The Geology of Anza Borrego
Some of the incredible geology at Font’s Point in Anza Borrego State Park.
Anza-Borrego Desert State Park is 640,000 acres in Imperial, Riverside, and San Diego counties. The park is a great bowl surrounded by mountains and is home to rocky slopes, alluvial fans, sandy washes, and deeply eroded “badlands” all of which can be explored by two-wheel or four-wheel drive vehicles or by hiking, biking, and equestrian trails.
Ancient tectonic and volcanic activity in or near the oceans of the western edge of North America (170 to 70 million years ago) produced magma that cooled to form igneous rocks, including granite. Granitic rock that has been heated and pressed becomes a metamorphic rock called gneiss. Material that was lifted and deposited in the oceans as sedimentary rock has also been heated into metamorphic rocks.
Later seismic activity (around 30 million years ago) was responsible for the fault zones we know today, including the Elsinore, San Jacinto, and San Andreas. Movement along these zones brought those deep volcanic rocks to the surface, and then lifted them to create mountains. The same movement caused many of the layers of ocean sediments to be stretched and tilted on these mountains. And those fault zones are still active, so the Anza Borrego landscape is still actively forming and changing.
The Salton Buttes are a series of lava domes that formed in the stretched and thinned crust in the last few thousand years. This thin crust also allows Earth’s heat to be so close to the surface, that the heat is used for geothermal power plants.
At the same time that the mountains are rising along faults, they are being worn down by wind, water (rain, snow, and ice), and gravity erosion forming badlands in three portions of the park. The eroded material is carried by streams into lower-lying basins, where it accumulates in layers to become sediments and sedimentary rock which may some day be lifted up to become new mountains, continuing the cycle. Fonts Point provides a breathtaking view of the badlands area.
Wind and water action on sandstone is also responsible for exposing the “pumpkin patch” field of sandstone concretions that were likely formed similar to a pearl in an oyster, with layers of shell, rock, or insects building pumpkin-size concreted balls of sandstone.
The Geology of Vegetation
The geologic diversity of Anza-Borrego provides for a terrific diversity of plants as well. While some plants, such as cactus, desert agave, and brittlebush can be found on rocky slopes, other plants such as wildflowers, Ocotillo or palms require sandy soils in washes and alluvial slopes, and shallow groundwater.
Palm oases have attracted animals and humans for thousands of years. While we may not want to drink the water from these underground springs, native animals including bighorn sheep, bats, and birds require that water to live. It’s easy to figure out where these oases will be located; California Fan Palms indicate the constant presence of water. The springs don’t always rise to the surface, but you can be sure that if there are palms, the water is not far away.
Some animal homes among the vegetation in the desert.
In the springtime, Anza-Borrego’s sandy desert floor provides spectacular spring-time wildflower viewing. Hundreds of flowering species of plants can be found in Anza-Borrego. Seed germination, as well as flower size and variety depend on sufficient moisture from winter rain and on warm temperatures in early spring. Both of these factors vary every year. When conditions are perfect, the desert can explode in a magical super bloom. This carpet of color is incredible and attracts a lot of people so bring your camera and your patience.
