The tablet version is unknown to me. I generated an old world, and then quit to test out the option to make it infinite. So when I did I found that the world ended up spawning with borders much like the far lands, 20 blocks above the ground beside it! The bordering Biomes were plains, Forest, and swamp, (on the inside as part of the original generation with the old world) but the whole area was engulfed in an extreme hills biome! (just on of the flat-ish areas of them though.) And finally, I looked towards the river in the original old world, and I saw that A glitched mountain spawned in the chunk over top of it! The Mountain spawned only inside of the one chunk, and only directly above the river biome's water!