Star-trails over sandstone hoodoos, time-exposure image. Star trails are formed by the apparent motion of the stars in the sky as the Earth rotates on its axis. Here, in the northern hemisphere, the star trails form concentric circles around the north celestial pole. Hoodoos are a type of rock formation that form from the erosion of sedimentary rock. Photographed in the sacred Blackfeet tribal area of Writing-On-Stone Provincial Park, Alberta, Canada.