DESCRIPTION:
Ape Cave Lava Tube,
Mount St. Helens, Washington

[Image,49K,JPG]
Ape Cave Lava Tube
-- U.S. Forest Service photo courtesy Gifford Pinchot National Forest

Warm clothing and flashlight or lantern are needed if you plan to explore the cave. Ape Cave is one of numerous lava tubes formed in the Cave Basalt. The basalt consists of pahoehoe flows that originated on the southwest flank of Mount St. Helens and flowed down the surface of older pyroclastic-flow deposits. Charcoal samples from two localities under the lava tubes yielded Carbon-14 ages of 1,860+/-250 and 1,925+/-95 years.

Ape Cave lava tube, which formed in one of these basalt flows ... (Cave Basalt flows about 2,000 years ago) ... is the longest lava tube (12,810 feet or 3.9 kilometers) in the conterminous United States (similar tubes are found in Oregon, California, and Idaho) and one of the longest in the world. The cave was constructed by a pahoehoe flow that crusted over; soon after, the molten lava on the inside drained away, leaving the outer crust in place. Lava stalactites and stalagmites and flow marks can be seen on the walls and floor of the cave. Lava stalactites, conical or cylindrical deposits of lava that hang from the ceiling of a tube, are formed by dripping; stalagmites are similar in shape and are formed on the floor of the tube by the accumulation of drips from the ceiling. Some time later, a sandy lahar flowed into the cave, possibly in A.D. 1480 or 1482 because the deposit contains white pumice granules that resemble W tephra. During the summer, a national monument interpretive naturalist leads tours through the lower part of the cave. Be sure to read the brochure (available at the cave entrance) to find out more about the cave and the equipment you will need if you plan to explore on your own. (Sturdy shoes or boots, warm clothing, and three sources of light are recommended.

... The Cave Basalt issued from a vent probably near the southwest base of the cone about 1700 years BP (Carbon-14); it contains 3.4-kilometers-long -- (Web note: approximately 2 miles) -- Ape Cave, the longest known uncollapsed segment of a lava-tube, as part of its 8.3-kilometers-long -- (Web note: approximately 5 miles) -- tube system.

Ape Cave: Named by members of an outdoor group called the Mount St. Helens Apes, this cave is the longest continuous lava tube in the continental United States. Cavers should wear warm clothes, sturdy shoes and have at least two sources of light. Lantern rentals are available at Apes' Headquarters.

Two different routes of exploration exist inside the cave and an above-ground trail (Ape Cave #239) connects the main and upper entrances. The lower cave is a relatively easy 0.75 mile hike that takes 1 hour round trip. The challenging upper cave is 1.5 miles long, requires climbing over large piles of rocks and takes 2.5 to 3 hours.

Trail Difficulty: Easy to Most Difficult

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