There's so many of them too! Wandered about 100m past one of my backcountry campsites last year and found one hiding. I honestly can't answer the geology question well, I know the arches are all sandstone and there's plenty of limestone here but my hasty search wasn't able to turn up much about the geologic history. Think it was once an ancient sea shore (bed?), so there's got to be plenty of fossils but I don't recall ever hearing much about them. This area has been inhabited since prehistoric times though, lots of artifacts and such for the archeologists. It was something of a pastime in the area to go down under the cliffs along the rivers and sift for arrowheads and other relics.