SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY
GLACIAL RECONNAISSANCE OF SEQUOIA NATIONAL PARK, CALIFORNIA
By FRANÇOIS E. MATTHES
Sequoia National Park, like Yosemite and Kings Canyon National Parks to the north, is on the broad west slope of the Sierra Nevada. It extends from an elevation of about 1,400 feet in the western foothills to 14,495 feet at Mount Whitney, the culminating summit on the main crest of the range at the east. Thus its altitudinal range, about 13,100 feet, is greater than that of any ether national park in the United States, south of Alaska. The park embraces the most southerly portion of the High Sierra, the scenic higher part of the range.
The main crest of the Sierra Nevada, at the east border of the park, bears many high peaks, no less than seven of which have altitudes exceeding 14,000 feet. Traversing the central part of the park from north to south is a secondary crest, the Great Western Divide, which is likewise an impressive range with peaks 11,000 feet to over 13,000 feet in altitude. This crest divides the park into two approximately even but dissimilar halves. The western half is occupied by the basin of the Kaweah River, most southerly of the southwestward-flowing master streams of the Sierra Nevada. The Kaweah Basin is an intricately dissected, rugged area of high relief. The eastern half is occupied by the upper basin of the Kern River. The Kern is unique among the master streams in that it flows directly southward, nearly parallel to the main crest of the range. Distinctive features of the upper Kern Basin, in addition to the high bordering mountain crests, are the impressive U-shaped Kern Canyon and the broad benches (ancient erosion surfaces) that border this canyon and its branches.
In the Pleistocene Epoch both the Kaweah Basin and the upper Kern Basin were occupied by glacier systems. These were the most southerly of the major glacier systems of the Cascades-Sierra Nevada chain. Being less favorably situated than those to the north, they were of smaller volume; nevertheless, glaciers of considerable size formed in both basins, especially the Kern, during each of the three glacial stagesthe Glacier Point Stage, the El Portal Stage, and the Wisconsin Stage. Information concerning the characteristics and distribution of the glaciers was sought by distinguishing and mapping the morainal deposits of each of the stages.
In the Kaweah Basin, the development of glaciers was limited by the fact that this basin heads not along the lofty main crest of the Sierra Nevada but on the Great Western Divide and on other secondary crests that are only part way up the Sierra west slope. Evidence is present in this basin for the earliest stage, the Glacier Point, but is extremely meager. For the next stage, El Portal, and the most recent stage, the Wisconsin, the records are far better. They indicate that during both of these stages the converging canyons of the Kaweah Basin became pathways for cascading ice streams. Even the larger of these streams, however, attained a length of only 10 miles. The ice streams therefore fell short of uniting to form a major trunk glacier corresponding to the ones in the main drainage basins to the north and in the Kern Basin to the east. There formed, instead, relatively small separate glacier systems, one or more in the headwater areas of each of the main branches of the Kaweah. Only locally, in their upper reaches, did these glaciers oversweep the divides; for the most part the glaciers were confined to the canyons, and these they filled only in part. The lowest altitude reached by ice in El Portal Stage was about 4,550 feet; in the Wisconsin Stage, about 5,200 feet.
The Kern glacier system, by contrast, was a great, many-branched ice body fed from ranks of cirques along the high bordering ranges. Since the Kern Canyon extends in a nearly straight line through the middle of the upper Kern Basin, and the tributary canyons branch from it like the ribs in an oak leaf, the Kern glacier system had much the same leaflike pattern.
The maximum extent reached by the Kern glacier system in the Glacier Point Stage cannot be determined with certainty, but the evidence would seem to warrant the inference that the glacier advanced approximately as far as its successor of El Portal Stage.
Records of El Portal Stage, though incomplete, can be interpreted with more assurance. The volume of ice was then greater in some places than the canyons could hold; the ice locally spread across intervening divides and over benchlands on either side of the main canyon to a total breadth of 4 to 6 miles, thus producing a central ice sheet about 30 square miles in extent. The overall length of the Kern glacier system was 32 miles; the terminus of the trunk glacier lay at an altitude of 5,700 feet in the bend of the canyon to the north of Hockett Peak (at lat 36°14', which may represent the southern limit reached by glacial ice in the Sierra Nevada).
Records of the Wisconsin Stage are for the most part very well preserved. They indicate that during this stage the Kern glacier system had less volume than during El Portal Stage and remained a sprawling ice body whose trunk and branches lay confined within their respective canyons as distinct ice streams, separated from one another by mountain spurs or low divides. The tributary glaciers were as much as 15 miles long. The overall length of the Kern glacier system was 25 miles. The farthest point reached by the terminus of the trunk glacier coincides with the south boundary of the park; the boundary posts stand at an altitude of 6,350 feet on the curving outer moraine that marks the extreme limits of the Wisconsin glaciation.
The time available for this reconnaissance did not permit subdividing the Wisconsin Stage into substages. However, morainal complexes were noted that may throw light on this subject as detailed investigations are undertaken in the future.
The more significant postglacial changes in the park include those which have served to modify the form of the Kern Canyon. The present U-shaped form of that canyon is not precisely the one that resulted from repeated glacial erosion. The walls, once smooth, are now furrowed by gullies, and talus slopes at their base define the curves of a new U-shaped form superimposed on the glacially eroded one. The low gradient of the canyon over a stretch of several miles suggests that the glaciated rock floor may have been excavated into a chain of lake basins that are now filled with sediments.
Last Updated: 03-Aug-2009