In a Customized World with only mesa biomes selected (No other settings changed) I found this strange patch of white sand in a mineshaft. It looks like it fell from the top of the mineshaft.
Environment
Java up-to-date, Minecraft PC, Windows 10
MC-102260
Red sand to plain sand mesa mineshaft
Resolved
Duplicate
0
Please attach the customized world preset, the seed and the coordinates.