Was Joshua Tree underwater?

When you wander through the sunbaked deserts of Joshua Tree National Park today, with its dry washes, jagged granite peaks, and iconic yuccas, it's hard to imagine this landscape as anything but arid. But hidden beneath its rugged beauty lies a story written in stone—one that reveals a surprising truth: yes, Joshua Tree was once underwater. In fact, its geologic history spans hundreds of millions of years and includes ancient oceans, volcanic eruptions, and the uplift of entire mountain ranges.

In this article, we’ll dive deep—literally and figuratively—into the geological history of Joshua Tree National Park to understand how this now-dry desert was once part of vast marine environments.

Joshua Tree National Park

The Ancient Seafloor: A Paleozoic Past

To grasp how Joshua Tree was once submerged, we need to journey back more than 500 million years into the Paleozoic Era. During this time, much of what is now the western United States, including Southern California, lay near or beneath a shallow inland sea.

◉ These warm, shallow seas were teeming with marine life such as trilobites, brachiopods, and corals.

◉ Sediments from this ancient ocean floor—sand, silt, and carbonate—accumulated over millions of years and eventually compacted into limestone, shale, and sandstone.

◉ Though most of these sedimentary rocks have been deeply buried or eroded away in Joshua Tree, traces of them can still be found, especially near the park’s southeastern edge.

Subduction and Uplift: The Role of Plate Tectonics

Around 200 to 250 million years ago, tectonic activity dramatically reshaped this underwater landscape:

◉ The Farallon Plate (a now mostly-subducted oceanic plate) began colliding with and sliding beneath the North American Plate.

◉ This process of subduction caused intense heat and pressure, triggering episodes of volcanism and mountain building.

◉ Magma from below the Earth's crust intruded into older rocks, cooling slowly to form the granite seen in places like Ryan Mountain and Jumbo Rocks.

This granite originally formed deep underground but was later exposed through erosion and uplift over tens of millions of years, as the region gradually rose above sea level.

Evidence in the Rocks

While Joshua Tree’s current landscape may not show obvious fossils or marine formations like those seen in coastal areas, geologists have documented rock types that support its underwater past:

◉ Metamorphosed sedimentary rocks found in the Little San Bernardino Mountains and parts of the Pinto Gneiss complex are remnants of marine deposits altered by heat and pressure.

◉ Marine limestone, now rare in the park, once formed from ancient coral reefs and sea creatures that lived in the shallow seas covering the region.

These rocks provide crucial evidence that the land now occupied by creosote bushes and cacti was once part of a vast seafloor ecosystem.

A Desert Born of Water

As tectonic forces continued to reshape the western United States, the seas retreated, and mountain ranges formed. Over the past 20 to 30 million years, Southern California experienced significant:

◉ Crustal stretching, especially along the San Andreas Fault system, which created basins and mountain ranges.

◉ Volcanic activity, leaving behind lava flows and ash deposits in the region.

◉ Erosion and sedimentation, which sculpted the landscape we see today.

Eventually, the region dried out completely, and desert ecosystems began to develop. The Colorado and Mojave Deserts—the two desert regions that converge within Joshua Tree—became dominant, and plants like the Joshua tree (Yucca brevifolia) began to thrive in the dry climate.

The Last Trace of Water: Ancient Lakes and Flash Floods

Even in more recent times, water has left its mark:

◉ During the Pleistocene Epoch (Ice Age), about 2.6 million to 11,700 years ago, the region experienced wetter periods.

◉ Ancient lakes may have temporarily filled some of the basins during these wetter climatic phases.

◉ Today, water still plays a vital role in shaping the landscape through flash floods, erosion, and ephemeral streams, especially during the summer monsoon season.

Conclusion: A Landscape Forged by Water and Time

So, was Joshua Tree underwater? The answer is a resounding yes—many times over millions of years. The park’s dramatic terrain, dry washes, and granite domes are the product of a dynamic geologic history that includes:

• Shallow seas
• Marine sediments
• Subduction and volcanic activity
• Uplift, erosion, and desertification

Understanding this underwater past gives new depth to Joshua Tree’s present. It reminds us that even the driest deserts were once oceans and that every rock and ridge tells a story billions of years in the making.