A Natural History of Reds Meadow

Located in the heart of the legendary Sierra Nevada Mountains, the Reds Meadow region contains some of the most majestic and awe-inspiring arrays of landforms in North America. From the stately basalt pillars of the Devils Postpile to the shimmering radiance of Rainbow Falls, visitors to Reds Meadow are deeply immersed into an exciting record of Earth’s most awesome processes.

Only 14 miles from downtown Mammoth Lakes, the Reds Meadow area is only minutes away. But before you pack up your hiking, fishing, or photography gear and head for the scenic beauty of the meadow and the Middle Fork of the San Joaquin River, take a minute to review the natural history of the region. From the birth of the Sierra Nevada Range to explosive volcanoes and rock-pulverizing glaciers, the history of Reds Meadows is one adventurous tale.

The Birth of the Sierra Nevada Mountain Range

The mighty Sierra Nevada Mountain Range began rising from the ancestral Pacific Ocean around 150 million years ago. Incredible amounts of pressure, generated from friction between two colliding tectonic plates, caused the Earth’s crust to buckle and rise at the western edge of the North American continent. At this tectonic plate boundary, termed a subduction zone, dense oceanic crust was forced below the westward migrating North American continent. At depth, the oceanic crust melted and rose, forming the impressive Sierra Nevada batholith granites and feeding numerous volcanoes throughout the region.

The mountain range gradually kept rising, albeit slow compared to modern rates of uplift in the Sierras, for the next 145 million years. Erosion of the mountains during the Cenozoic Era (which geologists use to refer to the past 65 million years of the Earth’s History) cause the accumulation of the thick sedimentary rocks of California’s Great Valley. Throughout most of this time, the Sierras were much lower in elevation than today. It is believed that up until 3 million years ago, the ancestral San Joaquin River, which now barrels over Rainbow Falls, flowed from central Nevada through the Sierras on its way to the Pacific.

Until geologically recent, the Sierra Nevada Range did not climb into the impressive altitudes we see today. Around 2 million years ago, the mountain range began rising at very rapid rates on the order of several millimeters per year. The elevation increase combined with an overall cooler global climate caused the creation of massive alpine glacial sheets. These glaciers deeply carved into the rocky mountainsides of the Sierras as they advanced and retreated, shaping the present configuration of the Reds Meadow valley.

The Devils Postpile

The legendary basalt columns of the Devils Postpile formed around 100,000 years ago, when lava rich in iron and magnesium poured forth from a fissure in the Earth’s crust near the headwaters of the Middle Fork of the San Joaquin River. This basaltic flow, which would have been similar to Hawaiian-style lava, traveled down the river valley until it reached a natural dam. This obstruction caused a great, 400 foot-deep lava lake to form at the present site of the Devils Postpile.

As a result of the lava lake’s great depth, the molten lava cooled to solid-rock basalt very slowly. When molten rock cools, it contracts (or shrinks) as it turns to a solid. This contraction creates cracks, or joints to a geologist, due to the difference in volume between molten and solid rock. In ideal conditions, cooling basalt forms joints in hexagonal patterns, which is termed columnar basalt. These joints start at the surface and migrate downward as the internal mass of the lava would have cooled, forming the thick pillars we see today at the Devils Postpile. The five to six-sided pillars average 2 ft. in diameter and are up to 60 ft. in length and may be viewed in “tile-like” cross-section from the top of the outcrop.

Since it’s formation, the basalt of the Devils Postpile has been relentlessly attacked by the erosive and destructive powers of weather, glacial ice, moving water, and earthquakes. The seasonal freeze-thaw cycle combined with sporadic earthquakes has very effectively broken apart the basalt. The base of the outcrop is littered with massive columns that have succumbed to the powers of nature and gravity.

The greatest force responsible for the configuration of the Devils Postpile is glaciation. Three distinct Sierra Nevada glacial periods, beginning around 70,000 years ago and ending 10,000 years ago, relentlessly dug into and carried away the dark basalt. Without the marvelous power of alpine glaciers, the spectacular columns of the Devils Postpile may not have been exhumed. At the top of the columns, which is open for visitor exploration, the basalt rock is polished and striated (the geologic term for scratches glaciers leave in rocks) from the waxing and waning of the most recent glacial ice flows.

Rainbow Falls

At Rainbow Falls, rushing water of the Middle Fork of the San Joaquin River cascades 101 ft. over a cliff of dark volcanic rock. At this location, which is only 2 miles south of the Devils Postpile, the cliffs are composed of rhyodacite, which is significantly different in composition than the basalt to the north. This volcanic rock formed in two stages beginning 70,000 years ago, when a vent to the east of the falls explosively erupted and ejected silica-rich magma into the San Joaquin River drainage. The first eruptive stage was quickly covered by the second round of magma, causing the lower volcanic rocks to cool and crack at a much slower rate.

Like the Devils Postpile, the current appearance of Rainbow Falls is a result of several erosional forces working in tandem. Glacial activity greatly shaped the broad valley through which the San Joaquin River flows. The most important mechanism for the shaping of Rainbow Falls is erosion from the river itself. Because the lower volcanic rocks, or the first volcanic event, exposed at the falls are more prone to erosion, the waterfall creates an undercutting effect. This undercutting has created the broad basin surrounding the waterfall and has caused the falls to migrate upstream. It is believed that undercutting has allowed the falls to recede up to 500 ft. upstream over the past 10,000 years.

Rainbow Falls aptly gets its name from the dazzling array of colors generated by summer sunlight passing through the thick cloud of waterfall mist. This optical display is a result of refraction, reflection, and dispersion of light as it passes through the small droplets of water suspended in the air (or mist) surrounding the falls. The best time to experience this wondrous phenomenon is during the early afternoon, when the sun reaches it’s apex for the day.

Fast Fact In 1911, President William Howard Taft issued a presidential proclamation that assigned National Monument status to the Devils Postpile area. This action was in response to public outcry, including that of John Muir, resulting from a proposal to build a hydroelectric dam in the valley.

Planning Your Visit

Reds Meadow and the Devils Postpile National Monument are open for the summer 2015 season. When visiting, make sure to stop by the ranger station near Soda Springs for information on accessibility and for the most current trail conditions. While you are there, make sure to pick up maps and brochures describing the area and don’t forget to inquire about guided tours of the Reds Meadow area.

Getting There

The Devils Postpile, Rainbow Falls and Reds Meadow are accessible from the terminus of California State Highway 203, 10 miles past Mammoth Mountain Ski Area’s Lodge. A mandatory shuttle bus transports visitors during the busy summer months in order to eliminate automobile congestion. The bus departs from the Mammoth Adventure Center and the Village at Mammoth.

Visitor Resources:

• Current operational hours and additional information: http://www.nps.gov/depo/planyourvisit/hours.htm
• For current road conditions: http://www.visitmammoth.com/about-mammoth/road-conditions/
• For Reds Meadow/Devils Postpile Shuttle information, visit: http://www.nps.gov/depo/planyourvisit/reds-meadow-and-devils-postpile-shuttle-information.htm
• For National Park Service informational brochures (PDF files): http://www.nps.gov/depo/planyourvisit/brochures.htm