HISTORY: Digging into geologic origin of Donner Summit and Truckee
Special to the Sierra Sun
FOR YOUR INFO
A batholith (from Greek bathos, depth + lithos, rock) is a large mass of intrusive igneous rock (also called plutonic rock), larger than 100 square kilometers (40 square miles) in area, that forms from cooled magma deep in the Earth’s crust.
Igneous rock describes rock formed under conditions of intense heat or produced by the solidification of volcanic magma on or below the Earth’s surface.
For a phenomenal panoramic view of Donner Summit, Summit Valley and Donner Lake, take Donner Pass Road to Donner Summit (Elev. 7135 ft). As part of the Donner Summit Historical Society 20 Mile Museum, walk behind Summit Haus and you will find perched on a rock the historic Donner Summit Overlook and signpost.
— Bill Oudegeest
Fifty million years ago there was no Sierra. There were jungles and crocodiles in the tropics of Nevada. North America met the Pacific Ocean in what is now eastern California. Donner Summit sat under the sea.
The Pacific Ocean sits atop the Pacific Tectonic Plate which is continually in motion sliding under the Continental Plate. This subduction melted the crust forming an immense granitic batholith of melted rock 60 square miles in size.
As the great batholith formed it floated on the earth’s mantle and then rose and solidified. The rising and volcanic activity that came with it formed the foundation of today’s Sierra.
Fifteen to 30 million years ago what would become the Sierra was covered with volcanic material up to a thousand feet thick.
About five million years ago a new round of subduction began. The resulting pressure pushed up the great granite batholith with its covering of volcanic material.
Over the eons, as the mountains rose, rain from natural weather patterns fell and eroded river channels into the sediments and volcanic rock that sat atop the batholith. The rains and erosion gave form to the new Sierra, our Sierra.
After millions of years the Sierra was almost complete but there was still one more episode. As the Sierra rose precipitation turned to snow. Each year some snow would not melt. More snow fell compressing the snow that was there. Eventually glaciers formed, perhaps a thousand feet thick. The South Yuba glacier, occupying what is now the South Yuba River, was 25 miles long.
The glaciers’ great weight, grinding downhill, scoured peaks of the rock overlaying the batholith and carved glacial valleys, river channels, and basins. The flowing glaciers followed the existing river channels. Summit Valley, the source of the Yuba River on Donner Summit, is one such valley. Donner Lake and natural dam at the east end is another.
Large granite rocks (glacial erratics), were deposited all over as the glaciers melted and began to retreat. Native Americans would later use mortars on the large glacial erratics to grind pine nuts and acorns.
With the end of ice ages the final geologic form of the Sierras was in place but Donner Summit was barren. Lichen and moss arrived covering and breaking down the rock and depositing organic materials. Eventually top soil formed and was colonized by a succession of plants. The forests and meadows of today sit atop the volcanic conglomerate and the great Sierra batholith.
Donner Summit was now ready for the arrival of man.
Bill Oudegeest has had a house on Donner Summit for more than 40 years. He is a retired public school teacher and administrator and one of the founders of the Donner Summit Historical Society. He writes and edits the Donner Summit Heirloom, has published two books on local history, written a variety of pamphlets and exhibits, leads hikes, and more.
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