North America is on a plate that reaches from the Arctic to the Caribbean and from near the California coast to the middle of the Atlantic Ocean.  This huge plate, about 60 miles thick, has a weak spot that is located right beneath Colorado.  Before 300 million years ago the spot where the Rocky Mountains are was an ocean.  Island ancestors of the Rockies pushed up from the sea floor, built a mountain range and eroded away.  The debris spread out over what is now the Colorado plains (see the plains on the right side of  Figure 4).
 
 
 
 

Figure 4.  The image above, is called a shaded relief map showing the topography of the state of Colorado.  You can see how the transition between the mountains and the plains is abrupt.  The chain of mountains cutting the state in two are called the Southern Rockies.  The Rocky Mountains extend all the  way from Mexico to Northern Canada (Sterner 1995).  Click here to go back to the Fundamentals page.

 

 Figure 5.  Diagram of the area surrounding Fort Collins before the mountains had formed (top) and after uplift of the Front Range had occurred.  each of these layers of sediment were deposited when Colorado was covered by an inland sea.  Sandstone is rock formed from ancient beaches, shale is formed from ancient deltas (where rivers dumped sediment into the sea), and limestone is formed when shell fish, coral, and other sea creatures fall to the bottom of the shallow sea.

    Eventually time and pressure created layers of sedimentary rocks out of the ancestral Rocky Mountains. When the present Rocky Mountains began to rise the overlying layers of sedimentary rock folded and buckled into vertical layers.  We can see a cross section of the folded layers in the Dakota Hogback that shows up in places all along the Front Range .  These are layers of shale and sandstone of various geologic epochs.  The "Devil's Backbone" that can be seen west of Loveland is a spine of hard sandstone that resisted the forces of weathering better than the other layers.  Horsetooth  Reservoir sits between two ridges of hogbacks.  Notice how the different rock types correspond to the shape of the ridgeline, why are sandstone ridges convex and shale ridges concave?

    Figure 6.  Diagram of the area surrounding Fort Collins after uplift of the Front Range had occurred.

Dinosaurs, Coal Mines, and Gold

    During the same period of time that inland seas were rising and retreating, dinosaurs were roaming the Earth.  Thirty-million years ago, the landscape of Colorado blanketed in dense rainforest.  Fossils of giant redwood like trees, insects, and mammals may be seen near Colorado Springs at the Florissant Fossil Beds National Monument.  These fossils were formed when huge volcanic eruptions buried the landscape in ash. 

    The Morrison Formation is a shale deposit that occurs west of Fort Collins.  There are also a variety of fossils that have been found in this deposit.  Shale is formed when fine sediments (silt) settle out in river deltas and in lakes.  When the bodies of animals and plant parts settle out with the silt, they may be preserved as fossils.  Organic material that becomes deposited in dense beds that become covered may also be transformed into coal.  Colorado has many coal mines.

   All this time the North American  plate continued to move westward overriding the Pacific sea floor.  Toward the end of this mountain building process, explosive volcanic activity brought lava flows to the surface.   You can find plenty of volcanic evidence in Colorado in the igneous and volcanic rocks that are common along the Front Range.   In fact if you imagine a long rectangular box about fifty miles wide  from Nederland (above Boulder) southwest to about Silverton this is known as the "Colorado Mineral Belt".  Most of the mineral mines are located in this belt.  This is where most of the gold, silver, lead and other metals came from.  In a liquid form, these metals were forced up from deep in the bowels of the Earth through cracks and fractures.

     By 36 million years ago these mountains had eroded into a narrow band of uplands.  Colorado and parts of Wyoming and surrounding states began to rise under a gigantic dome of pressure caused by plate activity.  As the dome rose a deep depression formed along the Front Range known as Denver Basin.  As the mountains eroded away vast quantities of rock and sand filled the basin.  How do we know this happened?
 
  The rock layer that caps Long's Peak is the same rock formation found at the bottom of the Denver Basin.  Long's Peak is 22,000 feet higher than the bottom of the Denver Basin.

    Geologists think this rise or buckling was the result of a weak spot in the Earth's crust.  When the plate movement slowed in California the buckling happened here, sort an accordion effect like what happens to a car when it hits a tree.  When this happened Colorado's highest mountains appeared.
This was also the time of maximum glacial activity.

     Why do they call them the Rocky Mountains?  If you had lived all your life in the smooth, tree covered mountains and hills of the Eastern United States, what would you call our Rockies at first sight?

     So far all we have discussed is mountain formation, orogenesis,  like the grand mountain landscape  of  Long's Peak and Meeker Peak west of us.  Now we can discuss the forces that wear mountains down.

      Even as the mountains are growing taller and taller the forces of nature are slowly wearing them down.  If you think about it, mountains grow because they are rising faster than they can be worn down.  This wearing down process is called weathering.  Even when mountains stop rising weathering continues.  We will discuss weathering and erosion further on the Erosional Processes page.

     Remember, this is the second set of Rocky Mountains.  Perhaps in that original set of Rocky Mountains there was another Front Range and another Long's Peak or another Poudre River and Big Thompson Canyon.  These are mysteries that weathered and washed down the mountains over an very long period of time.  The clues lie in the deep layers of sediments that extend from the toe of the mountains all the way to Kansas.  The slow process of weathering and erosion over time washed these mountains out onto our eastern plains.

       An interesting question might be what happened to all the water that transported the sediments on to the plains.  Most of it eventually found its way back to the sea.  However, all that coarse sand and gravel deposited in deep beds in the plains made an excellent place to store water.  That is how the vast Ogalala Aquifer  came to be.

    From what you have learned so far about mountain building;  how do you suppose animal fossils got so far from an ocean?