Mountain top very rich with fossils

This forum is predominantly American and I think You'd have better luck with local sources re fossil formation in that particular mountain.

Having said that I live in the UK not far from Dorset and what is known as the Jurassic coast. It is very rich is fossils as well, and many people do collect them and they're sold in the local gift shops. As for value it depends on the fossil, they're not diamonds, and a lot are so cheap they're given away as free gifts in comics and the like. Having said that I'm sure if you found some hitherto unknown specimen of something it might be worth a tidy sum

I'm not a Geologist, and I'm sure others more qualified will be along to give better advice.

Ive looked and there are quite afew geologic maps with key descriptions of fossil content in ach identified rock groups and even Formations.
I would acquire some of these maps and try to locate where youve found these fossils wrt the geology. Its really a fairly easy way to search out where youve found your specimens.
Then its a simple matter of getting some good quality reference fossil collections or books and compare what youve got with what geologic surveys have found.

Its kind like hunting for antiquities.

People all over the world have found seashells on various mountains. I live where Arizona, California and Mexico meet, altitude about 1100 feet (335 meters) and the gravel contains a lot of seashells, about half the size of a fingernail.

I believe the Op is interested in specific identifications and associations, rather than just "shells".
He already knows they are "fossil shells"

Well excuse me for existing.