RECONNAISSANCE OF THE GEOMORPHOLOGY AND GLACIAL GEOLOGY OF THE SAN JOAQUIN BASIN, SIERRA NEVADA, CALIFORNIA
By FRANÇOIS E. MATTHES
Reconnaissance of the San Joaquin Basin, situated in the central part of the west slope of the Sierra Nevada, California, was undertaken in order to extend southeastward the studies made earlier in the Yosemite region, and to test the soundness of conclusions reached in the Yosemite region, regarding the geomorphologic development and the glaciation of that area and to determine their bearing on the history of the Sierra Nevada as a whole.
For these purposes the San Joaquin Basin is, in several respects, the most revealing of the drainage basins of the west slope of the Sierra Nevada. In it, the erosional features are unobscured by volcanic flows except locally. Also, the granitic rocks are exceptionally extensive, and their prevailing massive character has profoundly influenced the topography. For example, the San Joaquin Canyon is consistently narrow, and exhibits neither anomalous Yosemite-like widenings nor, for the most part, the unusual sculpturing characteristic of the yosemites.1 Also, the resistant nature of the massive granitic rocks accounts for the preservation of an unusually fine record of the successive cycles of erosion, and an extraordinary wealth of clean-cut hanging side valleys. Furthermore, because the granitic rocks are practically continuous to the foot of the range, the ancient erosion surfaces and the array of hanging valleys likewise extend not merely throughout the glaciated upper course of the San Joaquin Canyon but also through most of the unglaciated lower course, to within a few miles of the foothills.
It should be noted, too, that the San Joaquin River, having a southwestward course conformable to the slope of the Sierra block, was subjected to the full rejuvenating impulse of each tilting movement. Its tributaries, on the other hand, have mostly northwesterly or southeasterly courses, at right angles to the master stream as well as to the direction of tilting of the block. The flow of the tributaries, therefore, was accelerated very little by the tilting of the Sierra block. In this contrasting relationship of master stream to tributaries the drainage pattern of the San Joaquin Basin is typical for most of the basins in the central and southern part of the Sierra Nevada, but probably in no other basin is the relationship so well shown.
The salient geomorphologic features of the San Joaquin Basin may be summarized as follows: The San Joaquin Canyon is essentially a narrow, youthful, branching gorge, which has been cut into the floor of a relatively broad, mature valley. The mature valley in turn is flanked by extensive undulating uplands, with a relief over wide areas not greater than 1,000 feet. In places, however, the uplands are surmounted by hills, mountain groups, and imposing mountain crests trending, in most cases, in a northwesterly direction, and having a relief of 1,500 to over 2,500 feetranges forming a part of the so-called High Sierra. At least one mountain crest has peaks that bear distinctively tabular summits.
These features are believed to be the products of four partial cycles of stream erosion, each initiated by a major uplift. They are interpreted as follows:
Expressive of the latest cycle, is the branching gorge or inner canyon of the San Joaquin River and its main branches. This gorge is regarded as being wholly of Pleistocene origin, and is correlated with the "canyon stage" of the Yosemite region. The benches and shoulders which flank the Pleistocene gorge in the lower and middle reaches of the basin, giving the San Joaquin Canyon a 2-storied aspect, are remnants of a mature valley developed in a partial cycle of erosion which preceded the canyon stage. The maturity of this valley is indicated by its broadly flaring cross section and its low, smooth gradient (deduced from the extended gradients of the lateral valleys that were left hanging in the latest cycle of erosion). To have attained its mature character, the valley must have required a period several times as long as that which was required for the carving of the Pleistocene gorge. It may be inferred that the partial cycle of erosion in which the valley was developed embraced all, or nearly all, of the Pliocene epoch. Accordingly, the Pliocene valley of the San Joaquin River may be assigned to the "mountain valley stage" of the Yosemite region.
The undulating uplands into which the Pliocene valley has been cut are erosion surfaces referable to a still earlier cycle. The forms of these uplands are so much more mature than those of the Pliocene valley that there is good reason to assign for their development a correspondingly greater span of time. Furthermore, they are clearly correlative with the uplands of the Yosemite region, and are traceable across the divide and continuous with the uplands of that region. From paleontological evidence found north of Yosemite Valley, in the Table Mountain district between the Tuolumne and Stanislaus Rivers, it appears that the Yosemite uplands, representing the broad valley stage, are of late Miocene age. Accordingly, the uplands of the San Joaquin Basin are also thought to have been produced in a cycle of erosion which began some the back in the Miocene and continued until the latter part of that epoch.
Finally, the hills, mountain groups, and mountainous crests surmounting the uplands are clearly monadnocks, and the tabular summits (Mount Darwin and Mount Wallace) of the one crest are, from the evidence of the great height to which these summits rise above the uplands, much older than the Miocene erosion surface. These platforms are referable to the most ancient erosion surface recognizable in the Sierra Nevada, thought to be Eocene in age.
The hanging lateral valleys fall into two tiers or sets, the one much higher than the other. Valleys of the upper set are hanging with reference to the mature Miocene erosion surface. Valleys of the lower set are hanging with reference to the Pliocene erosion surface (the flanking benches and shoulders of the San Joaquin Canyon). The 2 sets of hanging valleys attest to 2 rejuvenations of the master stream induced by 2 uptiltings of the Sierra Nevada. Valleys of the upper set were left hanging as a result of the uplift which initiated the mountain valley stage of erosion during the Pliocene; those of the lower set, as a result of the uplift which initiated the canyon stage of erosion during the Pleistocene. The 2 sets of hanging valleys are analogous to the 2 upper tiers of hanging valleys in the Yosemite region.
