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Geology
The story of the modern Wash begins tens of millions of years ago as Frenchman's Mountain (the large mountain range south of the smaller Sunrise Mountain) was arriving at the eastern edge of the valley after traveling 40 miles from the edge of the Colorado Plateau. Geologists explain that the mountain block traveled along as the earth's crust was being stretched apart in this region known as the basin and range.
Frenchman's Mountain
The reason that geologists know where the mountain came from is because of a unique rock known as "Vishnu Schist." This rock was formed in the Colorado Plateau and no where else on Earth, so the fact that there is "Vishnu Schist" embedded in Frenchman's Mountain means that the mountain had to come from the Colorado Plateau.
At some time between two and five million years ago, a stream channel formed along the south side of Frenchman's Mountain and connected with the Colorado River. However, as time wore on, the current climatic conditions of the Las Vegas Valley began to take shape. Approximately five thousand years ago, the stream channel along the side of Frenchman's Mountain became a dry desert wash channel that would only flow during rainstorms. This was the condition of the Las Vegas Wash when urban development of the Las Vegas Valley began in the early 1900s.