March 1, 2017

A day in the desert

Posted by Nanci Bompey

By Jenny Lunn

Anza-Borrego Desert State Park

Each year, AGU invites the Editors-in-Chief of the organization’s 20 journals to a retreat with members of the publications team and other senior staff to discuss strategic issues across the journals. This year’s meeting was held in San Diego, CA, and started with a field trip to the Anza-Borrego Desert.

Anza-Borrego Desert State Park is the largest state park in California extending more than 900 square miles. It is located about 70 miles east-northeast of San Diego. More than 100 different films and TV series have been shot in the park, with the landscape representing a range of locations, both on Earth and other planets.

The journey to the park’s eastern boundary just south of Ocotillo Wells was an interesting trip across climatic zones. We started off down by the sea at Mission Bay in San Diego, surrounded by tropical vegetation and sea lions honking their noisy accompaniment to our breakfast. Driving east on Interstate 8 we climbed into the Cuyamaca Mountains. At around 4,000 feet, we were completely enveloped in the clouds and there was snow on the side of the road.

Route from San Diego to Anza-Borrego

Turning north onto Route 79, we passed through Cuyamaca Rancho State Park, dominated by oak woodland forest, to the old gold mining town of Julian. Finally turning east on Route 78 we dropped down a steep and twisty canyon, with phenomenal cacti and other plants dotting the hillside, into a dry and desolate plain. Our bus stopped seemingly in the middle of nowhere and someone remarked that it looked as though we’d been dropped into the set of Breaking Bad!

Our guides for the day were volunteers from the Anza-Borrego Desert Paleontology Society, local enthusiasts who have spent many decades exploring and researching this region. We divided into 4×4 vehicles and set off in a convoy for an adventure up Fish Creek through Split Mountain, stopping off at the Wind Caves and Elephant Knees.

Anza-Borrego Desert is located on the boundary of the Pacific and North American tectonic plates. This is a transform boundary with the Pacific Plate moving northwards with respect to the North American Plate. Along much of its length, the plate boundary is a single, well-defined fault line but, in this region, it is a network of faults.

Fish Creek Wash

Over millions of years, these tectonic movements have stretched the continental crust, which has thinned and then sunk creating a large depression called the Salton Trough. A range of dramatic events have subsequently carried and deposited different sediments into this low-lying basin. AGU’s Director of Publications, Brooks Hanson, explained the complex landscape history succinctly to the non-experts with a deft hands-and-thumbs demonstration: “Sometimes the two plates don’t move parallel to each other and it creates a big hole and then stuff falls into that hole.”

Six million years ago, this low-lying basin was filled with tropical sea water, essentially a northern extension of what is now the Gulf of California. Meanwhile, to the north, the ancestral Colorado River was eroding the rock of the Colorado Plateau to create the Grand Canyon. This deposited an immense volume of sediment in the Salton Trough, forming a river delta. Over time, the landscape turned into a vast savannah, while sediments being eroded from the growing Santa Rosa Mountains to the west were deposited across the area.

Looking at oyster and scallop shells laid down 5 million years ago when this area was under the ocean

These broad phases of geological development deposited a range of marine, deltaic and riverine sediments, and left a fossil record of both marine and terrestrial organisms. In turn, these sedimentary materials have been subject to different kinds of weathering, and erosion by wind and water. This has created the badlands scenery of dry creek beds and alluvial fans, rocky canyons and bedding planes, strange plants and sea shells. Of course, this landscape is not static. It is an active tectonic zone where the processes of subsidence and uplift, tilting and faulting are ongoing, while erosion and weathering continue to shape the surface morphology.

This is a much-simplified version of a very complex geological landscape. I have visited a few deserts on other continents but this was quite different from anything I have ever seen before. Spectacular. Unusual. Terrifying. Awesome.

— Jenny Lunn is an Assistant Director in AGU’s Publications Department.