Lake Natron is an alkali endorheic salt lake of tectonic origin present in northern Tanzania.

Northern Tanzania.

From dry lake beds.

Natron is white to colorless when pure, varying to gray or yellow with impurities. Natron deposits occur naturally as a part of saline lake beds in arid environments.

It was generally found in the dry lake beds that had been left as a result of hydration and then recession of the water in the Nile Valley.

The simple answer is "dried up lake beds."