A river system then migrated northward and transitioned into either a large lake or marsh. As this occurred, bentonite-rich clay (formed in part from volcanic ash erupted from nearby volcanoes) and clayey cross-bedded sand with some interbeds and lenses of lime ooze were laid down. This eventually became the light purplish-gray claystone, sandstone, and carbonate rock of the Chinle's Monitor Butte Member. Burrows that measure 5 inches (13 cm) in diameter and 5 feet (1.5 m) long were excavated by lungfish into the Monitor Butte. This member easily stands out in the lower to middle slopes of the Waterpocket Fold's west face in such places as The Castle and along the cliff's just north of State Route 24 as it enters the park.