Tongass National Forest

Tongass Home » About the Tongass

Geology: Glaciers, Caves and More...

Southeast Alaska is a young land, a land shaped by fire, ice, and water.

It's part of the "ring of fire," the volcanic band that stretches around the north rim of the Pacific Ocean, although there are no volcanos active here at the moment. An excellent example of this fiery part of the Tongass's history is Mount Edgecumbe, a volcanic cone clearly visible from Sitka.

A more recent force is ice. Like much of the northern hemisphere, Southeast Alaska was covered by ice during the last ice age. Ice still covers thousands of square miles, mostly at higher elevations but extending clear to sea level in several places. The ebb and flow of glacial ice over thousands of years carved out deep U-shaped valleys. In the last 10,000 years, as the ice retreated the land it once covered lifted, "rebounding" in the absence of the glaciers' massive weight. Melting ice raised the level of the sea, too, and it poured into the deep valleys, forming the islands and fiords that make up the landscape we see today. GLACIERS AND ICEFIELDS

Nor were glaciers the only source of water that shaped the Tongass. The abundant rainfall, flowing over organic soil and into the limestone underlying many areas, has formed karst landscapes with deep and extensive caves, many older even than the glaciers. This recently discovered and recognized karst geology is proving to be a world-class resource. Exploration and mapping of the caves is revealing their extent and also yielding evidence of their prehistoric use by animals and humans. KARST AND CAVES

The broken and very unlevel terrain of most of Southeast means watersheds are relatively small. However, their health is critical to the health of everything living in the Tongass. WATERSHEDS

The geologic history of the region - young soils "growing" after retreating glaciers bared scoured rock - and its cool, wet climate have produced the Tongass of today, a land of towering trees where water could drain and stunted plants in the peat bogs that formed where water was trapped, of ice fields, bare rock, alpine meadows, and temperate rain forest.