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Collisions In The Eastern Sierra

Mount Whitney

Later this week I plan to drive north to visit friends and chase birds in the Eastern Sierras. I always enjoy travel to the region, in no small part because of the spectacular scenery. I’ve done some studying about the geology of the area and I’d like to share it with you.

Driving north on US-395 from where it splits from the I-15 (2800’) north of Cajon Pass (3776’), the scenery is that of a sparse Mojave Desert with little vegetation save for the occasional Joshua Tree and the desert scrub and sage that eeks out an existence here. Reaching Inyokern (2434’) about 100 miles further north, the southern Sierra Nevada Mountains appear in the distance, and the promise of spectacular scenery ahead teases the imagination. Sixty-five miles later you reach Lone Pine (3727’) and the valley floor narrows between the overthrust of the Sierra Nevadas to the west and the anticlinal Inyokern range to the east. Near Lone Pine is Owen’s (Dry) Lake, the final resting place for the Owen’s River which drains the valleys to the north. So geologically tortured is the region, the highest point in the lower 48 states (Mount Whitney) is a mere 13 miles to the west, and the lowest point in the western hemisphere (Death Valley) is 73 miles to the east. Here the valley floor has become a more dense version of the barren plains we passed further south, but with more sagebrush. Bishop (4150’), lies in the valley 58 miles further north, but the next 24 miles to Tom’s Place California, US-395 climbs dramatically to an altitude of 7090’. We see here the vegetation transforms from Mojave Desert the Great Basin Sage. Twenty-three miles further north on US-395, and a little east of Crestview (7520’) is where the Owen’s River begins its journey southward. Here there is increasingly younger volcanism at work. North for approximately 25 miles is some of the most spectacular terrain so far, as we pass Mammoth (7880’), June Lakes (7654’) and Lee Vining (6781’). The volcanic features in this region abound, culminating with Mono Lake which erupted violently only 270 years ago. This region has produced 11 major volcanic eruptions within the last 700 years. The king of eruptions in the region was the massive eruption that produced the 20 mile (east-west) by 11 mile (north-south), 3000’ deep, Long Valley Caldera 760,000 years ago. That eruption scattered ash and debris over most of the western USA as far east as Kansas and Nebraska and south to northern Mexico. The caldera between Mammoth Mountain to the west and Glass Mountain to the east, still has molten magma less than 4 miles below the valley floor.

We might ask “What caused all these volcanic rumblings and what caused the surrounding mountains to form?” Science tells us that 20 million years ago, there began a great tectonic collision of a westbound (east-west stretching, actually) continental plate creating the “basin-range” topography still present in the Great Basin, and an eastbound Pacific plate. The Pacific plate slid below (subducted) the continental plate and the resulting volcanism built a line of volcanoes which produced large plumes of granite that later hardened under the cones that eventually eroded away due to the movement of thick ice sheets that formed during the epoch that followed, leaving behind the over-the-top scenic valleys such as Yosemite. About 5 million years ago the continental plate began to subduct. This tilted the block of crust formed during the prior collision, sloping now toward the west and lifting the broken eastern edge to expose the dramatic east face of these mountains we’ve come to admire.

There is a different though related story about the Inyokern Mountains that rise to the east of the Sierra Nevadas. These mountains are also tall. In fact, their highest point, White Mountain (14,252’) is nearly as tall as Mount Whitney (14,505’). These mountains are part of an “anticline” structure. An anticline is produced when forces push towards each other and the crust is bent, or folded up, much like a rug pushed over the floor until it meets a wall and buckles and folds upward. The spreading action of the basin-range formation in the Great Basin was pushing the crust west faster than the subduction under the Sierras could swallow it. Hence the continental crust buckled to form this interesting range of mountains.

Travel to this region has never failed to provide me with nice images, be they wildlife or scenery. I’ll be sharing what ever I get , so keep an eye out!