In eating back the mountain headwall, alpine glaciers formed rounded depressions, called cirques. Unlike the narrow clefts left by running water, these broad, deep basins look as though they were made by ice-cream scoops gouging into the rock. Hidden, Ptarmigan, Iceberg, and Avalanche Lakes sit in well-developed cirque basins, and many mountains are dimpled by the beginnings of other cirques—the conspicuous amphitheater on the south shoulder of Heaven’s Peak, for example. Occupying all major drainage systems, glaciers modified the contour of the valleys, changing them from their narrow, stream-cut V-shapes into broad U-shapes. Into these wide main valleys, waterfalls plunge from higher, smaller valleys. Like rivers, flowing glaciers have tributaries. Lacking the ice mass and cutting power of the main glaciers, these tributary ice fingers could not bite as deeply into the bedrock. When the ice melted, hanging valleys were left stranded high above the main valley floor. Hidden Lake sits in one of these hanging valleys, and from it Hidden Creek plunges 750 meters into Avalanche Basin toward McDonald Creek. On my many previous visits to this pass I have been too busy enjoying the wildflowers, the weather, or the scenery to realize what an open textbook of glaciation is everywhere displayed. I stand here on a small saddle of a pass. Wherever glaciers met, passes, or cols, were created. A high, notched pass like this one (or Swiftcurrent or Gunsight) reveals recent connections. Broad, lower passes, such as Logan, resulted where the ice early overran the mountain ridge and had a chance to work longer. Where two glaciers worked on opposing sides of a ridge and failed to meet, they formed an arête—a thin, steep-walled remnant resembling a saw blade. Another ice age would probably consume the park’s many thin arêtes, such as the Garden Wall and Ptarmigan Wall; but it would also create new ones from existing ridges. Further testimony to the sculpting power of ice is presented by Mt. Reynolds, looming to the east. The most dramatic feature of a glaciated landscape is the pyramid-shaped mountain called a horn—and Reynolds is a perfect example. Horns were formed when three or more glaciers cloaked the mountain, excavating its sides toward its core and gradually transforming its original domed shape into a sheer-sided peak. Glacier has many remarkable horns, from the sleek spire of St. Nicholas in the south to exquisite Kinnerly in the northern Kintla valley. Sperry Glacier stares back at me from the flank of Gunsight. Glaciers found in the park today are not remnants of the last ice phase, which ended here about 8,000 years ago, but are newly formed, having come into existence some 4,000 years ago. They reflect a cooling trend in the present climate.