When a clay is rock?

In general, clays were considered as a soft material.

It’s true, clay is soft like gels when it is saturated with water; however, clay is a hard rock when it is dried. Look at the photo from Wyo-Ben’s iconic bentonite deposit in Wyoming, the layer of bentonite deposit (dark color), over tens of thousands of years, it still standed and stretched with other rocks above the ground.