Baked Pierre Shale

Ascend a high place with a panoramic view in or near Walsenburg, such as the Hogback in Lathrop State Park, or the mesas east of Walsenburg along CO Hwy 10 that have a near circular view. You can see the Great Plains that rose from the Mississippi River gaining elevation to eventually smash into and end their surge at the Spanish Peaks and Sangre de Cristo Mountains that had risen from the bowels of the earth reaching for the sky. Can you now see why Huerfano County vistas are rather unique?

The following two paragraphs are written views for you to gain an understanding of the geology of western Huerfano County that can be seen traveling to and from Walsenburg, Colorado.

The view North to East to South. In the Late Cretaceous Period, at the time of the dinosaurs, a sea encroached over Colorado and many other states. It was originally called the Cretaceous Sea and now it is called the Western Interior Seaway. The sea existed for million of years and deposited sediments of sand (sandstone), the shells of dead sea animals (limestone), and silt/mud (mudstone, or shale). The sediments and their rock components are the landscape of Eastern Colorado. The wide-ranging relative flat surface in Huerfano County is the Pierre Shale Formation.

The view South to West to Northwest. The two big mountains are the East Spanish Peak and the West Spanish Peak. Some books call them the Huajatolla - The Breasts of the Earth. The next section of peaks are the Sangre de Cristo Mountains, and several mountains that are not above timberline. Finally you see a mountain above timberline. This is Mt. Maestas. Highway US160 is actually on the south side of Mt. Maestas as it ascends North La Veta Pass. To the right and slightly behind is Rough Mountain. These mountains, excluding the Sangre de Cristo are igneous mountains. These mountains were not formed by volcanoes or rifts where lava pours out of a crack in the earth. They were formed by a huge bubble, 60 cubic miles or less, of magma coming from the mantle. Envision a lava bubble that never broke through the surface layers of the Late Cretaceous rock. They are called stocks. Weathering and erosion eventually exposed the stocks that are igneous mountains you see today in Western Huerfano County.

Mount Maestas is considered a sole injected stock where magma was injected along the plane of a thrust fault between the shale beds of the Late Cretaceous Period, and overlaying conglomerate beds of the older Pennsylvanian-Permian red beds. Think of a cake decorating bag with a rectangular nozzle making flat and smooth frosting. Seen from a distance did you notice how relative level and smooth the slopes of Mt. Maestas are?

Rough Mountain is an elongated stock of magma. Other forces caused the elongation. All the stocks in Huerfano County formed in the Tertiary Period. To continue the view to the Northwest you can see three more igneous mountains. They are Silver Mountain, a stock, Sheep Mountain and Little Sheep Mountain formed by a western moving sill (magma that flows horizontal between rock layers) that was eventually pushed up by a blocking force. There is a small rise about ½ mile northwest of Little Sheep Mountain that is colloquially called The Lamb.

You now have the ingredients for this EarthCache. Let’s open the cookbook.

You learned in previous sections what sediments were deposited by the Western Terminal Seaway. The sediment relevant to this EarthCache is the mudstone/shale named Pierre Shale of the Late Cretaceous. In the Tertiary period, a stock relevant to this EarthCache, rose and was named Rough Mountain. Take the layer of Pierre Shale and spread a thick layer of red hot magma over the Pierre Shale. The baking from the red hot magma gives you a phyllite cake.

Okay, so you want a more technical method. The mudstone over millions of years hardens to a rock layer named the Pierre Shale. Before the Pierre Shale was exposed by erosion magma was injected between the shale beds of the Late Cretaceous Period and overlaying conglomerate beds of the older Pennsylvanian-Permian Period. The magma began to cover the shale layer. At first contact the magma began to heat the Pierre Shale with its thousand degrees of temperature. The heat along with pressure and time changed the characteristics of the Pierre Shale by a process called metamorphosis. This is a change of the rocks chemical and physical properties. Once the magma cooled it became an igneous rock named phyllite. Believe me, the cake is tastier.

Parking information. There is approximately 800 feet of Hwy. US160 that is on public land. The small stretch of public land is about 3 ½ miles traveling eastward from the summit of North La Veta Pass. This area has a small parking area on the south side of the highway and a larger parking area on the north side of the highway. Your only public access to the EarthCache is by parking on public land and starting your hike to the EarthCache from there.

Hiking information. It is best if you have a map that shows the public Bureau of Land Management (BLM) land on Mt. Maestas. The parking area is in the southwest corner of the north half of Section 31, Township 28 South, Range 69 West. You may not want to ascend the steep slope adjacent the parking lot, otherwise you may do the 1-step forward ant 2-steps backward dance. You can find tree covered slopes an either side of the road cut within that 800 foot stretch of road that is public land. You are not allowed to physically cut your way through the gamble oak that is widespread of the south side of Mt. Maestas. Yup, you bull your way through the gamble oak first stripping clothing then skin. Continuing your hike to the notch on BLM land to phyllite layer is rather difficult and tiring. The hike consists of traversing 40 degree to 50 degree slopes with the latter part of the slope curving into the notch that is now gaining in greater degrees of ascent over loose talus.

The phyllite layer in this EarthCache is most likely the youngest layer of phyllite that is easily seen by drivers or hikers than any other phyllite in Colorado. It is an unappreciated gem in Western Colorado's geological box of treasures that Huerfano County is proud to showcase.

There are phyllite layers in the Uncompahgre formation in southwest Colorado and their age is almost a billion years. The phyllite layer in this EarthCache is only a meager 20+ million years old. All the stocks mentioned in this description may have phyllite layers which may not be exposed. Weathering and erosion may have removed the phyllite, or vegetation may cover it. The radial dikes of the Huajatolla spread vertically in the Tertiary through the early Cretaceous and the Pennsylvanian-Permian Period. Yes, dikes more than likely went through the Pierre Shale. Enjoy miles of hiking the base of the exposed dikes through many, many gamble oak groves looking for a phyllite layer.

Nearby is the Raton Mesa formation of basaltic rock from major flows originated in New Mexico that runs from Trinidad (Fisher’s Peak) to southeast Colorado following the Colorado and New Mexico border. This lava flow is less than half the age of the local stocks, and the lava flowed over the relatively flat Pierre Shale that more than likely metamorphosed the shale to phyllite. Enjoy miles of hiking the base of the exposed lava through gamble oak and other trees, brush and grass looking for a phyllite layer. Don’t forget all the eroded debris from the vertical lava flows covering what was the original base of the lava flows.

Eastern Colorado is typically all Late Cretaceous. Look at geological map of Colorado. From east of Pikes Peak and north of Raton Mesa there is no evidence of large igneous uplifts that would have allowed the potential for phyllite.

To log this EarthCache please email to me the answers for the following questions.

1 Pick up a piece of igneous talus and phyllite talus about the size of a softball. There are distinct characteristics on the surface of phyllite talus that the igneous talus does not or cannot have. Only what you can see on the phyllite talus is the distinct characteristics that are not on the igneous rock. Colors, shade of color, texture, minerals or crystals that you can see on the surface of talus are NOT the answer to the question. Rotate the talus to check all surfaces and probably check two or more chunk of talus What are the distinct characteristics that you can see on the phyllite talus?
2 What differences do you see between the talus rock that weathered and eroded off Mt. Maestas, and the talus rock that weathered and eroded off the phyllite layer?