Colorado’s Turbulent Past: Epilogue

We descended from our last camp site in the San Juan Mts and discovered another environmental consequence of the weathering of the volcanic and intrusive rocks from this area. As the plagioclase feldspar weathers it forms the mud we see everywhere that is low, but it also forms a thin soil on the underlying rock that is weathering in place to form regolith. The trees cannot put roots very far into this broken and weathering rock so they have very shallow roots, which allow them to be blown over by a strong wind or just fall over when they get too large. We encountered a fallen tree as we headed for Dove Creek and the highway; we think it was an aspen but none of us were experts.

We didn’t have a saw (not too well prepared) but a couple of motorcyclists riding the TAT had a small hand axe and we had a hand saw, which together were used to cut about half-way through the trunk. The winch on the lead vehicle was then used to finish breaking the fallen tree and drag it out of the road.

Our path took us through some Paleozoic rocks that survived on the flanks of the uplift associated with both Laramide and Tertiary magmatism. These older sediments had been eroded from the top of the San Juan Mts before the Tertiary volcanism but they are very thick to the south and west in New Mexico and Utah. I am not going to discuss these areas in this series of posts because we didn’t really travel through it at a leisurely rate but just headed to Monticello, UT where we ended our trip.

Here is our afterward group shot in front of the Abajo Mountains as we were preparing to head east to Louisiana, New York, and New Hampshire.

I like to end these trip posts with a summary. This journey  covered a long period of time, from the Proterozoic (~1.7 billion year ago) granites of the Wet Mts to the Miocene (~6 million years ago) basalts of the Raton Mesa in SE Colorado. It is typical of CO’s complex geologic history that these oldest and youngest rocks are near each other where our trip began on I-25.

These two regions not only represent the entire time interval we covered in this trip, but also the end members of the igneous rocks that are produced within the earth’s crust; the Proterozoic granites were formed deep beneath the surface from magma that originated from melting of continental crust during an ancient orogeny whereas the Miocene basalts flowed out onto the land from fissures. This basalt also represents a significant contribution of their source magma from ocean crust, which has led many geologists to propose that part of the Pacific Ocean’s crust was overridden by N. America and melted beneath the continental interior to produce basalt, which is typical of ocean crust only.

Between these two extremes of igneous rock, we find granitic rocks with intermediate composition from the Laramide orogeny (~60 million years ago), which began the slow process of raising the Rocky Mts to their present elevation, and both volcanic and plutonic igneous rocks that were produced as this event wound down between approximately 36 and 20 million years ago. Many of these igneous episodes produced gold and silver deposits that occur along deep fractures in the earth’s crust, originating from mountain building ~1.7 billion years ago.

My final comment on this trip refers back to the name of this blog; the most exciting and geologically interesting parts of this journey were only possible because of roads built over the last 150 years to extract gold and silver from these mountains. The few areas where we were able to travel over designated trails that weren’t much more than tracks were limited to meadows and mesas in the SW part of the state. For this journey, we might refer to the blog as “Rocks and Roads”.