"First, a little history, so you’ll know what you’re looking at: In ancient times, a forest covered a deep valley. Then, 3,500 years ago, Mount St. Helens erupted, sending a massive mudflow down through the canyon, filling it with volcanic rock. Over the years, the river carved a path through the rock, forming a canyon with waterfalls, deep cuts, and towers of harder rock—Lava Canyon.
Later mudflows covered all of that, and eventually forest grew back over the whole thing. Then, on May 18, 1980, Mount St. Helens erupted again, melting 70% of its glaciers in an instant and sending millions of cubic feet of mud and rock blasting down the side of the mountain at about 45 miles an hour."