natural resources
 

wildlife
 

Part of Idaho's Minidoka Bird Refuge, the City of Rocks is home to eagles, falcons, vultures, hawks, hummingbirds, jays, sparrows, doves, and the state bird, the mountain bluebird. Among the mammals that live within the park are elk, mule deer, mountain lions, coyotes, badgers, bobcats, porcupines, ground squirrels, and bats. Reptiles such as the sand lizard, watersnake, blowsnake, rubber boa, and the park's only poisonous snake, the rattlesnake (found only at lower elevations), also live within the City of Rocks. All plants and animals are protected by law and should not be disturbed.

 

vegetation
 

The range of elevations within the compact area of the Reserve combines with other factors to create varied patterns of vegetation and wildlife habitat. At high elevations the forests are of lodgepole pine, limber pine, and Douglas fir. Middle elevation forests are of quaking aspen, mountain mahogany, and cottonwood. Sagebrush, pinyon pines, and juniper dominate lower elevations. The Reserve boasts Idaho's tallest pinyon pines, at more than 55 feet. The nuts of the trees provide important proteins and fats for wildlife. In addition to the trees, spring and summer displays of wildflowers can be spectacular. Over 450 plant species have been recorded at the City of Rocks.

 

geology
 

More like mother and daughter than siblings, the composition of the Twin Sisters helps to explain how the City of Rocks came to be. The darker sister is made of rock that geologists call the Green River Complex. It is 2.5 billion years old and is some of the oldest rock in the lower 48 states. The lighter sister is made of rock in a far younger formation that geologists call the Almo Pluton. At 25 million years old, it is a relative newborn!

Both formations began as molten matter in the Earth's crust. Eventually the Almo Pluton was thrust up through the Green River Complex while both formations still lay beneath the Earth's surface and other layers of rock. As time passed, the overlying rocks and the formations beneath them cracked. Along the cracks and fissures erosion took place more rapidly and exposed the rocks of the Almo Pluton and Green River Complex.

The exposed rocks were then shaped by the forces of erosion; the tops of rocks are dissolved by rainwater and minerals such as iron oxide are redeposited to form crust-like caps. These caps are more resistant to weathering than the underlying rock, and this causes the formation of spires and pinnacles. When the caps erode, the inner rock can be molded by erosion into the many caves, arches, bath tubs, and hollow boulders seen at the City of Rocks.

When water seeps into cracks on the sides of spires and then freezes, frost wedging can occur. As it freezes, the water expands and can cause great slabs of rock to crack off. This process has already removed some of the layers of rock bearing 150-year-old signatures left by the pioneers. Today, many people see animals, faces, or buildings in the shapes of these rock formations. Only your imagination limits what you see in the City of Rocks.