Keep in mind that Rhino is intended to accurately model surfaces for manufacturing. I can’t see a situation where making a model that looked like the dry, cracked mud makes sense from a manufacturing point of view.
If this is for visualization purposes, the options outlined by others should be pretty good.
If you really need a model like that, I don’t think a NURBS based surface modeler is the best tool for the job. Potentially a mesh modeler would be a better fit.
How do you plan to use the model?
What is the problem you’re trying to solve?