California reader Jennifer shared by e-mail some photos taken on a recent hike into the Sierras, including this one of glacial gouging and polishing of rocks where no glaciers exist today.
In the fall of 1831, young Charles Darwin accompanied his geology teacher Adam Sedgwick on a field trip to the mountains of North Wales. Half-a-century later he wrote: "We spent many hours in Cwm Idwal, examining all the rocks with extreme care, as Sedgwick was anxious to find fossils in them; but neither of us saw a trace of the wonderful glacial phenomena all around us; we did not notice the plainly scoured rocks, the perched boulders, the lateral and terminal moraines. Yet these phenomena are so conspicuous that...a house burnt down by fire did not tell its story more plainly than did this valley."
We only see what we expect to see. Because the theory of the ice ages had not yet been invented in 1831, Darwin and Sedgwick were blind. Later, with a theory in mind, the evidence of ice was obvious and irresistible.