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Once upon a time there was a volcano in the Cascades. The volcano mouth was plugged with cooled lava from it's last eruption.
The volcano decided to try a new eruption so it spat out the plug. Since Hood is the nearest volcano to where the plug landed it seems to have launched the plug on a journey that took it to the Columbia Gorge (50 miles) where it buried itself deep in the riverbed.
The plug's new name is Rooster Rock.