Minecraft wiki entry for Shelters, associated text: Do not use sand or gravel, because unlike most blocks, they are affected by gravity. You can't place a ledge with them, as they fall to the ground. (However, you can put a dirt block on the side of a sand pillar, and that will stay up even if the pillar doesn't. In particular, if a creeper does notice you, and blows up at the bottom of your pillar, the rest of sand you're standing on will fall closer to the ground, taking you with it… and apparently, you were already low enough for monsters to notice you. (Also, a passing enderman might pull a block out of your pillar.) If you're really stuck and only have sand or gravel, make the pillar extra-tall, at least 20 blocks.