DONNER SUMMIT, ROCK SOLID!
DONNER SUMMIT! “The most marvelous square mile in American history!” I have heard this so many times lately, and it is true. The attention has always been The Donner Party, but there was also the Emigrant Trail; Fremont, Carson, and Bridger; native Americans and petroglyphs; the building of the Transcontinental railroad; and interstate 80, the nation’s first “all weather” highway. The list is practically endless.
But before all these wonderful accomplishments there was GRANITE. Before all these events, rivers flowed unabated from the Plains to the Pacific Ocean. Millions of years ago there was no Sierra Nevada mountain range, no impediment of rivers flowing west. What is so fantastic about our Summit is that it is only 3 million years old and still changing. In John McPhee’s book, The Assembling of California (Farrar, Straus, & Giroux) we can find out how Donner Summit and the Sierra Nevada were formed.
As a former Pulitzer Prize winner, author of twenty-seven books, and a long-time contributor to The New Yorker magazine, McPhee’s easily-understood-by-laymen’s-terms style allows us to understand a sometimes difficult subject. Plate tectonics, batholiths, and caprock andesite are not terms we use in our daily conversations, at least not for me.
“Often likened to a raised trapdoor, the Sierra has a long and planar western slope and…a plunging escarpment facing east.” “Hinged somewhere beneath the Great Valley (San Joaquin/Sacramento Valley - ed.), and sharply faulted on its eastern face, the range began to rise only a very short geologic time ago - perhaps three million years…and it is still rising, still active….”
We learn that in the geologic periods before the uplift, Pliocene era volcanic andesite flows “spread themselves over the terrain like butterscotch syrup over ice cream.” As the successive volcanic flows filled in the landscape to form a somewhat flat surface the trapdoor began to rise tilting up the hardened andesite which we can see in the road cuts along highways 40 and 80.
Although most of what we see of the terrain from Nevada to the Summit is volcanic cap, McPhee calls this a “veneer” to what lies below, the monster Sierra batholith of solid granite.
Science calls a batholith a surface of at least forty square miles and no known bottom. At the Summit we sit near or on top of the Smartville batholith which has a surface of about twenty-five thousand square miles. McPhee says that it “lies inside the Sierra like a big zeppelin.” And some geophysists “say that it is six miles down.” At almost any place along Interstate 80 from you can reach out and touch the batholith especially around Big Bend.
To skip ahead a few million years the Pacific Plate started pushing into the North American Plate as in Plate Tectonics. If you hold your hands in front of you with your palms down and thumbs folded under gently push your left hand up over your right at an increasing angle with the right going farther under the left. This is how McPhee explains the concept, and how the Sierra range and Donner Summit were formed, and pushed up so much granite, over millions of years.
McPhee did much of his work on this book with the aid of UC Davis Geology Professor Eldridge Moores. With Eldridge driving an old pick-up truck McPhee sat in the passenger’s seat taking notes. At unexpected times Eldridge would stop the truck along a I-80 road cut and take samples - “as cars shot past us like F-18s….” What McPhee learned from his association with Eldridge puts the mysteries of the formation of Donner Summit in a better light for the layman.
Assembling of California covers all of California, spending time on earthquakes and volcanoes; the formation of the coastal ranges and great valleys. For instance, “fifty million years ago the town of Davis would have been in mud at the bottom of the Farallon Ocean, some thirty miles offshore, on the continental shelf.” I feel that to fully understand the dynamics of Donner Summit and the Sierra having a “rock solid” foundation is a must.
To totally enjoy the book I would suggest reading McPhee’s Annals of the Former World, a geological road trip along Interstate 80 from New Jersey to San Francisco of which Assembling of California makes up the fourth chapter.
Other books by John McPhee include Uncommon Carriers , The Survival of the Bark Canoe, and Looking for a Ship.
Reviewed by David (Rocky) Africa