Later in Permian time, the Kaibab Sea invaded the land and laid down a limey ooze that later lithified to form the locally up to 200 foot (60 m) thick Kaibab Limestone. This is the same light gray to white formation that rims the Grand Canyon to the southwest (see Geology of the Grand Canyon area). The lower parts of the Kaibab were interbedded with sand and silt before its main component, limestone, was converted into chert-rich dolomite by the intrusion of magnesium. The formation contains fossils of invertebrates including brachiopods, bryozoans, crinoids, gastropods, and pelecypods. Outcrops of the cliff-forming Kaibab in Capitol Reef can only be seen in the deeper canyons located in the westernmost part of the park. Retreat of the Kaibab Sea by the Mid Permian exposed its seabed to erosion, resulting in 100 foot (30 m) deep channels and the creation of a gap in the geologic record called an unconformity.