During Pliocene time a cluster of basaltic and basaltic andesite shield volcanoes was built in the
Three Sisters region. Included in this cluster are the North Sister, Little Brother, Husband, Wife,
Sphinx and Broken Top, the radial dikes and conduit fillings of which have been laid bare by
glacial erosion. The view that this arcuate line of peaks marks the rim of a caldera formed by
decapitation of an enormous central volcano, Mount Multnomah, is shown to be erroneous. During
the Pleistocene, andesites and dacites were erupted, principally by the Middle and South Sisters,
while new basaltic cones were growing elsewhere. During Recent time, still other cones of basaltic
lava and scoria were formed and vast flows were poured from some of them, notably from the
Belknap Craters near McKenzie Pass, while showers of pumice and viscous domes of obsidian were
erupted by neighboring vents. Some of these eruptions ended only a few centuries ago; nowhere
in the High Cascades has there been more volcanic activity within the last millennium. Taken as a
whole, the magmatic history closely resembles that of the Crater Lake region and the post-Miocene
activity of other parts of the Cascade Range farther south.Gerald W. Williams Collectio
