Limestone
Limestone commonly forms from calcium carbonate deposited in marine environments, including material produced by shells and other organisms. Some limestone layers preserve marine fossils.
A JOURNEY INTO DEEP TIME
Ancient Seas
and Fossils
Discover the evidence of ancient oceans and the organisms that lived in southwest Montana long before today's mountains existed.
THE STORY PRESERVED IN ROCK
When Montana Was Beneath the Sea
Much of what is now southwest Montana was covered by shallow seas during different periods of Earth's distant past.
Sand, mud, and the remains of marine organisms accumulated on ancient seafloors. Over time, these sediments became layers of sandstone, shale, and limestone.
Later mountain-building events uplifted and deformed these rocks, exposing evidence of environments that existed hundreds of millions of years ago.
GEOLOGICAL EVIDENCE
How Ancient Seas Left Their Mark
Sedimentary rocks are among the most important records of ancient environments. Their mineral composition, layering, textures, and fossils help geologists reconstruct the past.
Sandstone
Sandstone forms when sand grains become compacted and cemented together. Ancient beaches, rivers, and shallow marine environments can all produce sandstone.
Shale
Shale forms from fine-grained sediment deposited in relatively quiet water. Its thin layers can preserve delicate evidence of past environments.
Fossil-Bearing Layers
Fossils may occur within sedimentary formations, providing clues about ancient organisms, water depth, environmental conditions, and geological age.
ANCIENT LIFE
What Fossils Can Tell Us
Fossils are preserved remains, impressions, or traces of organisms that lived in the past. They help scientists understand the evolution of life and changes in Earth's environments.
Marine Invertebrates
Fossil-bearing marine rocks in southwest Montana can preserve evidence of organisms such as brachiopods, corals, crinoids, and other invertebrates that lived in ancient seas.
Trilobites
Trilobites were marine arthropods that lived during the Paleozoic Era. Their distinctive segmented exoskeletons are important fossils for understanding ancient marine ecosystems and the ages of some rock formations.
Trace Fossils
Not all fossils are body parts. Burrows, trails, and other traces can record the activities of organisms that lived on or beneath ancient seafloors.
Geology Connection
Finding a marine fossil high in the mountains does not mean the sea reached that elevation. The rock may have formed near sea level and later been uplifted by tectonic forces.
MILLIONS OF YEARS OF HISTORY
A Changing Geological World
Southwest Montana's geological record spans enormous periods of time. The following broad intervals help place its ancient environments in context.
Precambrian
Older rocks preserve evidence of ancient sedimentary environments and geological processes that preceded abundant complex animal life.
Paleozoic Era
Marine environments repeatedly occupied parts of the region. Sedimentary rocks from this era may contain fossils of ancient marine organisms.
Mesozoic Era
Changing seas, rivers, and terrestrial environments left additional sedimentary deposits across parts of Montana.
Cenozoic Era
Uplift, faulting, erosion, and sediment deposition contributed to the modern mountain-and-valley landscape.
EXPLORING THE GEOLOGICAL RECORD
Where Can You See Ancient Rocks?
Exposures of sedimentary rock occur throughout southwest Montana, including mountain slopes, roadcuts, and natural outcrops.
Geological maps can help identify the ages and types of rocks exposed in different areas. Specific public viewing locations will be added as we develop this guide.
Fossil Collecting and Public Lands
Fossil collecting rules depend on land ownership, the type of fossil, and applicable federal or state regulations.
Some common invertebrate and plant fossils may be collected in limited quantities for personal use on certain federal lands, subject to agency rules. Vertebrate fossils and scientifically significant specimens are generally subject to much stricter protections.
Before Collecting
Verify land ownership and current regulations with the managing agency. Never assume that collecting is permitted simply because fossils are visible at a location. Respect private property and protected sites.
LEARN MORE
Geological Maps and Fossil Resources
Scientific publications and geological maps provide more detailed information about southwest Montana's ancient environments.
Montana Bureau of Mines and Geology ↗
We will add locally documented fossil examples, original photographs, geological maps, and additional scientific references as this section develops.