UC Irvine Reserves
|Jepson Nomenclature (PDF)|
The Burns Reserve is located in a region of rugged hills and mesas at the easternmost extremity of the San Bernardino Mountains. The San Bernardinos, highest of Southern California's Transverse Ranges, began uplifting about 5 million years ago in response to tectonic stresses along the San Andreas Fault. The Morongo Basin is a depressed block that lies between the Pinto Mountain Fault to the northwest and the Morongo Valley fault to the southeast.
When the San Bernardino Mountains rose, they created a rain-shadow to the north and east. As the region dried and soils eroded, the granitic bedrock was exposed. Regolith erosion, chemical weathering, and thermal spalling molded the bedrock boulders that characterize the Burns landscape today.
Pinto gneiss, an intensively metamorphosed rock of pre-Cambrian age, occurs on the reserve and on either side of the Morongo Basin. In a few places, limestones that contain Cambrian fossils are present, but most of the area's limestones are late Paleozoic. Granite, granodiorite, and gabbro intruded into these older rocks at various times. The latest intrusion - the White Tank quartz monzonite - was emplaced in the Cretaceous, 83 million years ago. It cuts through all older rocks and comprises the ridges of the reserve. A few miles north and east of the reserve, the huge, pale, monzonite boulders are capped by Tertiary sandstone mesas overlain by black Pliocene basalt.
The Burns Reserve lies near the northern margin of a giant granite pluton. As the pluton was elevated and eroded, dikes of finer grained granites and veins of milky quartz were exposed. Such veins in nearby sites were mined extensively for gold in the late 1800s.
No permanent streams exist on the reserve; water runs in the washes only after heavy rains. In some years, seeps emerge along a small fault in the northern branch of Railroad Canyon.
Earthquakes have rocked the region around the reserve, most recently in 1992 during the Landers earthquake (magnitude 7.3). The Landers-Mojave earthquake zone passes within 8 kilometers (5 miles) of the reserve in an extended region of distributed faulting and deformation. Research following the Landers earthquake suggests the presence of a young, active fault that obliquely cuts across several older strike-slip faults [Nur et al.: Science 261:201 (1993)].
University of California, Natural Reserve System
|Last Updated 10/03/02|