During my visit to Tuolumne Meadows this summer, I joned a ranger walk at Pothole Dome. It’s north of the road at the west end of the main meadow.
During the glacial periods of the last two million years, huge glaciers filled the wide basin that now contains Tuolumne Meadows–part of a continuous icefield that covered the upper Tuolumne River drainage. The ice is gone now except for a few small glaciers clinging to the shady side of peaks.
Two hundred fifty foot-high Pothole Dome has been shaped by the glaciers that covered it then. It’s gentle on the uphill side and drops off steeply on the downhill side. Much of the south side slopes steeply–too steep to walk up, except for the lower portion.
Below the south side we stepped inside a screen of trees and looked up at the wall. Reflecting in the mid-morning sun were a regular series of shallow indentations containing smoothly polished surfaces. How did they get there?
We climbed the steeply slanting face for 15 feet or so to examine them from a closer viewpoint. Then off to the side we saw a huge pothole worn vertically into the rock. It must have been at over ten feet in diameter. Embedded in the sandy floor were four or five smoothly rounded boulders as much as four feet in diameter. This could only be the work of a powerful river. But on the side of a dome?
Our guide explained what had happened, as pieced together by geologists who have visited large living glaciers in other parts of the world.
Freed by the warmth of the sun, meltwater joined to form small streams on the surface of the glacier. Pouring into fractures, they found their way to the base of the glacier, joining and joining again to form powerful rivers under the glacier, capable of moving boulders and swirling them around and around in depressions to wear such potholes.
Higher on the dome, ice containing grit wore and polished the rock like sandpaper. This polished surface still remains in places.
As we circled the done and climbed the moderate southeastern slope, we passed a small lodgepole pine growing where erosion along a narrow joint had created a long trough a few inches wide where enough decomposed granite had accumulated to hold its roots. With solid rock on both sides, this pine had been slowly growing here for who knows how long, its roots snaking along the joint to find soil and nourishment. The force of life!
As we climbed, we passed dozens of rounded boulders of different compositions from the light-colored rock of the dome– left here as the glacier melted back. Just south of the highest point of the spacious summit is such a boulder four feet high. Angluar black hornblende crystals speckle its surface, absent in the rock of the dome. Another next to it has large feldspar crystals up to an inch in length. These out-of-place rocks could only have been snatched from different bedrock far up the basin, carried in or on the glacier and dropped here as the ice finally melted back.
We raised our eyes. The broad green meadow stretched before us, circled by forest which continued on up the spacious basin. To our right, the slope of the Cathedral Range curved smoothly upward below sharp summits, creating the broad, even “U” shape and round bosses worn by tens of thousands of years of ice–enough to cover the the basin with up to two thousand feet of gleaming white.
Standing there, gazing at this grand scene, the thought was hard to comprehend. Yet the stark evidence of its reality was all around us and under our feet. What a world we’ve been made part of! Its size, power and grandeur are beyond our ability to imagine. Yet I’ve been put in it and made a part of it. What a thought and what a blessing!
What do you feel when you look at mountains that have been shaped by glaciers, and consider what it took to do this?