The Keystone Thrust And The Polyphase Laramide Orogeny The Keystone Thrust is a famous as geological curiosity which has it's strata turned upside down. Normally we would expect that younger strata are found on top of older strata. The Keystone Thrust defies common sense. The dark red streaks that run through light gray limestone can be seen from the far reaches of the Las Vegas Valley. These streaks or strata are topped by the older limestone's.

As with many of the exposed strata in this region, and much of mountainous Western North America the Keystone Thrust was 'constructed' by a process whereby huge tectonic plates shifted, ground into each other and did all kinds of things to mess up the clean geological sequence. For most of the Paleozoic Era, this part of the Southwest was under a deep ocean. Sediments accumulated over the vast time span resulting in the limestone formations we see today. Around 225 million years ago (MYA), during the Mesozoic Era, the movements of the Earth's crust caused this now deep sea-bed to rise slowly. As the 'plates' which make up the Earth's crust continued to move and the sea-bed continued to rise until it became a set of huge lush plains. Streams criss-crossed these plains eroding them in some places and depositing debris in others. Today we see this debris as petrified logs and other fossilized matter. The 'Chinle' formation is what we call this layer or 'strata'. It is seen at the base of the Wilson Cliffs and elsewhere throughout the Southwest. The tectonic plates upon which the surface of the earth floats on, continued to move, push and pull the western North American continent. By 180 MYA, this area had become a vast red, desert. It continued that way until about 66 MYA. At that time the shifting of the continental plates again began to lift, stretch, compress and pull the Earth's surface. This activity continued in earnest until approximately 23.7 MYA. Since then there has been isolated activity. That and the process of erosion has created the Red  Rock Canyon that we see today. The term 'orogeny' refers to the process of mountain building. The shifting and collision of tectonic plates causes strata to be a) pushed against other strata , causing some of the strata to rise; b) pushed under other strata, causing the top strata to rise, or volcanic activity. In western North America the set of tectonically related events that created much of the topography from Northern Mexico to Alaska is referred to as the 'Laramide'. The effects are visible today from Southern Nevada to the Northern Rockies. The most significant activity occurred in Early Tertiary and late Cretaceous times. The continental plates were basically pushed pulled and shoved around. Once level strata were pushed up making them angular and tilted. At other times the strata were pulled apart causing whole valley's to slowly drop. The result today is the north - south orientation of the mountains and valleys in the Great Basin.

As the various strata were pushed together, then pulled apart and then shoved back together again, we get the Keystone Thrust. Through all this movement over the countless years, the older Paleozoic limestone's which were created from ocean bottoms, was folded over the younger Mesozoic sandstones during the Tertiary or Cenozoic Era. It was originally thought that the Laramide occurred only at the break between the Tertiary and Cretaceous Era's. Now it is considered to have been active at different places at different times since the Jurassic to Oligocene times, 36.6 to 23.7 MYA. Thus the 'Polyphase Laramide Orogeny' is the mountain building, resulting from shifting tectonic plates, that went on in Western North America from Jurassic times to as recently as 23.7 MYA. The result of this symphony of events is the Great Basin topography and places like the Keystone Thrust and Red Rock Canyon.

* This is a mouth full. It might be easier to say it than to explain what it means and how it relates to the Keystone Thrust. How did I do?