Mount St. Helens, Washington
Glacier Extents During Swift Creek Time

A glacier originating on the south flank of the old Mount St. Helens volcano moved down the valley of Swift Creek. The cirque in which this lacier originated is apparently carved into the rocks of old Mount St. Helens (Hopson, oral commun., 1972) and is now mostly obscured by young lava flows and flowage deposits, but part of its headwall is still visible at an altitude of 1,200-1,500 meters, about 4 kilometers directly south of the summit of the volcano. End moraines formed by this glacier are crossed by U.S. Forest Service road N818 about 0.5 kilometers north of its junction with road N83. There, about 8 meters of drift is exposed beneath 2-3 meters of tephra and pyroclastic-flow deposits. These moraines evidently do not mark the greatest downvalley extent of the glacier, because till having weathering features similar to those of the moraines is exposed about 1.5 kilometers farther south, near the entrance to Ape Cave. Till is also exposed in the west valley wall of Swift Creek ... where it seems to be part of an end moraine that was subsequently buried by several flowage deposits. The oldest flowage deposit is coarse and displays crude internal stratification, and my be either alluvium or a mudflow of glacial origin. Above it is a succession of pyroclastic-flow deposits.

Glacial drift and moraines in the upper part of the Swift Creek valley were formed by a glacier originating on the southeast flank of Mount St. Helens in the area north of June Lake. Faceted spurs and lateral moraines containing over 60 percent andesite and dacite fragments are present on both sides of a valley south of June Lake, but farther south, near the base of Marble Mountain, the drift is covered with young flowage deposits. Glacial drift is not exposed in deep cuts farther downstream, and the glacier probably terminated near the base of Marble Mountain.

The glacial drift that is interbedded with the Swift Creek assemblage at Mount St. Helens probably was deposited during the Evans Creek Stade of the Fraser Glaciation. This correlation is based on a comparison of such relative weathering features as depth of oxidation and thickness of weathered rinds on stones in glacial drift in this area with those described in other areas of western Washington (Crandell, 1969). The Evans Creek Stade was originally thought to have occurred between about 25,000 and 15,000 years ago (Armstrong and others, 1965). Subsequent work has suggested that alpine glaciers in southwestern British Columbia began to advance after 20,000 years ago (Halstead, 1968), and alpine glaciers on the west side of the Olympic Peninsula evidently reached their maximum extents before about 18,800 years ago (Heusser, 1974).

Glaciers of Fraser age also developed in cirques at altitudes above 1,100 meters in the mountains adjacent to Mount St. Helens, but most were less than 10 kilometers in length. Small glaciers existed along the ridge at the head of Cougar Creek northeast of Cinnamon Peak, where till and end moraines are present in several cirques. Small glaciers also extended down the north and west sides of Cinnamon Peak and terminated in the Kalama River valley about 2 kilometers north of Merrill Lake.

Exposures of glacial drift in the vicinity of Mount St. Helens and in the Lewis River valley west of the volcano show that the area was glaciated at least three times prior to the Fraser Glaciation and prior to the formation of the Swift Creek assemblage. These earlier alpine glaciers extended at least 50 kilometers down the Lewis River valley than did the glacier of Fraser age. During these earlier glaciations all the area at, and adjacent to, the present site of Mount St. Helens, except perhaps the highest peaks, may have been covered by glaciers.