The High Sierra

So, basins are made by glaciers. They're the empty rock containers of the upper ends of glaciers that are now gone. Their floors were therefore under ice for thousands of years. The ice scraped the floor as it flowed down and away. The ridges walling the basin, often in a horseshoe shape, were plucked at by the freeze-thaw cycle of water and ice; chunks of rock broke off after water seeped into cracks, then froze and expanded. When newly broken-off rocks fell on the ice, or got caught in its texture lower down, they were carried downhill and away. The ice mass resting against the curve of the headwall thus slowly ate back into it. Now that the ice is gone, or confined to little residual strips in the shade, the basin floors left behind are typically bare rock, dotted with boulders