Yosemite trails offer look at park’s geology

Glaciers carved the valleys and fascinating granite shapes at Yosemite.

Today’s fascinating geography of Yosemite National Park arose from a complex number of geological events that date to well over 540 million years ago. Prior to that time, the area where the park sits marked the edge of the North American continent. Rivers carried sediment from inland into a shallow sea; over hundreds of millions of years, this sediment hardened into rock, ultimately claiming the sea for the continent.

As the North American tectonic plate collided with another plate, those rocks – metamorphosized into marble, quartzite and slate – rose above the seabed. Chains of island volcanoes formed in the ocean off the continent and gradually was rode its plate into and combined with ancient North America. These rocks can be seen in isolated sections of Yosemite’s central and northern section as well as the neighboring Emigrant Wilderness of the neighboring Stanislaus National Forest.

Much of the granite exposed today at the national park formed deep underground during these plate collisions about 210 million to 80 million years ago. As plates continued to strike one another, the heavier ocean floor drove beneath the lighter continental land mass, beginning lifting up the Sierra Nevada mountains about 10 million years ago. Over time, softer rock eroded away, exposing much of the granite now seen at the park.

A series of ice ages beginning about 3 million years ago covered Yosemite in glaciers. As these hefty masses of ice pushed through the mountains, melting and advancing again and again, they carved out the deep valleys and shaped the granite. In some cases, the glaciers sheared massive chunks of the exposed granite domes and left creeks and rivers dropping hundreds and sometimes thousands of feet down newly created cliffs. The glaciers’ action can most dramatically be seen in Yosemite Valley, especially the Cook’s Meadow Loop and Lower Yosemite Fall Trail. You can hike up through some of the rock currently being cut away by the Merced River on the Mist Trail.

Learn more about national park day hiking trails in my Best Sights to See at America’s National Parks guidebook.