Boulder, Colorado has very interesting and unique geology compared to many other places in Colorado, or even the country. As you can clearly see the Flatirons are very unique for a rock, at least for the size they are. Even though the geology in Boulder looks all very different, it is mainly one single type of formation of rock you see at such places like Sanitas or simply on the Flatirons, this is known as the Fountain Formation.
In the Boulder area the oldest rock you can find is around 1.7 billion years old, which means it is from the Precambrian age(rock age is measured with carbon dating, which measures the radioactive elements contained in the rock). Rocks from the Precambrian age are either granite or metamorphic. In Boulder’s case for example you will only find metamorphic rocks, which basically means the sediments, which can be things like sand or mud, were pulled into a subduction zone in the ocean, then through heat and chemical reactions metamorphic rocks were formed.
Then about 600 million years ago, which is known as the Paleozoic Era. Colorado’s Precambrian age rocks were flooded, as the rest of Colorado, by the ocean. For 300 million years the lands of Colorado were covered with water, this left many rock sediments in the area for future rocks to be made. Then at 300 million years ago, known as the Pennsylvanian age two very large mountain ranges were pushed up by earthen forces, these are known as the Ancestral Rockies. Due to the weathering of the ocean waters the two new mountain ranges were composed of mostly sandstone and shale. Sandstone is a red rock, with a sandy surface, this is created from sediments of sand. Shale is a very fragile rock that is very flat and this is created from mud sediments. All this, and the addition of lots of iron, which makes it have a red appearance, creates the Fountain Formation.
Around 260 million years ago water from the east came in, this water brought lots and lots of sand with it, in turn this sand created the Lyons formation. Today we see Lyons Sandstone used in many parts of the Colorado University campus. Due to the massive amounts of sand now in the Colorado region, the Ancestral Rockies were burred by sand. Over the next 150 million years Colorado was constantly getting more and more deposits from ocean streams coming in and coming out. This lead to the deposit of many types of sand and gravel. This sand and gravel ended up creating the Dakota Hogback, a fine place to see the Dakota Hogback is on one of the trails at Mountain Sanitas, very beautiful.
Around 65 million years ago, our mountains began to finalize themselves. The previously buried mountains were pushed up through the earth. Many streams became very fast moving rivers, this ended up pushing many smaller pieces of debris towards the plains of Colorado. The old mountains that had just been uplifted, were injected with hot molten magma that spewed from the earths core. This magma caused the mountains to get a very unique look to them. Once the hot molten magma reached the top of the mountain it erupted out the top in the form of a volcano. There are only a few select places you can still see any trace of volcanic ash or rock here in Colorado.
This is how Colorado became what it is. Through the every changing times of the past, Colorado had many different influences by the ocean mainly. All of these influences combined to create the wonderful geology of Boulder, Colorado.