In looking at areas like Monument Valley, I am struck by the height and areas of some of the great mesas (flat topped rock formations). I understand that they were formed by erosion , by water. The mesas formed because tey were covered by a hard rock shell, while the areas around were of softer rock (and hence eroded away). Given that the Southwest has been a desert for 100’s of thousands of years, could these mesas be incredibly old? Perhaps billions of years?
OK, caffeine is starting to take effect.
It does rain in the southwestern deserts. People drown in the deserts. I used to live in the Mojave Desert, where I learned to fly. At a few thousand feet it’s easy to see where the water has been. (And is. Edwards Air Force Base’s lake beds are flooded annually.) In addition to the admittedly sparse (but still present) rain, water comes from miles away in the mountains. By the time it gets to the valley it can be moving at quite a clip. Flash floods. Of course water seeks the lowest point; so the washes are ‘reused’ year after year, eventually forming gullies. Given enough time, these can form canyons. And it’s not just water moving downhill. The water carries sand, soil, rocks, and so forth that are abrasive. Wind also has it effects, blowing abrasive sand in dry times.
IANA geologist, but this is how I understand the process.
Johnny L.A. makes an excellent point. While the Monument Valley area is arid, there is still precipitation. A quick search found estimates that range from 5-10"/year. One citation is here: http://pubs.usgs.gov/wri/wri994074/PDF/WRI99-4074plate01.pdf. So, there is still water to carry on erosion. Also, freeze-thaw cycles during the winter can cause significant amounts of erosion without a whole lot of water being involved.
Cool animated maps here showing relative changes from dry to wet since 1750. Even over this relatively short period, precipitation varies quite dramatically. .Paleoclimatology | National Centers for Environmental Information (NCEI)
Another line of evidence is the age of the rocks exposed in the mesas and buttes. The oldest rocks exposed in the area belong to the Cutler Formation, which is Lower Permian (Wolfcampian) in age Geolex — Cutler, or somewhat less than 300 million years old. The mesas you see cannot be billions of years old because the rocks they are made of are younger than that.