A surface that slopes down into the passage from a lava tube’s wall is an apron. In contrast to cutbanks, aprons tend to occur on the inside edge of a meandering lava tube passage where the flow’s velocity is lowest, and thermal erosion is reduced. This in turn allows more lava to accumulate. In the photo, there are aprons on both sides of the passage but the lefthand one is more pronounced. Often aprons go to the floor, but these ones are undercut, possibly by a later flow episode.
AUTHOR: Dave Bunnell