and more schistose than in the Ritter Range.
Between Mono and Bear Creeks, metavolcanic rocks of Mesozoic age reappear in a long narrow belt which extends southeastward along Goddard Canyon (Jenkins, 1938). This belt reaches far beyond the limits of the San Joaquin Basin, continuing, with some interruptions, across the Kings River Basin to the headwater area of the South Fork of that river.
The rocks of this belt, presumably Mesozoic in age, have not been fully mapped or studied in detail. They crop out along Goddard Canyon and make up a considerable part of both the Goddard Divide and the LeConte Divide. Where observed in the course of this reconnaissance they consist predominantly of intricately folded, nearly vertical metavolcanic schists striking northwestward.
Other smaller masses of metamorphic rock lie widely scattered within the San Joaquin Basin, some in belts, others without apparent system. The mountains of Kaiser Ridge, west and northwest of Nellie Lake, are composed largely of quartzite which disintegrates into angular blocks less than 2 feet across, contrasting with the much larger and more rounded granite boulders of nearby areas. The peak (altitude 9,622 feet) north of the lake is composed partly of quartzite, partly of granite; the extreme summit is granite. Farther east, Kaiser Peak itself is composed of thinly bedded quartzite and schist. On the north side of the peak these rocks are cut by thick sills of granite and aplite. The quartzite on the east spur of Kaiser Peak breaks down into distinctive small fragments. The Twin Lakes are bordered on the north by a belt of crystalline limestone forming an arc-shaped outcrop more than 1 mile long, the concave side of which is toward the northeast. The peak east of Potter Pass is composed, in its eastern part, of closely plicated grayish-green schist.
Small bodies of metamorphic rock, mostly schist, were also observed on the east side of Silver Creek, 3/4 mile above its junction with Fish Creek: at the east end of Junction Bluffs; 1/2 mile northwest of Clover Meadow Ranger Station; on Green Mountain, 1-1/2 miles north of Soldier Meadow; and in Shakeflat Creek basin, as well as on the ridge to the southwest of it, where the local rock is largely schist and quartzite. Similar small bodies of metamorphic rock undoubtedly occur at many other places in the San Joaquin Basin.
The volcanic rocks, scattered from the Sierra crest to the western foothills, occur in many small patches whose aggregate area is only a minor part of the San Joaquin Basin. In age they range from Miocene to late Pleistocene or perhaps even to Recent. Unlike the extensive flows in the northern half of the range, they have interfered but little, and only locally, with the orderly development of the valleys and canyons.
The principal areas of volcanic rocks lie on the main divide adjacent to the Middle Fork Canyon, and farther south within the canyon. This region, included in the southeastern part of the Mount Lyell quadrangle, has been mapped by Erwin (1934) who distinguished 5 volcanic units, 2 of late Miocene age and 3 of Pleistocene age.
The older Miocene unit, consisting of basalt flows, includes a relatively large occurrence east of the Middle Fork Canyon (southeast of Agnew Pass); also remnants south of Iron Mountain between the North and Middle Forks of the San Joaquin River, in the upper East Fork of the Granite Creek basin, in the vicinity of Clover Meadows, and elsewhere. At the Middle Canyon locality the basalt rocks are exposed in cross section, giving a bold, steplike profile to the east side of the canyon. The flows vary in number from place to place; in the thicker sections they "do not exceed 36" (Erwin, 1934, p. 46). North of Agnew Pass their aggregate thickness is estimated to be more than 1,500 feet. The flows, which have columnar or blocky jointing, permit surface water to seep through to the underlying metamorphic rocks, whose surface is followed to Middle Fork Canyon. Here, emerging from beneath the lavas, the water gives origin to most of the streams on the east side of the canyon. The other lava caps in the region are of similar character but smaller.
The younger Miocene unit includes andesitic flows, with a maximum thickness of about 1,000 feet, which overlie the basalt on the Sierra crest, east of Middle Fork Canyon. A few miles farther southeast, andesites also make up the bulk of Mammoth Mountain, an isolated and deeply eroded preglacial volcano which Mayo (1941, p. 1068) has termed "the most impressive volcanic edifice in the region."
The three Pleistocene units are basalt flows, tuffs, and pumice. There are several occurrences of the basalt, the most important being a Y-shaped mass immediately west of the Sierra crest. The converging arms of the Y, each about 3 miles long, extend southward from the head of Pumice Flat and westward from Mammoth Pass, and unite just below Reds Meadow to form the upright, which extends south-southwestward about 4 miles farther. This mass includes the Devils Postpile (Matthes, 1930b) (fig. 9). A short distance below the Postpile a cliff of the basalt gives rise to Rainbow Falls, 150 feet high, the most notable waterfall in the San Joaquin Basin (fig. 10). The basalt appears to have descended from the vents at Red Cones and Pumice Butte, and from a fissure near Mammoth Pass.5 It issued after the El Portal glacial stage onto the glaciated surfaces of Middle Fork Canyon, accumulating to depths of 100 to 700 feet. When the Middle Fork glacier readvanced in the Wisconsin stage, it removed all but the more obdurate parts of the lava (Matthes, 1930b, p. 5).
The other Pleistocene volcanic units consist of an area of crystallithic tuff in the vicinity of Satcher Lake and Reds Meadow, and a scattering of pumice pellets, as much as 1 inch in diameter, widely distributed throughout the region and particularly abundant between Agnew Meadows and Pumice Butte. Erwin considers the pumice to be "younger than older glacial material."
In this reconnaissance several volcanic cones and many small patches of volcanic rock were noted. Black Point, 1-1/2 miles west of Huntington Lake, is a small volcanic cone. Another cone, Brown Cone, lies to the north of Kaiser Creek; still another is situated in nearby Cow Meadow. The latter is the remnant of an andesitic eruption crater, the west and north sides of which are best preserved. About 3 miles farther southeast is another small andesitic volcano which is probably interglacial, because granite erratics left by the Wisconsin stage rest on its lava. The volcano stands about 100 feet above the surrounding country, and contains a shallow lake in what appears to be a compound crater.
Of the many scattered patches of volcanic rock observed, mention may be made of several in the lower South Fork Canyon; also those in the vicinity of Pincushion Peak, particularly west of the summit; those at the northwest and of Mono Ridge, one of them giving rise to the eminence called Volcanic Knob; several extending from Onion Spring Meadow southwestward almost to the South Fork of the San Joaquin River; the patches in upper Big Creek Basin which form Chinese Peak, Red Mountain, and Black Peak (fig. 47); an occurrence in Chiquito Creek basin, 1 mile northwest of Placer Ranger Station; and the patches which partly cap the summit of Squaw Dome. In the case of Chinese Peak, three large dikes radiate from the central summit.
Though none of the occurrences mentioned has been studied in detail, most of them apparently are andesitic. However, patches of olivine basalt (Miocene or Pliocene) which cap Table Mountain and a few other foothills in the Friant and Academy quadrangles are described by Macdonald (1941, p. 266 and folded map).
Last Updated: 27-Jul-2009