Sometimes gravel only cover the top layer of the terrain. Seed: 3264540676790285439.
Relates to MC-199004, MC-199005 and MC-199007.
Basalt Deltas only overwrite one layer of blocks into basalt, looking slightly unnatural
New nether forest biomes replace the top layer of blocks with the respective nylium block, even if the blocks being overwritten aren't netherrack; leading to unnatural looking terrain
Soul Sand Valleys replace the top blocks with soul sand/soil despite the blocks being overwritten being something else