I open my eyes, slowly. The sun has begun to slide over my hammock and into the hood of my sleeping bag. I push the fabric away. The sky is already a pastel blue, but the pine trees below it are lit by dabs of early morning gold. The soft roar of a gas burner off to my left means that Prof. Grove has just started the first pot of coffee. My toes are warm in their down covers. I don’t swing them into their boots until the last brush of gold threatens to fall from the trees, and I can snap a photo of the now-bright Yosemite sky.

It must be a sin to be this happy on a Monday.

Down the long mountain-slopes the sunbeams pour, gilding the awakening pines, cheering every needle, filling every living thing with joy. – John Muir, My First Summer in the Sierra

I have read Muir’s books often, always smiling with wonder at the love and grace with which he describes the mountains he walks through. His words seem especially apt now, for without his beautiful Sierra morning I would likely never have had one of my own. Muir’s writing and activism drove the preservation movement which led to the creation of Yosemite National Park. Today, this exceptional landscape – once a pasture for sheep or, as he once called them, “hoofed locusts” – is filled with family campgrounds, ambitious climbers, and (starting just a mile or so from the roads) a seemingly endless alpine wilderness.

I came to Yosemite as part of an MIT geology field trip – a bittersweet experience, as I have just started my Ph.D. at CU-Boulder and this was to be my last week with the MIT department. The fifteen of us – undergrads, grad students, an outgoing postdoc and Associate Dept. Head Tim Grove – had nine days to look at the landscape and geology of Eastern and Northern California. The undergrads have described many of our educational adventures in ‘Getting High in the West’.

Granodiorite in Yosemite

Yosemite was built from granite, and is being scraped away by glaciers. Both features make it bright and beautiful.

The raw material, granite, forms when magma cools underground, where heat escapes only gradually to the surface, and its crystals have millions of warm years to grow and interlock. Silicon, oxygen, aluminum, sodium, calcium and titanium are arranged. Clear quartz and white feldspar grow alongside black hornblende, shiny biotite and sphene. The final rock varies slightly with the ratio of elements within it – most of Yosemite Valley is actually made of diorite, granite’s darker and less silica-rich cousin.†

The diorites here are unfractured and exceptionally strong. Yosemite Valley is famous for its cliffs. Half Dome and El Capitan both boast thousands of vertical feet of sheer rock, making them pilgrimage sites for climbers around the world. No photograph can adequately capture the scale of these monoliths (but National Geographic’s photo gallery is a great attempt).

Most rocks would crumble from the strain. Yosemite’s diorite stands tall, except for the top edges of its rocky domes, where the stress of curved slopes and surface temperature changes cause it to fracture and fall off in layers like the skin of an onion. A cliff, however, is only half rock. The other half is empty space. To make a mountain, you must take a valley away.

Glacial polish over Tenaya Lake, Yosemite

The high Sierra has a mirror finish. The ripples on Tenaya Lake shine in the August sun, and so do the sides of the mountain above.

We walked up on the slopes above the lake. At Prof. Grove’s urging, I trained my eyes on the black slivers of hornblende under my toes to look for changes in the rock’s composition. I didn’t see the rest of the valley’s story until I slipped on it.

Granite, in most places I’ve wandered on it, easily holds up my hiking boots. Here, my feet spin like I’m in dancing shoes. Glacial polish – the reason the rocks shine on the hills – is best found by the fingers and the feet. It’s not a climber’s friend. It’s smooth, like a granite counter-top sanded flat right there on the mountain. It feels a little like rubbing your fingers on coarse paper. Once you’ve felt glacial polish you find it everywhere.

Glacial polish above Tenaya Lake, Yosemite


Look at the rock slab under our feet in the bottom-right of the photo above. It’s patchy. The dark patches are rough places, mottled by shadows and by lichen growing in the cracks. In the light, polished patches, there’s hardly a crack. Water doesn’t stay on their surfaces, and plants can’t grip them. They’ve stayed smooth and fresh for over ten thousand years, since the last time that glaciers descended from the High Sierra into these valleys, where their icy sides scraped and strained against the sides, excavating loose and fractured rocks and rubbing the walls smooth.

Glaciers’ traces are visible everywhere in the Sierras. Many scientists have painstakingly cataloged them: glacial polish; deep U-shaped valleys; erratic boulders and moraines (the last two are are carried miles by the ice and dropped on seemingly random plains and hilltops). The first of these was John Muir (c1865), who spent years walking the mountains and demonstrating that Yosemite Valley was once covered in ice.

Glacial striations in Tuolumne meadows

Today, we know that the Sierras have been glaciated, revealed, and re-glaciated many times. The names of these glaciations – such as Tioga, Tahoe, and Sherwin – roll out over hundreds of thousands of years. Their remnants are beautiful. Glacial polish; deep U-shaped valleys; erratic boulders and moraines (which are carried miles by the ice and dropped on seemingly random plains and hilltops) are everywhere in the Sierras. †For more geochemical details about granitic rocks in Yosemite, see Reid, Evans & Fates, 1982. For a related field guide, see ‘Magmatic Evolution of the Tuolumne Intrusive Complex’ by Memeti, Paterson & Mundil, 2014 (fieldguides.gsapubs.org). ‡ A (now refuted) hypothesis, that Yosemite Valley was formed in an earthquake, was put forward by Josiah Whitney c1865. Many books and articles have been published on the subject since. Mary Hill’s ‘Geology of the Sierra Nevada’ is a particularly accessible overview.