The rock formations in the Rocky Mountains and Great Plains is said to be different yet have similar amounts of certain minerals because the rock of Rocky Mountains is igneous rock while the rock of the Great Plains is sedimentary rock, Though the rocks may have some certain amount of minerals but they are formed in different ways. When sediment is firmly pressed and cemented together, it leads to formation of sedimentary rock but when Magma cools it form igneous rock when it's hardens.
