RIDGE-FORMING, ICE-BOUNDED LAVA FLOWS AT MOUNT-RAINIER, WASHINGTON

Large (0.3-4 km(3)) andesite and dacite lava flows at Mount Rainier, W ashington, sit atop or are perched along the sides of high ridges sepa rating deep valleys. Early researchers proposed that these ridge-formi ng lavas flowed into paleovalleys and displaced rivers to their margin s; entrenchment of the rivers then left the lavas atop ridges, On the basis of exceptional flow thickness, ice-contact features, and eruptio n age measurements, we propose that the lavas flowed beside and betwee n valley glaciers that filled the adjacent valleys in the Pleistocene, When the glaciers retreated, the flows were left high on the adjacent ridges. These lavas were never situated at valley floors and do not r epresent products of reversed topography, Instead, ridge-forming and p erched lava flows at Mount Rainier and at many other high stratovolcan oes illustrate the ability of ice to dam, deflect, and confine flowing lava.