This is a very cool find.
It has to be part of a Machine--Kiln that was broken up and disgarded.
Many parts of a "Machine" had a limited lifespan but eventually the entire kiln had to be remade--just because of use.
These pieces of the Kiln always carried some glass on them.
The folks back then just dumped what they considered as junk into the rivers--or landfills.
Go back and find some more--dig deep.
Good luck!
I Just love this stuff!
Marble--On!!