William Clark's "Pirimids" illustrate erosion and deposition, with columns showing layers of sediment including sandstone, gravel, and larger rocks eroded from ancient hills and deposited in valleys millions of years ago. Time and pressure cemented these sediments into formations hundreds of feet thick, and some of the "pirimids" are topped by harder stone capstones that protected the rock beneath from eroding as quickly as the surrounding formation. Today such formations are called "hoodoos," an African word for a thing or person that brings bad luck. William Clark wrote, "…passed a remarkable rock resembling Pirimids on the Left Side." on August 31, 1805.