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Pinnell Mountain National Recreation Trail

 A short column of schist protrudes from the top of a narrow ridge.

 

A tor composed of schist on a ridge near Pinnell Mounain.

Geology of the Pinnell Mountain Trail

Hikers on the Pinnell Mountain Trail walk across some of Alaska's oldest rocks. Schist, the predominant rock type along the trail, forms the prominent tors jutting from narrow ridge tops. This complexly folded and deformed rock dates to the Precambrian-Cambrian periods, 700 million to 2 billion years ago, when only the simplest life forms flourished.

Other rock types are visible in a few locations. The area surrounding the North Fork Shelter is composed of granite that has weathered into blocky boulders. On the east side of Table Mountain, the trail briefly parallels an intrusion of light gray rhyolite. These rocks are thought to be 50 to 70 million years old much younger than the schist.

The trail is also an excellent place to view unusual landforms created by Alaska's frigid climate. Although this area has not been heavily glaciated, cold temperatures have shaped its landscape, most notably through the process of solifluction. The scalloped waves of vegetation seen on many hillsides are called solifluction lobes. They are formed as frozen ground thaws from the surface downward each summer. The melted water cannot percolate into the frozen sub-soil (permafrost) and, instead, saturates the thawed surface soil and overlying vegetative mat. This sodden ground then "flows" slowly downhill.

Solifluction lobes on a tundra hillside
Solifluction lobes are visible on the tundra slope below the snowfield.

Other permafrost-related features include patterned ground (groups of circular or polygonal depressions), stone stripes (alternating stripes of fine and coarse rocks on a slope), and felsenmeer (expanses of angular boulders pushed to the surface by frost).