To Shiny Rails

Wait, what?! Rocks and railroading together! Yes, please. Papa and Matteo took a ride on the Cumbres and Toltec Scenic RR‘s once-a-year Geology Train today. We took all day to go 65 miles behind a steam locomotive, stopping often to hear about the geology we were looking at. There were a handful of geologists on the trip that each talked when their specialty was appropriate. We also had a railroader aboard that shared some info in that regard.

Our California friends may find it interesting to know that much of the geology in this area (south central Colorado and northern New Mexico) was created by the same structure and event that made for much of California’s geology – the subduction of one of the Pacific tectonic plates under the North American plate. As the subducting plate moved east, similar events continued to occur. The front range of the Rockies is about as far east as the affects of this subduction went.

Over the millenia, the San Luis Valley of Colorado was subject to various episodes of compression, extension, and volcanism. There was an ancient Alamosa Lake that extended over the entire valley up to 200 feet deep that also contributed to the rocks and stratigraphy of the area. Volcanism was the most recent regional phenomenon about 35 million years ago. The “cap rock” in many areas are welded tuff, lavas, or the like.

Beneath that volcanics layer is a very much older formation. This makes for a “Great Unconformity” (a huge missing time period between layers). In this case, the underlying Pre-Cambrian rocks are over 1.6 billion years older than the layer immediately above. For comparison, the unconformity often noted at the Grand Canyon is likely less than 500 million years. The older Pre-Cambrian mountains of the area were completely inundated by the younger volcanic activity (about 35 million years ago). We walked through these 1.6+ billion year old rocks when we walked through the tunnel toward the end of the trip.

Coming down Cumbres Pass into Chama, New Mexico it was recent (geologically speaking) glacier activity that contributed to much of what is seen. The railroad grade down the first part of this descent is 4%! The Cumbres Pass is just over 10,000 feet high making it the highest operating railroad pass in the U.S.

We crossed the Colorado/New Mexico border eleven times along the route!

All the photos are here.