Hiking the Tahoe Rim Trail

The Carson Range
Gorp.com


To most earth scientists the Carson Range may be little more than an appendage of the northern Sierra Nevada. It is, however, respectable in its own right, with dimensions very similar to those of southern California's San Bernardino Mountains. The Carson Range splits from the Sierra Nevada proper at Carson Pass, then arcs gracefully northward past Tahoe, ending indistinctly just south of Interstate 80's borderline town of Verdi, about 57 miles north.

The range is very old, becoming the entity it is today perhaps around 80 million years ago when, during one or more episodes of faulting, the crust of the earth hereabouts was stretched and crustal blocks subsided. The Carson River's valleys, east of the range, came into being, as did other east-side basins and valleys such the Mono Basin and Owens Valley. Likewise, the Truckee River's valley came into being, but sans Lake Tahoe. The lake is a geologically young feature, brought about when, roughly 4 million years ago, faulting was reactivated in the river's drainage, accompanied by volcanic eruptions. The outpouring of lava constructed a dam across the river valley, and over time Lake Tahoe ponded up behind it. It is this massive accumulation of lava that today makes up the north-rim lands of the Lake Tahoe Basin.

The rocks of the Carson Range are very similar to those of its western counterpart, lands along the Sierra Nevada crest. Both are dominated at their north and south ends by volcanic rocks, with mostly granitic rocks in between. But when you compare the granitic Desolation Wilderness landscape with the granitic central Carson Range landscape, you quite a difference. This is due mainly to the rain shadow cast by the high ridges of Desolation Wilderness. Most storms approach the Sierra from the west, and they unleash most of their precipitation usually snow as they rise up the western slope, because air cools as it rises, and the colder the air is, the less water vapor it can hold. By the time a storm has worked its way from the Sacramento Valley, near sea level, 9000 feet up to the Sierra crest, its air mass has cooled about 30 degrees Fahrenheit and its water-vapor capacity has halved. Half of its water vapor has been converted to precipitation. When the storm reaches the Carson Range, it is carrying much less water, and as a result this high country receives only about half the precipitation received by Desolation Wilderness.

How does this precipitation difference affect landscape development? During the Pleistocene epoch, or"Ice Age," snowfields covered Desolation's uplands and were the source of huge glaciers that scoured out loose bedrock on lands east and west of the ridges. With considerably less precipitation than its counterpart, the Carson Range spawned glaciers only at its high-elevation north and south ends. While admirable in size, these glaciers were no match for the giant Sierra-proper glaciers, and only one sizable lake basin was created through glacial erosion that of Star Lake.

The central Carson Range was unscathed by glaciers, though during the Pleistocene it probably had some very decent snowfields. Because it was not scoured by glaciers, its soils were not removed. Whereas most of Desolation's soils where they exist! have formed from glacial deposits left when glaciers finally retreated about 13,000 years ago, the central Carson's soils never were eroded in the first place. The range has had soils for tens of millions of years. Over this time, soil creep, landsliding, and stream erosion have removed soils, but the soil cover has continually been replenished by newly weathered fragments of bedrock.

An advantage of old, deep soil is that it can hold more water than do new, discontinuous patches of soil in Desolation. While most creeks are rapidly drying up in Desolation after the snow melts, the creeks in this area may still flow, since abundant water can be stored in deep soil. I stress "can" because in actuality, much of the Carson Range is quite dry. There are at least two reasons for this. First, slopes often are fairly steep, and on such slopes soil creep the downward movement of grains occurs sufficiently fast so that a thick soil never develops; the steeper the slope, the faster the soil creep, and the thinner the soil. Second, lying in a rain shadow, the range's plants receive less precipitation, so the density of trees, shrubs, and herbs is less. Where vegetation is dense, as in the Sierra proper, plants can hold soil creep to a minimum. Furthermore, where vegetation is dense, a lot of plant litter accumulates, which can retard soil creep, but perhaps more important, the litter can be converted to humus. This humus in the soil can retain a lot of water. The Carson Range's granitic soils are mostly grains of quartz and feldspar, and water flows easily among the grains. Therefore, only on gentle slopes or canyon bottoms, where granitic soils are thick, are you likely to find perennial streams.


Published: 29 Apr 2002 | Last Updated: 15 Sep 2010
Details mentioned in this article were accurate at the time of publication

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