Most mountains were formed by tectonic plates pushing against eachother. This is how Montana’s mountains were formed and… how fossils that were once part of a sea floor, like the one I found (see Sunday, August 01, 2010 blog post), got to the top of a mountain. So how did these sea-dwelling organisms end up on a mountaintop in Montana? . . . Blame it on plate tectonics. About 100 million years ago, massive segments of Earth’s crust (crustal plates) moved eastward from the area of the Pacific Ocean, pushing into the western edge of North America. This caused the rocks containing the sea fossils to rise from sea level, forming huge mountains such as those found in this part of Montana. A similar process is happening today in the Himalayas where fossils of ancient sea creatures can be found among the world’s highest mountains.