#laramide

 Once they’re exposed to the elements, the weaker layers weather away, leaving the stronger layers to stand high as angled hills or even as almost individual layers as seen here. Like many places in western North America, these rocks record sedimentary processes associated with the growth of the Rocky Mountains. The most prominent outcrops in the park are made of 2 formations – the Fountain formation which was deposited between Pennsylvanian and Permian time (~325 to 275 million years ago) and the Lyons Sandstone, deposited during the Permian (~275 to 250 million years ago). The rocks standing the tallest in this photo are made of Lyons Sandstone, a formation deposited in desert-like conditions. That unit contains cross bedding and erosional unconformities as are commonly found in sand dunes and is made of well-cemented, quartz-rich sand. That sandstone is likely about the same age and tells a similar origin story to the Coconino Sandstone near the top of the Grand Canyon; the formation of a giant desert while Pangaea was assembled (http://tmblr.co/Zyv2Js1NYukEi). Before these sandstones were heavily deformed, other sedimentary units were deposited on top of them. A chalky, limestone layer sits directly in contact with those units and then it is followed by an unconformity representing about 50 million years of missing time. That limestone probably represents the intrusion of the actual ocean into this area followed by its retreat and erosion before new sediments began arriving. In the Jurassic and Cretaceous, several additional sedimentary units formed. The Morrison Formation is found throughout the western U.S. and is famed for containing dinosaur fossils – true at this park as well. The Morrison contains sediments eroded from a growing mountain range to the west that were carried by rivers and deposited near the shorelines of lakes and the ocean as it rose onto the continent. Cretaceous-aged sandstones and even a shale sit atop the Morrison – both formed by this same interaction between rising sea levels and sediments shed from the growing mountains. The sandstone represents sediments close to a beach and the shale represents deeper, quiet water; sediments deposited when the Western Interior Seaway covered much of the Great Plains. Finally, to form these hogbacks, the rocks had to be tilted. The earlier pulse of mountain building in the Rocky Mountains happened to the west of this site. It was known as the Sevier Orogeny and it formed stacks of faulted rocks similar to those seen in mountain ranges like the Andes today. The deformation during that mountain-building event stayed fairly close to the Western U.S. coastline for tens of millions of years, building high mountains but not extending into the continent. That deformation pattern changed during the Cretaceous, when an event called the Laramide Orogeny occurred. The Laramide and the Sevier orogenies represent 2 distinct styles of mountain building in the Western U.S. The Sevier Orogeny involves faults forming in sedimentary rocks, building a mountain range style that we call a fold and thrust belt, with lots of folded and faulted sedimentary rocks. The Laramide was different. During the Laramide Orogeny, some of the deepest rocks in the continent; the ancient, crystalline granites and metamorphic gneiss basement rocks were faulted. These faults extended much deeper than those of the Sevier Orogeny and they also moved much farther inland. For example, the Sevier Orogeny faults can be found in areas like Idaho and Western Montana while the Laramide Orogeny extended all the way to the Black Hills in South Dakota, about 500 kilometers to the East. Both of these mountain building pulses were driven by subduction of an oceanic plate called the Farallon plate off the western coast of North America. The same subduction zone also caused the formation of the large igneous rock provinces in the Sierra Nevada Mountains, Idaho, and even Western Colorado. Although they had the same cause, something different must have happened between these 2 mountain building events. Some geologists believe the Farallon plate became shallow as North America approached a ridge, causing the Farallon plate to push on the bottom of the continent and creating the Laramide Orogeny. Others believe the Laramide Orogeny occurred when part of a large oceanic plateau was subducted beneath North America; pieces of that plateau could have been too buoyant to sink into the mantle and similarly pushed upward on the bottom of the crust, creating the Laramide faults. During the Laramide, crystalline basement rocks like those found on Pike’s Peak to the west were pushed upwards. As those rocks moved, the sedimentary layers sitting on top of them were twisted and bent upwards as well. Flat-lying sedimentary rocks that once sat on top of these deeply buried granites were folded and tilted, in some cases to nearly vertical. That’s how these rocks acquired the tilt they see today. Finally, once they were exposed at the surface they were exposed to weathering. Weathering by rain and wind will often grind away at fine-grained, weak rocks like shales while sandstones tend to stand taller. Sandstones are mostly composed of the mineral quartz, which is very hard and does not erode easily, and the grains of those sandstones are often cemented by that same quartz, giving those layers extra strength not found in the limestones or shales. Thus, coarse grained sandstones stand tall in this park and create many of the hogback features that led to it being designated a National Landmark. -JBB Image credit: Dixie Lawrence https://flic.kr/p/6BeES3 References/Read more: Geologic map: http://bit.ly/1G5Ky6G http://bit.ly/1He9Fbn http://bit.ly/1UxwUnV http://bit.ly/1IHzx1V http://bit.ly/1JSsBO5 http://abt.cm/1J2UhNc