Saturday, September 23, 2023

An Aerial Tour of the Stanislaus Table Mountain

This is a short blog series of informational articles from my college faculty website that is soon to be extinct (arcane unsafe software, they say). Way back in 2002, the parent of one of my students offered me a flight of my choosing, and I knew it had to be Stanislaus Table Mountain. It is one of the more famous geological features of our region, and it is best appreciated from above. Please buckle your seatbelts, and comply with the no smoking signs...

Our flight takes us from Oakdale, a small town at the foot of the Sierra Nevada, to the Sonora-Columbia area in the Sierra Mother Lode. Our objective was to get a bird's-eye view of the Stanislaus Table Mountain, regarded by many as one of the finest examples of an inverted stream in the world. The "mountain" formed around 9 million years ago, when a latite lava flow streamed westward from vents in the vicinity of the Dardanelles, near Sonora Pass at the crest of the Sierra Nevada. 

The latite is dark-colored with phenocrysts (crystals) of plagioclase feldspar scattered throughout. It superficially resembles basalt, but is more closely related to more silica-rich lavas like andesite and dacite. The lava flowed down a river channel carved out of the tuffs and mudflow deposits of the Valley Springs  and Relief Peak formations, ending somewhere just west of Knights Ferry, having traveled some 60 miles. The lava resisted erosion better than the softer surrounding rock, and the flow eventually was exhumed, forming a winding, sinuous ridge, especially in the vicinity of Jamestown and Columbia.

Soon after takeoff, we approach Knights Ferry. The modern Stanislaus River makes a prominent loop, with orchards and agricultural fields on the left-side flood plain. The higher terraces are drier, and are used primarily for grazing. The exposed rocks in the barren areas are mostly Mehrten formation, which consists of volcanic mudflow deposits around 4 to 9 million years old.

North of Knights Ferry, exposures of the Gopher Ridge volcanics are visible. These are metamorphic rocks dating from the Jurassic period. These rocks formed as island arcs (volcanic islands like Alaska's Aleutian Islands) on the oceanic crust of the Pacific Ocean, but were scraped off against the edge of the North American continent as the rocks were subducted. The rocks have been metamorphosed, and turned almost vertical by intense east-west pressure. They are more resistant than the surrounding slates, and so stand out as a prominent ridge. The town of Copperopolis is barely visible in the upper right corner of the photo.

Farther along, we pass the site of the Harvard Mine. The open-pit mine was active from 1986 to 1994, producing about 660,000 ounces of gold from about 17 million short tons of rock. The lake in the pit is about 300 feet deep. Just prior to closing down, the miners recovered a huge mass of crystallized gold, weighing more than 40 pounds. The gold is on display at the Ironstone Winery in Murphys. The body of water in the background is New Melones Lake.

Our turnaround point was just east of the town of Sonora. The town had its beginnings in 1848 when gold was discovered by Mexican nationals who had lost their citizenship as California was signed over to the United States. The Mexicans were soon displaced ("invited to leave") by American miners.  The original town is visible in the center-right part of the photo, while the newer urban development is visible in the center and left-hand part. Scars from the on-going construction (completed long ago) of a bypass can also be seen.

Turning west, we start to see the inverted stream of Table Mountain. Highway 108 passes along the lower left hand part of the photo. Very little soil has developed on the top of the old lava flow (note the lack of trees, and widely scattered grassy areas). The sinuous nature of the flow is becoming evident.

A look out the back of the plane offers the best view of the flow. The barren-looking surface of the flow is actually a unique ecosystem of native wildflowers that are largely free of the invasive European grasses that have overwhelmed the original grasses over much of the region. It is a fascinating hike, especially in the spring after a few good rainstorms. A relatively recent hike to the top of the lava flow can be seen here:

Looking west, with Knights Ferry in the far distance. The flow is wider, especially where some of the lava backed up into some ancient tributary streams. Tulloch Lake on the Stanislaus River is visible to the right.

The rest of the flight was a bit more mundane, as we buzzed my house and returned to Oakdale.

My thanks to Ken Iwahashi, the pilot on our journey. 

Addendum: If you are wondering what the latite rock looks like, I went out today and got a few shots of it.


D Schwartz said...

Very cool. Are there other well known and prominent inverted streams in the US?

Garry Hayes said...

There are several in the Sierra foothills both north and south of Stanislaus Table Mtn. St. George, Utah has three or four right in the middle of town. Zion National Park has a wonderful example in the park west of the actual Zion Canyon.