Silt Block

Silt Blocks were added in 1.1, and falls under the soils category. Like Sand Blocks, Silt Blocks are subject to gravity and will deal damage to any player or mob they fall on. Silt Blocks they look similar to Ash Blocks but are a lighter color.

Location
Below cavern level mostly around the point where the lava pools are underground.

Uses
Silt can be used in the same way as sand, and can be used to fill lava pits. It can also be used to function in various traps, since it causes damage to whatever it falls onto.

Trivia
Silt blocks were probably based off the gravity effected Ash blocks introduced in patch 1.0.5, however the gravity-Ash blocks were removed in the 1.0.6 patch.