The white stuff is a type of deposit called evaporite. The water from any stream or river contains some dissolved solids. If that water flows into a basin in an arid area, the water continues to evaporate away, leaving more and more of the dissolved minerals behind. Over long periods of time, quite a bit of evaporite can build up.
This deposit is mostly salt (and yes, I tasted the ground), but it also contains gypsum, anhydrite, sylvite, and other minerals. The additional mineral content gives it a different taste from pure table salt, causing some people to say that it tastes saltier than salt.