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.)
From: Pringle, 1993, Roadside Geology of Mount St. Helens National Volcanic Monument and Vicinity: Washington Department of natural Resources Division of Geology and Earth Resources Information Circular 88, p.64.
Ape Cave is open year 'round, although during the winter months it requires some extra hiking/snowshoeing to reach. There is a small information visitor center at the cave that is only open in the summer months. During the summer, a recreation parking pass is required. When snow comes, a sno-park pass is required and cavers must walk/snowshoe about one mile from the Trail of Two Forests parking area. For more information, call the Mt St Helens Monument Headquarters in Amboy at (360) 449-7800.
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