Tunnels will occasionally lead from the surface down into this layer and there you might find even larger caves, sometimes half-covered with lakes of Mineral Water or Tar, where Mirus and Warmworms can spawn on blocks of Siltstone, Stalactite, Tourmaline and even a number of Ore-Nodes. Usually you will be able to spot patches of Saltrock embedded in the walls or ground of such caves.
Still you will have better chances to find Saltrock in large amounts by digging mine-shafts. You are going to need to equip an Obsidian Mining Cell or better to dig through this layer and/or to take blocks of Saltrock.
Taking Saltrock for the first time will unlock the crafting-recipe for Salt.
You can place Saltrock like it is or you can use it for a few crafting-recipes, like for Iron Mining Cells.