In a subtle dance of arid solitude, two southern California deserts come together at Joshua Tree National Park. One is the Colorado Desert--an extension of the Sonoran Desert--and features natural gardens of ocotillo and cholla cactus. The other end of the park engages the Mojave Desert, the special habitat of the Joshua tree as well as some of the most interesting geologic displays found anywhere. After the area became a national monument in 1936, local and regional residents were the primary visitors. As Southern California grew so did park visitation; Joshua Tree now lies within a...