Book uncovers prehistoric Merced, Central Valley, Yosemite

Most of the people who travel along Highway 140 east toward Yosemite don't realize the history they are passing by -- and over.

It's not just the history of Gold Rush country. It's also a prehistoric window on how the area was formed millions of years ago.

Authors Allen Glazner and Greg Stock have taken a comprehensive look at the geology of Yosemite National Park and Highway 140, the road that gets visitors to the park. Their book, "Geology Underfoot in Yosemite National Park," excavates the geology in the foothills and Yosemite.

For people interested in how the Valley, the foothills and the Sierra Nevada came to be, the book is a comprehensive read, and easy to understand.

Merced lies in a mostly flat valley, but in the foothills on the way to Yosemite the history of the area can be seen in rocks.

One of the most common sights along Highway 140 are what are known as "tombstone rocks." According to Glazner and Stock, the small fins of rocks that stick up out of the rangeland east of Merced are actually schists, a word that means to flake off. The tombstone rocks, which start about eight miles east of Planada, show how the elements have caused the rocks to flake. The rocks are also home to many multicolored lichen, making the rangeland even more striking.

Tucked among those tombstone rocks are numerous white quartz veins sticking out of the ground. They were produced by hot fluids that circulated through the metamorphic rocks. And those quartz veins are where the gold miners found gold.

Visitors along Highway 140 can see massive formations of slate all along the highway. Across from Dial's Rock and Fossil Shop, near Agua Fria Road, there are outcrops of slate hanging high over the road.

Glazner and Stock write that these rocks have been squeezed and tightly folded like a stack of fan-folded printer paper. Some of the rocks are slanted from the pressures that formed them when an underwater sea covered most of the area.

Granite makes up most of Yosemite Valley, from lofty Half Dome to the polished rocks that cause rapids on the bed of the Merced River.

What we know as Yosemite Valley began about 25 million years ago, when the Sierra Nevada started increasing in height. A cooling climate one-to-two million years ago combined with a lot of rain and snow to form the first Sierra glaciers.

The glaciers scoured out Yosemite Valley, along the Merced River, and left behind the famous Yosemite landmarks, such as Half Dome and El Capitan.

The very edge of glaciers came down into the San Joaquin Valley. The rocky, pebble-strewn ground of eastern Merced County was once the edge of a glacier.

Before reaching Yosemite Valley and its landmarks, visitors can see the geology that helped make the area famous.

Quartz, and the gold veins that go along with it, can be seen along Highway 140. Just east of the Highway 140 and Highway 49 intersection, a big vein of quartz can be seen in the cutout of the road.

And along with the quartz, visitors to the area can also see green rocks on both sides of the highway. The green color is caused by green minerals such as chlorite, epidote and serpentine. Just a bit farther north on Highway 140, where the road follows the canyon of Mariposa County's Bear Creek, a parking area on the east side of the road gives access to beautifully water-polished greenstones along the creek.

Gold panners still haunt the creeks and rivers near Mariposa, looking for the gold that sometimes washes down from the mountains. For anyone interested in how that gold got where it is, or just wants to know how Yosemite Valley was formed, Glazner and Stock's book is a good place to start.

Reporter Carol Reiter can be reached at (209) 385-2486 or creiter@mercedsun-star.com.

This story was originally published July 22, 2010 at 1:11 AM with the headline "Book uncovers prehistoric Merced, Central Valley, Yosemite."