My son found one small and one large (about 12" in length) similar fossils in sandstone (I believe) at the top of the Bookcliffs, just north of Grand Junction, Colorado. They are very interesting impressions and we were curious as to what may have caused them.
Any help would be appreciated. Many thanks.
Crinoid.
PS: Hi @Michael_Callen , great to see you 
Thanks Dan! At first, I thought you were calling me a name. So, then, would this be a colony of them, such that each little divot would have represented one animal? Or is it the stalk of the animal? Again, thanks!
Crinoid vary a lot, but this would be one animal with a stalk-like “foot” (?) where it attached to the seabed (or something).
I would NEVER call you names … but I might edit your title. 
Yes, that might happen once or twice. 
Thanks, again, Dan.
To clarify: a crinoid stem is made of a long stack of calcite disks, and you’re looking at a mold of that stack. Paleozoic limestones commonly contain abundant stalk fragments and individual disks.
Thank you, John!
I’m not convinced these are crinoid stems - where’s the disc-like morphology? It’s much more lumpy and disorganised.
You’re right. Magnified, it’s not disk-like at all. But I can’t think of what else it might be. It’s an impression, part of a circle in cross-section, with lots of little oval pits, some of them perhaps superimposed on others. Doesn’t seem like an ichnofossil.
Various questions: how is it oriented to the bedding plane? Is this a Cretaceous sandstone?
My son Scott, who found the fossils, works for the forest service as a firefighter, so he doesn’t know much about geology. It does seem that it is cretaceous sandstone. It was found upon the cap of the south-facing portion of the Book Cliffs. As per Wiki, it seems to agree with your suggestion: Book Cliffs - Wikipedia
They [crinoids] doesn’t necessarily have to be disc-like.
Image fixed!
ETA: not the Doctor Who Crinoids either. 
Sorry, what doesn’t necessarily have to be disc-like?
Crinoids. Will edit
It’s confusing because the crinoid stalk shown on your diagram is disc-like.
Clarification: it isn’t the crinoids themselves that are disk-shaped, it’s the individual pieces of the stem (though some are occasionally pentagonal or star-shaped). The stem, in those crinoids that actually have stems, is of course stem-shaped. As in your illustration.
stem-pieces copy.pdf (37.2 KB)
Anyway, we seem to have established that the fossil isn’t actually a crinoid stem.
You mean disc-shaped at the end, right?
I don’t know what you mean by “at the end”. But the crinoid stem is made up of a stack of disk-shaped skeletal segments, as shown next to that penny above. That’s what I’m talking about. That’s the only disk-shaped part of a crinoid. (Though the arms, the “flower” part, may also have disk-shaped skeletal segments.)
You were talking about them being “disc-shaped”… I’m assuming a typo here.
No typo. The individual pieces of the stem are disk-shaped, while the stem itself is a string of those pieces and is more or less tubular. As in your picture.
Okay, I see. Thank you for your clarification. So then, the individual pieces of the stem are typically disc-shaped, whereas these appear more spherical. Any other plant or animal that you can think of that would leave this kind of impression, assuming that this is not a crinoid stem?
