Concerns about stability. The ice sheet covering West Antarctica is the last great marine ice sheet. Its bed lies below sea level and slopes down inland from the coast. This profile is based on Thwaites Glacier, West Antarctica. In the top panel, the ice sheet is in equilibrium; influx from snowfall (q) is balanced by outflow. A small retreat (lower panel) will provoke changes in both the influx and the outflow. If these changes act to promote further retreat, the ice margin is unstable.