function concaveGroundTransition(heading, direction)
  local bounds = boundingBox()

  local wallCheckRegion = {
    bounds[1] + heading[1] + 0.125,
    bounds[2] + heading[2] + 0.125,
    bounds[3] + heading[1] - 0.125,
    bounds[4] + heading[2] - 0.125
  }


  -- Walls must be higher than one block off the ground
  if self.groundDirection[1] < 0 then
    wallCheckRegion[1] = wallCheckRegion[1] + 1.0
  elseif self.groundDirection[1] > 0 then
    wallCheckRegion[3] = wallCheckRegion[3] - 1.0
  end

  if self.groundDirection[2] < 0 then
    wallCheckRegion[2] = wallCheckRegion[2] + 1.0
   elseif self.groundDirection[2] > 0 then
    wallCheckRegion[4] = wallCheckRegion[4] - 1.0
  end


  if world.rectTileCollision(wallCheckRegion, "Dynamic") then
    if setGroundDirection(heading) then
      heading = headingFromDirection(direction)
      world.logInfo("hit wall at %q", heading)
      return heading
    end
  end

  return nil
end