Lava River Cave is one of Oregon's longest uncollapsed lava tubes at 5466 feet long. About 100,000 years ago, it carried 2000° F. lava from an upslope vent to lower areas on the flanks of Newberry Volcano. The trench at the entrance formed after lava drained from the tube and part of the roof collapsed. Collapse blocks litter the first 1000 feet, beyond which nearly a mile of cave remains much as it did when the ancient lava river left it. Farther inside, a section passes beneath U.S. 97, where the ceiling is about 31 feet above the floor and the roof is about 50 feet thick. Flow lines, including a prominent lava shelf with lavacicles on its underside, formed where lava held a constant level and left ridges on the walls, while thin glassy glaze, wrinkled glaze, and other remelt features are abundant. The cave also contains vertically stacked passages, where late in the eruption lava eventually drained only through the lower tube as levels fell. In the lower section, sediment slowly washes through roof cracks and partially fills the cave, forming delicate sand gardens as water droplets erode the sand fill. Near the end, sediment thickens until it fills the tube from floor to ceiling, and in 1936 two explorers dug a trench more than 300 feet long but could not extend the open cave.