I really have only guesses as to what this is.
The stone appears to be like petrified volcanic mud.
I would imagine different lumps of another kind of clay in the mud
when the stone was split eons ago it exposed the different material.
I personally think the boot print is actually the evidence of a holdfast form kelp or some prehistoric marine growth.
Pure conjecture on my part.
I did collect a casting for a museum.
They sent me a box of different types of material to form a cast.
Any geologists on the forum? Or marine biologist.