Shrinking steadily from their period of greatest extent in the middle of the last century, these modern glaciers finally stabilized in the late 1940s and since then have shown only a slight increase in area. Movement distinguishes glaciers from icefields, and the movement of ice is a force on as well as a feature of a landscape. A glacier excavates by abrading and plucking at the rock. Alternately melting and freezing, ice at the headwalls plucks out blocks of rock. Ultimately the rocks are deposited along the sides or at the feet of the glacier as moraine debris. But as they move in the grip of the ice, they constantly abrade the rock surfaces they encounter. Polished rock beds of past glaciers show striations—grooves gouged by rock fragments imbedded in the moving ice. Flow rate of a glacier depends upon the thickness of the ice and the degree of slope. Under tremendous pressure, ice becomes plastic, like thick taffy. Unlike kilometer-thick continental glaciers, which may move a hundred meters a day, small alpine glaciers seldom progress more than two or three centimeters per day. Although a glacier moves, it gets nowhere if in a state of equilibrium—when annual melting equals annual accumulation. Snow mass gained at the sun-shielded headwall is usually lost as melt at the exposed snout. Glaciers such as Sexton or Weasel Collar, whose snouts perch on cliff edges, also lose mass by calving. Thunder you hear on a late-summer day near such a glacier may actually be the sound of ice pushed off from the lip of a cliff. Walking back to the visitor center, I suddenly stop where the trail skirts the steep moraine of Mt. Clements. From the opposite side of the moraine five mountain goats have appeared. Spotting me on the trail below, they also halt. But before I can get to my camera they are off in a stiff-legged gallop, running in single file along the crest of the moraine to the distant safety of the mountain face. Moraines are ridges of rock debris piled up along the edges and terminuses of glaciers. Like a bracelet lying against the wall of this mountain, the circle of steeply piled debris marks the extent of a small, recently vanished glacier. Ghost of the power that once resided here, a stagnant icefield lies beneath the confining walls of the moraine. The recent accumulation of these rock fragments is a mighty accomplishment, attesting to the force of moving ice. _continued on p. 38_ [Illustration: The Mountains of Glacier Lying astride the Continental Divide in the Northern Rockies, Glacier is above all else a mountain park. The special beauty of its lakes, streams, and forests derives from the microclimates and varied topography and soil produced by mountain-building and mountain-eroding forces.] [Illustration: Overthrust Mountains