the ancient rocks of the Belt Sea that form our mountains have much less limestone (limestone is mainly a byproduct of sea life) and fewer fossils than the younger rock exposed in most of the Rockies;
(the Belt Sea; form; ) (our mountains; have; much less limestone (limestone is mainly a byproduct of sea life) and fewer fossils than the younger rock) (limestone; is mainly; a byproduct of sea life) (the younger rock; exposed; L:in most of the Rockies)
the Belt Sea our mountains limestone the younger rock
form have is mainly exposed
much less limestone (limestone is mainly a byproduct of sea life) and fewer fossils than the younger rock
a byproduct of sea life
L:in most of the Rockies