Thousands of years of cutting and polishing by water and wind erosion have produced the chaotic rock formations and slotlike ravines you'll discover in Anza-Borrego's Calcite Mine area. During World War II, this was an important site -- indeed the only site in the United States -- for the extraction of optical-grade calcite crystals for use in gun sights. Trench-mining operations throughout the area have left deep scars upon the earth, seemingly as fresh today as when they were made.
Be aware that you will need an Anza-Borrego Desert State Park parking pass before starting any trek to the Calcite Mine (or to other off-pavement destinations in Anza-Borrego). This can be obtained in Borrego Springs and at several other sites; call 760-767-4205 or 760-767-5311 for more information. Drive approximately 20 miles east from Borrego Springs on County Highway S-22 and park in the roadside turnout at mile 38.0, or go 0.1 mile farther east to the Calcite jeep road intersection, where parking is more limited. The Calcite road ahead is open to high-clearance four-wheel-drive vehicles -- but hiking offers both good exercise and more flexibility in choosing a return route.
Follow the jeep road as it dips into and out of South Fork Palm Wash and continues northwest toward the southern spurs of the Santa Rosa Mountains. Ahead you will see an intricately honeycombed whitish slab of sandstone, called Locomotive Rock, which lies behind (northeast of) the mine.
About 1.4 miles from S-22, the road dips sharply to cross a deep ravine. Poke into the upper (north) end of this ravine and you'll discover one of the better slot canyons in Anza-Borrego. At 1.9 miles, where the road ends, look for small calcite crystals on the ground and along the walls of the vertical mining trenches.
On the return, try this alternate route: Backtrack 0.5 mile to the aforementioned slot ravine. Proceed downstream along its bottom. As you pass through deeper and deeper layers of sandstone strata, the ravine narrows until it allows the passage of only one person at a time. There's a "pouroff" at one point, necessitating a bit of a jump. When you reach the jumbled blocks of sandstone in Palm Wash at the bottom of the ravine, turn right, walk 0.3 mile downstream, and exit the canyon via a short link of jeep trail that leads back to the Calcite road.