According to a free map at the Joshua Tree Visitor Center, the boundary between the Mojave and Colorado Deserts runs right through the Cottonwood Mountains. So I guess these plants were found generally along that divide. Feel free to correct me if I misidentify any of them.
Mojave Yucca along the ridge.
Early season Ocotillo bloom.
Hedgehog cactus?
Silver Cholla cactus?