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The Origin Story of Southwest Idaho

By: Sierra Laverty, Adult Education Specialist | 04/22/2021

Welcome home! You live in a rare, complex ecosystem. The water you drink is the result of a series of ancient lake beds. The air you breathe is the output of hundreds of miles of sagebrush to the south, and young forests to the north. The food you eat is diverse and rich, owing its start to the most valuable soil type in the world.
For Southwest Idaho residents, there is literally no place like home.
The Boise Metropolitan Area is located in the Great Basin Sagebrush Steppe. While many Idahoans don’t understand what that entails, it affects nearly all parts of our day-to-day lives. The Idaho Botanical Garden is inviting you to look closer at our little corner of the Sagebrush Steppe over the next two years, as we introduce new classes, exhibits and other shenanigans that center on something we all have in common: home.


Can you trace your roots back 2.5 billion years? Idaho can.

The oldest boulders at City of Rocks look and feel different from the others. They’re darker, jagged, and rougher. They are 2.4 billion years older than the lighter, smoother nearby rocks, so no wonder they seem to be showing their “age” a little more. They come from a time period when the only living creatures were single-celled, microscopic organisms, and serve as a glimpse into the continental crust that forms the foundation of our state.

Idaho lay underwater off the coast of North America for about 540 millions year through the Paleozoic and well into the Mesozoic Era. We can see evidence of Idaho’s ancient marine past-time in oceanic fossils of trilobites, corals and brachiopods, and lots of sand and sediment based rock beds that layer over one another deep under the Treasure Valley. As the supercontinent Pangea formed, Western Idaho was the ocean floor of a shallow, tropical marine ecosystem just above the Equator.

Right behind the Lewis & Clark Native Plant Garden, Tablerock and it’s sandstone are a relic of Lake Idaho, an ancient lake that existed from about 10 to 3 million years ago and covered most of Southwest Idaho—which has been underwater more than once.

The Pacific Northwest (islands, at the time) eventually joined North America. The western edge of Idaho emerged out of the sea, and new, towering mountain ranges and volcanoes formed that would later disappear completely. Finally above water, Idaho would be home to small, herbivorous burrowing dinosaurs, palm-like scale trees, dire wolves and towering oaks—all of which are now extinct. Before geologic forces created the Cascade Mountain Ranges (and with it an enormous rain shadow), the inland Pacific Northwest including Southwest Idaho was wet and forested. Between about 30 and 1 Million Years Ago this area was under siege of volcanic flows, and in some places those flows were several miles thick. These events set the stage for a new ecosystem between two great mountain regions began to form—the steppe of the Intermountain West. 


We would like to thank geologist, Dr. Terry Maley for helping us with the technical info here!

Learn More

  1. What is a Steppe? (blog post)
  2. Native Plants for Your Yard (class)
  3. Idaho Geology (e-book)
  4. Geology Underfoot in Southern Idaho (book)
  5. Stories from the Sagebrush Sea