Following disappearance of the large Pleistocene glaciers streams returned to the valleys and began to cut new valleys within the old. Because post-Pleistocene time has been of such short duration these new valleys are small youthful gorges. Interesting examples are Sunrift Gorge, where Baring Creek has cut a narrow channel into the upper part of the Appekunny formation; and the gorge at Hidden Falls on Hidden Creek in the Grinnell Valley. Sunrift Gorge lies only a few feet north of Going-to-the-Sun Highway at Baring Creek bridge, and Hidden Gorge is a stop on the guided trip which Ranger-Naturalists conduct from Many Glacier Hotel to Grinnell Lake. Both of these channels have very smooth, straight sides because they have been eroded along vertical fractures known as joints. The latter are common throughout the mountains and are responsible for the smooth surfaces on some of the highest cliffs. The gorge of Avalanche Creek near Avalanche Campground is another example of post-glacial stream erosion, only here the whirling action of sand and gravel-laden water has carved out a number of cylindrical potholes in the stream course. Some of them, though only 6 to 10 feet across, are 20 or more feet deep. Since we know that the streams did not begin to cut these gorges until the large Pleistocene glaciers had disappeared from those sites, approximately 10,000 years have been required for their formation. Thus the average maximum rate of down-cutting has been of the magnitude of 0.002 to 0.003 inch per year. With these figures as a foundation it is not so difficult to comprehend that the much larger valleys of the park could not have been eroded in less than several millions of years.