Levees

  As a stream of lava moves through an existing tube, the sides tend to cool first and create a free-standing, vertical remnant along the edge. Its height is increased by splashing and frothing, and the top is often irregular. The inside edge is typically smooth but may show grooves or flow lines. Often levees are associated with gutters, which form between the levee and the tube wall.

Photo #2 shows a set of the largest levees this author has ever seen