I live in Boulder, Colorado.  It is a beautiful place, at the foot of the front range of the Rocky Mountains.

Some people say that the only thing missing here is ocean.  But in its past (before people were around to appreciate it), Boulder once sat on the sandy beaches of an ocean.  At times it was covered by a shallow sea. You can still see remnants of that ocean sand in sandstone formations, fine beach sand at and wave patterns in rocks on hikes in the area.


The land we see today in and around Boulder has gone through hundreds of millions of years of changing climates – from ocean, swamp, lush tropical climates and redwood trees, desert and sand dunes, mountains that grew and were worn down and new mountains

The Boulder flatirons are made of sedimentary rocks from the Cretaceous period (70-135 million years ago, when the dinosaurs roamed), upended as flat slabs about 8 million years ago.


Further up the canyon, you can see Pre-Cambrian rocks.  That means they were formed over 570 million years ago! It was a time when the very first soft-bodied marine organisms were on the earth. The Pre-Cambrian period probably included two or more additional mountain-building and mountain-erosion periods dating back 1.5 to 2.5 billion years ago.


Colorado was not in the Northern latitudes then.  Until the Pennsylvanian period (270-310 million years ago), Colorado was part of a land mass that was south of the equator and was covered by a shallow sea!

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A supercontinent formed about 250 MYA.  (Colorado did not arrive at its present latitude until about 75 MYA!)  In the Pennsylvanian period, the Ancestral Rockies began to grow The rising of the mountains pushed the sea east.  The area became a desert.

By 250 MYA the Ancestral Rockies were flattened.  After the seas receded, great sand dunes covered the landscape.

Around 180-220 MYA, during the Jurassic & Triassic, the climate was wet and dinosaurs roamed in Colorado.  The climate was like Northern California today and tall forests with giant redwoods grew.  Coastal swamps covered the landscape by 72 MYA.

From 10-40 MYA, in the tertiary period, there was a lot of volcanic activity in Colorado.  A new period of uplift began about 18 MYA, which gave birth to the present-day Rocky Mountains.

The Ice Age had its affect too.  Several times in the last 1-2 million years, glaciers covered much of North America, including the mountains of Colorado.  They scoured the surface of the earth.  Glaciers acted like slow-moving rivers, picking up and churning all sizes of rocks and dumping them elsewhere as they moved.  Bowl-shapes in the mountains are remnants of glacier-carving.

The Interactive Geology Project of the University of Colorado has a  library of videos that make the geologic history of Colorado come alive, e.g.,:

Sources:; University of Colorado History museum; Prairie, Peak & Plateau (Colorado Geological Survey Bulletin)

Recommendations: Roadside Geology of Colorado (Mountain Press Publishing Co)