LAS VEGAS (AP) - A prominent sandstone arch at Valley of Fire State Park in southern Nevada has collapsed.Park rangers said it appears Natural Arch was claimed by forces that will eventually destroy about 300 others in the park: gravity and erosion.They said horseback riders notified them about the damage Wednesday, and no one has reported seeing it fall. While it's unclear exactly why and when the arch collapsed, there's no evidence of vandalism, rangers added."Maybe someone tried to take a picture on the rock, which we don't recommend, but there's nothing here that proves this was done on purpose," …