The most significant volcanic event in historical times was the simultaneous eruption of Mount Katmai and Novarupta in June 1912. Novarupta's eruption produced a pyroclastic flow that covered a nearby valley with ash as much as thick. At the same time the summit of Katmai collapsed into a caldera. As the valley deposits cooled, they emitted steam from fissures and fumaroles, earning the name Valley of Ten Thousand Smokes. As heat has dissipated from the deposits the steam vents have subsided and the valley has been eroded. At present streams have cut canyons as much as deep, but only wide. Katmai is a stratovolcano, in height, with a large summit caldera. Several glaciers originate from the mountain, and one in the caldera is one of the few to have formed in historical times. The caldera floor is about below the rim. The mountain stands on Jurassic sedimentary rocks, and its volcanic components are less than thick. Apart from the 1912 eruption, no significant activity has occurred in historical times. Novarupta is described as a Plinian pyroclastic vent with a plug dome, a diameter crater with a , diameter dome in the middle. Its only historical activity was the 1912 eruption.