Ever have a contractor come and dig a trench or a foundation for you? He gives you a price for the job, but adds a disclaimer about how that price does not include unexpected events like hitting rock. Hitting rock slows the whole project down and drives up the cost because trenchers and backhoes don’t dig through rock all that well. He’ll probably have to break out the dynamite and blow stuff up to get the rock out of the way. Fun, but costly.
Here’s my rock clause: If I hit rock in your project, I’ll tell you right away, and together we’ll find the most expedient, least expensive way to blow the rock up and get it out of the way. We’ll even make it fun, if we can. But it will change the cost of your project.