The Ordovician and Silurian periods do not have geological evidence in the Grand Canyon, leading to a disconformity. This 65 million year gap formed deep rivers in the Muav Limestone that were later filled with freshwater limestone which is today called the Temple Butte Limestone. This layer forms cliffs, is grey to cream colored, and has many fossils, including backbones and freshwater fish plates. Another unconformity is found on top of this layer. The next section, the Redwall Limestone is dark brown to bluish grey limestone and dolomite and has white chert (a primarily silica sediment) nodules in it. This formed from a recession of a sea in the area in the early Mississippian period. Simple to complex marine fossils have been found here. The entire region was uplifted in the late Mississippian period, leaving this section to be eroded away. The color mainly comes from red beds (red colored sedimentary layers) filled with iron in the above layers that then drip down onto the limestone from rain. Karst topography, or the dissolution of soluble rocks, occurs in the Redwall Limestone. Caves, sinkholes, and underground drainage systems are formed this way. As these grew and collapsed, they were filled with the Surprise Canyon Formation: a conglomerate, sandstone, limestone, and dark purple siltstone formation.