An intervening period occurred in the Mid Ordovician (about 450 Ma) when a sheet of quartz-rich sand blanketed a large part of the continent after the above-mentioned units were laid down. The sand later hardened into sandstone and later still metamorphosed into the thick Eureka Quartzite. This great white band of Ordovician rock stands out on the summit of Pyramid Peak, near the Racetrack, and high on the east shoulder of Tucki Mountain. No American source is known for the Eureka sand, which once blanketed a belt from California to Alberta. It may have been swept southward by longshore currents from an eroding sandstone terrain in Canada.