We addressed your question to Lewis Kiss, a geologist and collections coordinator with the Las Vegas Natural History Museum. "Geostatic pressure pushes [mountains] up and there is going to be a valley between them," he told us. "Even if there was not a river to carve it out of the mountains, there is still an incline."
"'Valley,' he says, "is a scientific term used in geomorphology," the science which studies geographic shapes and how they are formed. "A valley is a depression in the earth's crust that is longer than it is wide," not necessarily formed by rivers. The Las Vegas Valley was formed 25 million to 30 million years ago, Kiss says, when the earth cracked east to west, causing its crust to sink. Its origin has nothing to do with water. "The valley is a technical term used in all geosciences and technical sciences." It’s also part of geomorphology, which Kiss further describes as "what you see when you travel … the current status of the struggle between the earth’s internal and external forces."
Valleys formed by rivers tend to have a deeper, V-shape. "When it turns into a flat-floored valley, that’s the most mature state of a valley’s life," Kiss says. He adds that they can happen anywhere, even undersea (where there are no rivers), caused by tectonic shifts – as was the case with the Las Vegas Valley.
