The depression between two parallel and closely spaced retaining walls (lava levées) in which a flow of lava is confined. The retaining walls are composed of cooled blocks of lava from the flow itself. As the lava flows, the height of the flow surface pulsates and lava may overflow its retaining walls; this adds a coating of chilled lava to the walls, thereby increasing their height and the effectiveness with which they constrain the flow.