Along some parts of Glacier National Park‘s Highline Trail, you might spot some swirling patterns in the rocks. These formations, found in other regions of the park as well, are called stromatolites. They’re fossilized mounds of cyanobacteria, which once dotted the shores of an ancient sea.
1.4 billion years ago, the now cold and mountain filled Glacier National Park was beneath a shallow inland sea called the Belt Sea. The sediments flowing in that sea build up over time to form the sedimentary rocks found throughout the park today.
Over time, cyanobacteria grew layer by layer to form cabbage-like mounds. Now when we see their cross-section in a rock outcrop, they look like a sliced-in-half cabbage. These rock outcrops exist in several areas throughout the park, giving visitors a look into the past.
The stromatolites photosynthesizing over millions of years helped to allow the emergence of complex life on Earth through the production of oxygen, literally changing the composition of the atmosphere.
The belt sea didn’t just cover Glacier National Park, either. It spread across what is now Idaho, Montana, Canada, and beyond.