China Wall: A Tale of Two Rocks

Drive up Foresthill Divide to China Wall Staging Area
 and you pass over an ancient river channel that still contains gold.

China Wall sits atop a ridge between the North and Middle Forks
of the American River in Placer County, California

Perhaps this story starts in late November 2009, with the snowshoes I bought at Costco. Yes, it was an impulse buy. I saw them and had to have them. They were only $69.99!

I first tried them out in the snows at Donner Pass. Then I learned of a place called China Wall on Foresthill Divide. It's mainly a staging area for snowmobilers, but it's also popular with cross country skiers and snowshoers.

My first trip to China Wall, the day after Christmas, was purely exploratory. I took a solitary walk eastward along the ridge of Humbug Canyon. For about 1/8 mile I followed a black bear track, which appeared to be one day old.

Black bear track

Journey to Cape Horn Tunnel, January 2, 2010:

I returned to China Wall on January 2nd. Although the parking lot had several vehicles and people, my journey to Cape Horn Tunnel was solitary. I walked westward along the ridge. Off in the distance - 6.17 miles, bearing 121 degrees - was the start of Euchre Bar Trail, my favorite trail about which I will write later. Halfway downhill, I saw a bear track in the snow.

Euchre Bar Trail in the distance

The entrance to Cape Horn Tunnel is a water-filled hole by the dirt road. The tunnel was built to reach the gold bearing gravels in an ancient river channel.

I poked around a bit, and from the hillside near the tunnel entrance I picked out two rocks. One was metasedimentary sandstone and the other was quartz from a perpendicular vein. Hmmmm. How did the sandstone, which began as mere sand at or below sea level, get turned to rock and ultimately reach this elevation of 4,340 feet? Why is there a perpendicular quartz vein amidst the sandstone? And what about that ancient gold bearing river channel in front of me? Well, let's find out!

Entrance to Cape Horn Tunnel

How Was the Sandstone Rock Created?

This story starts about 450 million years ago, in the Paleozoic Era, specifically the Ordovician and Silurian Periods. There was a continent named Laurentia that looked nothing like modern North America. Off its western coast was a chain of islands forming what is called the Antler Arc. Sand collected either along the western shore of Laurentia or at the base of one of the islands. Erosion piled sediments atop the sand. Over time the sand was pushed further and further downward. Heat and pressure changed the sand to sandstone.

Plate tectonics moved the sandstone from deep below sea level to that hillside (although, not yet to the 4,340 ft level). Subduction began in the Mississippian Period. The heavier oceanic plate met the lighter continental plate and slid underneath it. The sandstone and the other sediments atop the oceanic plate were scraped off and crammed against the continental plate.

These rocks are part of a formation called the Shoo Fly Complex. (Shoo Fly was a small town north of Quincy, California.)

How did the quartz vein intrude the sandstone? And why is it perpendicular to the ground?

In the Mesozoic Era, when dinosaurs roamed these parts, the ancestral Sierra dominated the landscape. Its mountains towered possibly as high as the Andes, and the ocean lapped at its base. Subduction created this ancient range. The descending rocks, heated to the melting point, created a range of volcanoes, the ancestral Sierra Nevada, against which rested the Shoo Fly Complex.

The rocks descending into the trench contained water. The superheated water - containing silica and bits of gold - rose through perpendicular fissures in the upheaved rocks of the Shoo Fly Complex. Silica cooled to become the perpendicular quartz vein amidst the sandstone. Men would later search for the gold.

And there's an ancient gold bearing river channel?

When the subduction process ended, the ancestral Sierra could rise no higher, and gravity and wind and water slowly brought the mountains down. Over the march of geologic time, by pebble and grain, the range was deposited into the ocean. Today the level Central Valley holds the remains of these mountains.

In this process, rivers ran westward to the sea. The waters flowed over those gold-bearing quartz veins. The rivers flowed over the ages, turning this way and that, wearing away hillsides, moving and pushing boulders and rocks, and depositing gravel and sand. And little by little came more and more gold.

Here comes our own Cenozoic Era, and in the Tertiary Period, specifically the Miocene and Pliocene Epochs, volcanic activity commenced again. The pyroclastic flows coming from the east took the easiest paths downhill - they followed the riverbeds. When they cooled, they left a hard cap over the gold bearing gravels.

And there were many pyroclastic flows. At Gray Eagle Shaft near Foresthill, the miners went down 364 feet to bedrock, and encountered four separate layers of andestic tuff.

Erosion continued, but it was the softer Ordovician-Silurian hills that were melting away. What were riverbeds became ridges.

And how did that little sandstone rock reach an elevation of 4,340 feet above sea level?

We finally reach the creation of the modern Sierra Nevada. The Basin and Range Province sits on a hot spot that is causing it to spread apart. A few million years ago, in the Pliocene, the westward expansion of the Basin and Range tilted a large block. That tilting, which continues to this day, created the Sierra Nevada mountain range.

That's what lifted the sandstone rock. That's also why the drive on the ridge over the ancient riverbeds from Foresthill to China Wall is on a slight uphill grade.

If there's still gold in those ancient riverbeds, why not just go after it?

The early miners didn't know about plate tectonics, but they figured out the ancient riverbeds quickly enough, and soon they were digging tunnels underneath the volcanic flows to reach the gold bearing gravels.

A lot of gold was taken from these gravels. And gold is still there waiting to be discovered. It just costs a lot of money to look for it. And then there are the environmental regulations to contend with. Since the returns usually don't exceed the cost, today there are only a handful of active mining operations in the area. The rising price for gold, however, is generating more interest in exploration throughout the old Mother Lode.

Just remember, as you drive up the divide to China Wall, that gold remains below in those ancient gravels.