Minecraft wiki entry for noise_settings, associated text: y_above conditional surface rule to check if it is above a specified y-coord
 anchor: the minimal y-coord where this condition is true.
 above_bottom: the y-coord above the bottom of the world, mutually exclusive with absolute and below_top.
 absolute: the exact y-coord, mutually exclusive with above_bottom and below_top.
 below_top: the y-coord below the top of the world, mutually exclusive with above_bottom and absolute.
 surface_depth_multiplier: [needs testing]
 add_stone_depth: [needs testing]