Goat Rocks - Geography and Geology

Geography and Geology

The complex is located 18 miles (29 km) north of Mount Adams, a large stratovolcano. It is located in a zone of intermittent volcanism which has produced many small vents. Over time, overlap occurred, and the vents now make up the Mount Adams volcanic field, Indian Heaven, and even parts of Goat Rocks.

The peaks are the eroded remnants of a stratovolcano, termed the Goat Rocks volcano, which about 2 million years ago might have been similar to the current large stratovolcanoes in the Cascade Volcanic Arc, reaching over 10,000 feet (3,000 m) high. Once the volcanic eruptions came to an end, the ongoing forces of glacial erosion stripped away the outer layers of volcanic ash deposits and lava flows, uncovering the rocky lava spines where magma had cooled and hardened within the conduits of the volcano. The current eroded volcano has numerous summits reaching about 8,000 ft (2,400 m).

The Goat Rocks area is notable for its extensive glaciers, despite the modest elevation and southerly location relative to the rest of the Washington Cascades. Five glaciers mantle the north and northeast slopes of the peaks, along with numerous smaller permanent snowfields. The highest point on the Pacific Crest Trail in Washington is at 7,200 ft (2,200 m) on the northwest flank of Old Snowy Mountain, where the trail crosses the edge of the Packwood Glacier.

The remnant core of the Goat Rocks Volcano is Egg Butte, located north of Old Snowy Mountain in the valley carved by the ice age Packwood Glacier. Similar to the lava spines remaining on the ridges to the south and east of Old Snowy Mountain, the dense core became exposed after the Packwood Glacier removed the softer ash and broken flows around it.

At the base of the valley lies Packwood Lake, formed behind the terminal moraine created by the Packwood Glacier, evidence of the large amount of volcanic material stripped away by the glacier.