By the same method employed in connection with study of the Yosemite Valley, the data provided by the hanging valleys of the San Joaquin Basin were used to reconstruct the longitudinal profiles of the San Joaquin River and its 2 branches for each of the 2 stages of erosion indicated. The abundance and distribution of the hanging valleys made possible the reconstruction of the former river profiles throughout a much longer section than in the case of the Merced, and farther down toward the foothills.
The mountain crests surmounting the Miocene erosion surface are features inherited from the system of northwestward-trending ridges that occupied the place of the present Sierra Nevada in Late Jurassic and Cretaceous time. The northwestward-trending valley troughs between these crests similarly are features dating far back into Cretaceous time, and still control the drainage pattern of the tributary streams to a marked degree.
The main drainage divide, at the head of the San Joaquin Basin, does not coincide with the top of the great east escarpment of the Sierra Nevada except at two very short stretches. Evidently the divide is an ancient erosional feature, which parted northeastward flowing waters from southwestward flowing waters long before tectonic movements formed the escarpment. The escarpment is probably in large part of early Pleistocene origin, and is a feature produced as a result of the downfaulting of the Owens Valley graben.
During the Pleistocene epoch, the higher parts of the San Joaquin Basin were repeatedly mantled with glaciers. As a result, glacial features were superimposed on a landscape inherited from Miocene and Pliocene times. For tracing and mapping the courses of the ancient glaciers, and for determining their farthest limits, reliance was placed on the testimony of glacial deposits rather than on that of sculptural features, and a systematic survey was made of the moraines built by the individual glaciers. On the glacial map constructed from this survey, distinction has been made only between the Wisconsin and pro-Wisconsin stages. However, at several localities the field data appear to warrant future subdivision of the Wisconsin moraines into those of early Wisconsin the and those of late Wisconsin time, corresponding to the Tahoe and Tioga stages of Blackwelder. The pre-Wisconsin moraines are in all respects closely similar to those in the Yosemite region, and are therefore referred to the El Portal stage, believed to correspond to Blackwelder's Sherwin stage at the east base of the range and thought to be not younger than the Illinoian stage of the continental glaciation. No morainal deposits belonging to the Glacier Point stage, the earliest stage of glaciation recognized in the Yosemite region, are shown on the map but a few small, isolated patches of an ancient moraine were discovered in the central part of the basin, and these may well be representative of the Glacier Point stage, in view of their elevated positions.
The San Joaquin glacier system of the El Portal stage was by far the largest system of confluent glaciers in the Sierra Nevada, measuring more than 50 miles in length along the crest of the range, and 30 to 35 miles in breadth. Of the 1,760 square miles comprised in the San Joaquin Basin, almost 1,100 square miles were covered by glaciers. This ice mass, taken together with the ice masses in the basins of Dinkey Creek and North Fork of the Kings River, formed a mer de glace 1,500 square miles in extent. Yet it was not an icecap, strictly speaking, for its surface was broadly concave rather than dome shaped. It consisted of a large number of confluent glaciers that had descended from the surrounding peaks and crests, filled the canyons, and overflowed on to the intermediate uplands.
The glacier system was essentially bilobate, as it comprised two great primary branches that descended Middle Fork and South Fork canyons. The branches united below the junction of the two forks, to form a very broad trunk glacier that overflowed the main San Joaquin Canyon and spread widely over the central part of the basin. The glacier ended in a rather blunt tongue, only a few miles long, which lay in that part of the main canyon separating Chiquito and Kaiser Ridges. The length of the San Joaquin glacier system, measured from the head of the South Fork branch to the terminus of the trunk glacier, was nearly 60 miles; the length measured along the Middle Fork branch was almost 45 miles.
Big Creek basin was occupied by a glacier system of its own. This system attained a length of about 20 miles and an areal extent of about 105 square miles.
During the Wisconsin stage of glaciation, in the San Joaquin Basin as elsewhere in the Sierra Nevada, the volume of ice was considerably smaller than in the earlier stage, and the ice-covered area was, therefore, much less extensive. The San Joaquin glacier system had, of course, the same general pattern as in the earlier stage, for it reoccupied most of the cirques and followed the same valleys and canyons, but it took the form of many discrete glaciers which became confluent only in part. Thus, large upland areas which were ice-covered in the earlier glaciation remain bare throughout the Wisconsin stage.
By far the greater part of the ice which formed in the San Joaquin Basin was again part of a bilobate system, and this pattern was the more pronounced because the ice was largely confined within the canyons or broad valleys. The trunk glacier in the main canyon, formed through union of the Middle Fork and South Fork glaciers, was only a short and feeble tongue. The maximum length of the San Joaquin glacier system, measured from the head of the South Fork to the terminus of the trunk glacier, was 47 miles; the length measured along the Middle Fork branch was 32 miles.
A number of glaciers, which in the El Portal stage were tributary to the San Joaquin glacier system, in the Wisconsin stage fell short of joining that system, and throughout their existence remained separate ice bodies or became confluent only with adjacent glaciers. This was the case with the Granite Creek glacier, the Big Creek glacier system, and the many relatively small glaciers on Chiquito Ridge, Kaiser Ridge, and other ridges.
Last Updated: 27-Jul-2009