The Sierra Nevada runs nearly 400 miles along the eastern edge of the state, varying in width from about 50 miles to 80 miles at its widest. The northern portion of the range is heavily forested with lower elevations - the significant peaks are few and just manage to poke up from the surrounding forests. Starting at the northern end of Lake Tahoe, the peaks begin to rise above 9,000ft and the drama of the Sierra begins to unfold. Ski resorts abound in the popular Lake Tahoe region. The more pristine mountain areas are encompassed within the Mt. Rose, Granite Chief, and Desolation Wildernesses. Between Lake Tahoe and Yosemite National Park the peaks rise higher still, up to about 11,000ft. Volcanic rock dominates this area, making for poor rock climbing opportunities, but scenic hiking and scrambling. Most of the high country in this area is protected within the Mokelumne, Carson-Iceberg, and Emigrant Wildernesses. Starting with the northern boundary of Yosemite NP and extending south to the Golden Trout Wilderness lie the highest peaks in the range, the finest rock climbing, and the most inspiring scenery. Called the High Sierra, this region makes up the heart of the range and its signature attractions - an almost limitless number of peaks and some of the best granite features found anywhere on the continent. Here the eastern escarpment of the range is dramatic, in places rising more than 10,000ft in a few short miles from the Owens Valley below. Many fine peaks above 12,000ft find their home here. Though most have seen some technical rock climbing routes established, the possibilities have even now only been touched on. Comprising the tail end of the range, the Southern Sierra extends to Tehachapi Pass and is a drier, lower elevation region. The peaks in this region are still challenging and the rock climbing opportunites superb. Visitors here are afforded more solitude than found in the more popular areas of the range.
The two largest metropolitan areas in the state are the Bay Area (San Francisco/Oakland/San Jose) and the Greater Los Angeles Area. The third largest population center is San Diego, but folks there mostly follow the route taken by those in the southern portions of the LA area. Folks in this southern area generally take US395 straight up to the eastern side of the range. There is a stoplight at Kramer Junction (with SR58) which can create significant traffic congestion at busy times. If coming from the San Fernando Valley or areas thereabouts, SR14 through Mojave and then US395 is the usual route to the east side. Lone Pine takes about 3-4hrs to reach from LA. If travelling to the west side, I5 to US99 is a straight shot north. If heading to the Lake Tahoe region, going up the west side is faster, but the east side is far more scenic. In either case, the drive to Tahoe is a long one, 7-8hrs by the west side, another hour or so on US395. In winter, the east side route is sometimes closed above Bridgeport during snow storms.
From the Bay Area, access is a bit more complicated. Easiest of all to reach is the Tahoe region, a relatively short drive (~3hrs) on I80/US50. I580 is usually used from the North Bay to reach US99 and places south of Lake Tahoe. From the South Bay, SR152 across Pacheco Pass is usually quicker. Both of these routes see high traffic on Friday (outbound) and Sunday (returning) evenings. If trying to reach the east side, the usual route is SR120 through Yosemite during the summer (~5hrs to Mammoth). When SR120 is closed for the winter, access to the east side is much longer, requiring a drive south to SR178 or SR58, or north across US50 or SR88 to US395 and then south. As this can take 8hrs or more, only the hardiest of weekend warriors will make this gruelling winter pilgrimage.
Most of the remaining population centers in the state are in cities along US99, having obvious access to the Sierra. The two most populated regions just outside the state are found in Las Vegas and Reno. The latter sits right on US395, while residents of Las Vegas will have a longer drive (~5hrs), crossing Death Valley via SR190.
In addition to the major roads listed above, a variety of other roads provide partial access to the range from both the west and east sides. On the west side, SR140 and SR41 provide access to Yosemite as far east as Yosemite Valley. SR168 provides access to Lake Thomas Edison and Florence Lake (between Yosemite and SEKI NPs), though in winter it is closed past Huntington Lake. SR180 and SR198 service western approaches to Sequoia-Kings Canyon NP (SR180 to Cedar Grove is closed in winter). SR190 and SR155 provide additional access to Southern Sierra from the west side. On the east side, there are many roads leading to trailheads on the steep escarpment, most of which are closed in winter.
More on Sierra Nevada Highways from sierranevadaphotos.com
Get current road conditions from CalTrans
In geologically recent times, the Pleistocene Ice Age brought significant glaciation to the range. Rivers of ice carved out their former stream channels and created the sheer walls and hanging valleys found in Hetch-Hetchy, Yosemite Valley, and Kings Canyon. As they retreated, the glaciers clung tenaciously to the north facing slopes of the higher peaks, fracturing and forming the steep north faces found on many of them. The glaciers that remain in the Sierra are but the remnants of this colder clime. Doing very little mountain carving in this day and age, most Sierra glaciers sit quietly in the shadows of the north faces, biding their time until the next Ice Age emerges.
Learn more from the USGS and the Yosemite & SEKI NPS sites