They're soils, but they're special soils [1] that can harden to a brick-like consistency (called a "duricrust") upon being dried out and exposed to the air [2,3].

The unusual durability of such soils has made them a target for study in their own right, using cosmogenic isotopes to estimate erosion rates over Myr timescales (which turn out to be incredibly slow; 0.16–0.54 m/Myr) [4]

[1] https://www.cambridge.org/core/journals/clay-minerals/articl...

[2] https://en.wikipedia.org/wiki/Duricrust

[3] https://www.sciencedirect.com/science/article/pii/S016913681...

[4] https://www.sciencedirect.com/science/article/abs/pii/S00128...