Boulder Field, Silver Divide
John Muir Wilderness

Camera: Olympus E-500: 80 mm EFL, ISO 100, 1/50 @ f 8

Boulders like these are commonplace in the Sierras. You might wonder how they got to be where they are. They are too far from the nearest mountain peak to have rolled in. One clue can sometimes be seen on the granite expanses on which the boulders sit. In places where the surface has not been weathered away, you can see many parallel scratches on the rock surface, running in the same direction as the valley. These scratches were made by ancient glaciers, rivers of ice hundreds of feet deep, grinding their way slowly down the valley. As they moved, the glaciers plucked rocks off the glacier bed and carried them along. Later, when the glaciers retreated, they left their load of rocky bits behind, creating the boulder-strewn landscapes we see today.

Look along the ridge of the Silver Divide and you will see a section that looks like a huge quarry dug into the mountain. Just to the right of the quarry, you can see a part of the mountain that shows how it was once smooth and rounded. The entire mountain had been worn smooth by a glacier passing over it, long ago. The quarry is the work of a more recent, much smaller glacier that once sat in the shade of the mountain. This glacier quarried rocks out of the pocket where it sat by the action of freezing and thawing, with some help from gravity. Mountaineers call the hole left by the work of the little glacier a "cirque," and the boulders strewn all about are called "erratics."

- Richard Kreis