Wednesday, July 19, 2023

Devils Postpile

Devils Postpile
1.3 miles loop, 250 feet elevation gain
Difficulty: Easy
Access: Devils Postpile bus to trailhead (unless arriving before 7 AM), Devils Postpile National Monument entrance fee and bus fare required

California's Devil Postpile National Monument, nestled in the Sierra Nevada outside the town of Mammoth Lakes, is home to one of the most perfectly geometric examples of columnar basalt in the American West. Although such hexagonal rock columns are not rare in the western US, Devils Postpile is an exemplar of this type of unique volcanic rock. This is one of the top attractions in the Eastern Sierra and should not be missed; the hike to reach the formation is easy and short and includes views of both the front cliffs of the Postpile as well as a chance to study the neat geometric patterning from above. Devils Postpile can only be visited during summer, when Reds Meadow Road is open past the Mammoth Mountain Ski Resort to reach the national monument.

Due to its status as one of the top attractions around Mammoth Lakes, visitors looking to go to Devils Postpile in the summer months must ride a shuttle bus from the Mammoth Mountain resort. Buses run during the day from Mammoth Mountain, making stops at Agnew Meadows, Devils Postpile, the Rainbow Falls Trailhead, and Reds Meadow; tickets can be purchased online in advance. The Reds Meadow Road is only open to passenger cars outside of the bus system’s operating hours; visitors hoping to drive to the Devils Postpile Trailhead must pass the entrance kiosk at Minaret Summit before 7 AM.

Devils Postpile is a long way from any major city (Reno, Nevada is perhaps the closest) but is a short drive from the town of Mammoth Lakes, the primary ski and outdoor destination of the Eastern Sierra.

I visited Devils Postpile on an early July morning: as the bus system was in effect, I found myself arriving at Minaret Summit at sunrise to be able to drive to the national monument. From the center of town in Mammoth Lakes, I followed Minaret Summit Road to the northwest, crossing the saddle and passing the entrance kiosk to enter the San Joaquin watershed. The entrance kiosk was not staffed at 6 AM but it is staffed later in the day- expect to stop at the kiosk on your way out to pay entrance fees. At this point, the Minaret Summit Road became the Reds Meadow Road and began descending into the Middle Fork San Joaquin River valley. While the road was paved the entire way, the stretch of road just past Minaret Summit was hairiest: here, there were no lane divides, with two-way traffic on a windy mountain road (including occasional buses) accommodated on a single lane road. After reaching the bottom of the valley and passing the Agnew Meadows turnoff, the road became an easier drive, with defined lanes the rest of the way down to the right turnoff for Devils Postpile. Taking the turnoff, I followed the road down to a large parking lot, which was already half full just after 6 AM on the Fourth of July weekend. There was plenty of parking and bathrooms at the trailhead.

The trail to Devils Postpile left the parking area and headed south along the Middle Fork San Joaquin River, quickly entering a pleasant riverside meadow. After crossing the meadow, the trail reentered the forest but stayed by the riverbank, with partial views of the river to the right.

Middle Fork San Joaquin River
At a third of a mile from the trailhead, I came to a junction with the Upper Postpile Loop. I stayed on the lower trail, which immediately after came to the foot of a spectacular wall of hexagonal basalt columns- the Devils Postpile. On the left end of the postpile wall were a group of warped columns that had clearly been subject to more geological forces since the time of their origin. These twisted columns allowed me to gaze directly onto the bases of the hexagonal columns.

Devils Postpile
Beneath the postpile wall, a pile of broken hexagonal columnar basalt talus littered the hillside; the extreme geometric nature of these rocks made them look artificial, as if this were the dumpster for some darker-covered concrete columns rather than a natural wonder. I walked a hundred feet south along the trail, enjoying impressive direct views of the postpile throughout.

Devils Postpile
How did such a unique geological occurrence come to this valley at the headwaters of the Middle Fork San Joaquin River? The postpile's ultimate origin is shared with the origin of the rest of America's Western Cordillera: the subduction of the Farallon Plate beneath the North American Plate. This subduction event created the Rocky Mountains as well as the magma intrusions into the crust that were responsbile for Devils Postpile. About a hundred thousand years ago, one such magma intrusion erupted in the Reds Meadow Valley and formed a lake of mafic (basaltic) lava; as the lava lake cooled into rock, the lava began to contract, forming boundaries known as cooling joints that were perpendicular to the surface of the cooling lava. As these cooling joints extended through the lava, they formed hexagonal columns as the lava cooled into basalt. Later erosion by glaciers and the Middle Fork San Joaquin River exposed the columns and created the cliffs of basalt that we see today.

The trail passed by the base of the postpile and then began a short ascent, reaching a second junction with the Upper Postpile Loop at a half mile from the trailhead. This time, I took the left fork to follow the Upper Postpile Loop, which I took for the return trip to the trailhead. The Upper Postpile Loop continued ascending a small hill and immediately provided me a closer opportunity to study the columnar basalt. The columns here were at an angle, which indicated that geological forces have warped the original basalt formation: the original cooling joints (and thus the columns) would have all formed vertically.

Columnar basalt
The loop trail ascended 200 feet above the Middle Fork San Joaquin River before it began to drop downhill again, reaching an interesting view of the columnar basalt from above at 0.8 miles into the hike. Here, glacier polish had left a flat, smooth surface of hexagonal basalt tiles, where each tile was the cap of one basalt column. This was a rare and unique opportunity to see a cross-section of the geometries of these columns, which turned out to not all be hexagonal: while six-sided columns were the most common, I also spotted a number of 5 and 7-sided columns.

Smooth caps of the Devils Postpile
After marveling at these bizarrely cleanly-shaped rock columns, I finished up the Upper Postpile Loop with a final descent back to the main trail next to the Middle Fork San Joaquin River, closing the loop at just under 1 mile; from there, I followed the trail along the river just one-third of a mile north back to the trailhead and parking area. I coupled this hike with a later hike to Rainbow Falls to see the two main attractions of the national monument and enjoy its fascinating geologic history.

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