^^^ I think Dleg conflated two ideas - something I am routinely guilty of myself.
First thought,
In terms of state-licensed recognition of work, many states grant registration as a matter of a GENERAL LICENSE. For instance, my professional stamp says Professional Engineer - it doesn't indicate my discipline of registration or hint at the body of knowledge that I may possess. However, if you look at a state like California, they provide registration by discipline. So, your stamp will say Civil Engineer or Geotechnical Engineer (as an example). I am not familiar enough to tell you the ins-and-outs of states that register by discipline but know that is how it works in a nutshell.
Again, most states license by engineering profession rather than engineering discipline, so it is just the few states that you have to look into how thier regulation of the profession affects what you can stamp.
Second thought,
While it is true that most states grant registration by "general acceptance", you are still required to follow certain guidelines regarding your proficiency and competency in the areas you provide engineering services (e.g. certify or stamp documents). These standards are typically codified in the state's rules, so obviously the standard for competency or proficiency will vary from state to state.
Again, you will need to see how your education and experience match up with the competency and proficiency standards for the state you wish to practice.
Last thought,
My background mirrors yours in many ways - I am an environmental engineer by education/experience that dabbles a bit in geotechnical work. I took the Civil PE Exam, in part, to ensure portability of my license. So, I guess part of my rationale for claiming competency in some areas of civil engineering is based on passing the Civil PE Exam. I would also need to draw upon experience and education to further those arguments depending on assignments that I accepted.
I realize this post is a bit of a ramble, but I hope it helps give you a start with thinking about how to resolve your questions pertaining to certifying/stamping various engineering works.
JR