As a scenic seeker venturing into this beautiful portion of the mighty Sierra Nevada, it will be important to remember that underlying all that scenic beauty is GEOLOGY!  It’s all geology. Geology, however, as a science, cannot explain why the incredible views are scenic, but rather it attempts to explain the underlying landscape and how it came to be.  If you like rocks, and you are attracted to tremendous forces and you have plenty of time, you will love the geology encountered on this 300 million year old drive. You will encounter rocks left from an ancient shallow sea, marvel at exposed granites that were formed miles below in the earth’s crust, wonder at the volcanic ridges of ash and mudflows that line the highway, stare into the deep river valleys and note the remnants of huge glaciers long gone.

Basically, geology is a slow mover punctuated with great rapid events like earthquakes, volcanic explosions, slides, or floods.  The following brief narrative timeline will give you a sequence to keep in mind as you wander over the pass or pick up a metamorphic rock or marvel at a huge granite boulder perched over the Stanislaus River Canyon.  A scenic seeker already has developed powers of observation and a lively imagination and these will be tempered by the inferences that can be drawn from the physical evidence of geology.  Have fun!

The Paleozoic Era: (600 million to 225 million years ago)

A shallow sea covers the area of inland California.  It barely connects to the Pacific Ocean.  Mountains rise to the east where ancient rivers carry sands, gravels and mud down to be dumped into this sea to depths of 12,000 feet in some places.  As you head east out of Arnold, the metamorphic red rocks and soil you see in the road cuts are what remains—perfect for your imaginings.

Mesozoic Era (225 million to 60 million years ago)

Triassic Period—Soon granites are seen in the road cuts just before the Calaveras Big Trees Park entrance.  Granites form miles below in the earth’s crust, due to the Pacific Ocean plate having been forced below the westward moving North American continental plate due to the breakup of the old continent Pangaea.  The friction and depth have all created great pressure and heat, causing some melting of the Pacific plate so that molten magma begins to plume upward through the denser overlying rocks.  This molten magma does not surface so it cools very slowly—like, millions of years.  Crystals have time to form and veins of gold and silver can well upward and deposit in fissures and cracks along with quartz and other minerals. 

Jurassic Period—With all the heat and pressure created by this rude granite intrusion, the overlying ancient sea bed of sands and muds are changed (metamorphosed) into the slates, schists, and quartzites. Remember the red rock? Is this what you imagined?

Cenozoic Era (65 million years ago to the present)

Early Tertiary (65 million years ago) A time of erosion begins and continues to the present.  An ancient Sierra Nevada range is worn down to rolling foothills stripped of much of the overlying rock deposited by the sea.  This sediment in turn is deposited to form the Great Central Valley.

Later Tertiary (35 million years ago to 3 million years ago) An Active volcanic period begins where the molten material forming from the melting oceanic plate actually reaches the surface, sometimes with some great explosions.  The entire landscape is eventually covered in ash, mudflows, and lava.  Volcanoes form peaks.  During times of quiet great valleys are formed by eroding rivers and streams, only to be filled again by yet another volcanic eruption.  The present Sierra Nevada Mountain Range began to uplift with a major westward trend creating new river drainages.

Quaternary Period (1 million years ago to present) Uplift continues to the present.

Pleistocene epoch (a million years ago-10,000 years ago) The so-called Ice Age begins.  Ice advances and retreats many times.  The entire Sierra crest is capped in ice with fingers of it spilling down the several canyons as huge glaciers.  Much of the volcanic cover has been eroded except on the peaks and ridges. When the glaciers finally leave, there remains much exciting evidence for you to find:  erratics, glacial polish, u-shaped valleys, striations, and cirques.

By David Gano, from Guide to Ebbetts Pass, 2010