I find a lot of these Michigan Lightning Stone (septarian/concretions) here locally. Often times there is a visible line, where one part is the clay part and the bottom (i'm guessing) is this material that has tubes and odd shapes. Sometimes fossil is clearly visible like the worm looking one below (2nd pic). Can anyone tell me more about these types of stones? I'm very interested in their formation but have been unable to find much information.
The top pic shows the darker clay on top and the line where it appears to be fossil. The material is still clay like but has a lot going on. It looks similar to the crinoidal limestone death floor stone I find, but without the obvious fossils.