Coming back to the trail--a hundred or so feet away,--on the left hand side returning to the spring, is a gigantic sloping granite block, perfectly polished with glacial action, and black as though its surface had been coated in the process.

Near here the trail _ducks_ or markers are placed in a deep grooving or trough three or four feet wide, and of equal depth, while to the right are two other similar troughs working their winding and tortuous way into the valley beneath.
In Chapter VIII an idea is given of the movements of the great glaciers that formed Desolation Valley and all the nearby lakes, as well as Glen Alpine basin.

These gigantic ice-sheets, with their firmly-wedged carving blocks of granite, moved over the Heather Lake Pass, gouging out that lake, and Susie Lake, in its onward march, and then, added to by glacial flows from Cracked Crag, the southern slopes of the Tallac range, and the Angora Peaks, it passed on and down, shaping this interestingly rugged, wild and picturesque basin as we find it to-day.

How many centuries of cutting and gouging, beveling and grooving were required to accomplish this, who can tell?
Never resting, never halting, ever moving, irresistibly cutting, carving, grinding and demolishing, it carried away its millions of millions of tons of rocky debris in bowlders, pebbles, sand and mud, and thus helped make the gigantic moraines of Fallen Leaf Lake.

The ice-flow itself passed along over where the terminal moraine now stands, cutting out Fallen Leaf Lake basin in its movement, and finally rested in the vast bowl of Lake Tahoe.
To the careful student every foot of Glen Alpine basin is worthy of study, and he who desires to further the cause of science will do well to make a map of his observations, recording the direction, appearance, depth, length and width of all the glacial markings he discovers.