The bug
In 14w20a, Mojang added caves naturally cutting through sandstone, sand, hardened clay and mycelium upon generation (see https://twitter.com/dinnerbone/status/466887474367954944).
However, this new feature did not extend to the cave generation that spawn ravines, so you still get unnatural-looking formations where the ravine cuts into grass but not other blocks (see image attachment). This is why you never see surface ravines in desert, mushroom or mesa-type biomes.
How to reproduce
Sand and sandstone
Seed: 1
/tp 197 62 -4 70 24Linked issues
Attachments
Comments 6
I provided screenshots to show that ravines don't naturally cut through hardened clay or mycelium. I used a customised world to prove the mycelium, details in the screenshot.
TL;DR: Fixed in 18w08a.
In 18w08a, ravines changed. They don't generate where they did in previous versions (using the same seeds and locations), so I had to find new surface-ravines or would-be surface-ravines to see what they cut into.
Sand & Sandstone:
Ravines now cut through these. Seed: 1
Location: /tp @s -10360 75 39780
Red Sand & Terracotta:
Ravines now cut through these. Seed: -635205980
Location: /tp @s -805 70 -188
Mycelium/Snow Blocks:
Can't find surface ravines or would-be surface ravines in these biomes. Much harder now that we can't use Customised Worlds to generate a world of just that biome. But even if they don't cut through those, MC-16132 exists for that.
Conclusion: Fixed in 18w08a.
migrated
Please provide seed and coordinates.