The eastern Snake River Plain is a predominantly rhyolitic province, analogous to the present-day Yellowstone Plateau volcanic field but older and further evolved. The wide-spread Quaternary basaltic lavas that now blanket the eastern Snake River Plain appear to have erupted after major rhyolitic activity ceased and represent less than 20% of the total volume of volcanic rock near the north-central margin of the plain. The rhyolites show a general age progression that becomes younger northeastward toward Yellowstone. Three major ignimbrite sheets, comparable in magnitude to those of the Yellowstone Plateau field, have been correlated on both sides of the eastern Snake River Plain from the Arco and Pocatello areas to margins of the Yellowstone Plateau. These 4.3- to 6.5-Ma-old ignimbrites are referred to herein as the Heise volcanic field. Studies of facies changes and lateral variations in the three major ignimbrites of the Heise field permit identification of related calderas that are now buried beneath the plain. Petrologic studies suggest that, although the ignimbrites overlap compositionally, each was derived from an isotopically distinct magma that contained a significant component of crustal material.