<P> River otters may have disappeared from the park in the late 20th century, and muskrats are extremely rare . Beavers cut willows, cottonwoods, and shrubs for food, and can significantly affect the riparian vegetation . Other rodents, such as antelope squirrels and pocket mice, are mostly omnivorous, using many different vegetation types . Grand Canyon bats typically roost in desert uplands, but forage on the abundance of insects along the river and its tributaries . In addition to bats, coyotes, ringtails, and spotted skunks are the most numerous riparian predators and prey on invertebrates, rodents, and reptiles . </P> <P> Raccoons, weasels, bobcats, gray foxes, and mountain lions are also present, but are much more rare . Mule deer and desert bighorn sheep are the ungulates that frequent the river corridor . Since the removal of 500 feral burros in the early 1980s, bighorn sheep numbers have rebounded . Mule deer are generally not permanent residents along the river, but travel down from the rim when food and water resources there become scarce . </P> <P> The insect species commonly found in the river corridor and tributaries are midges, caddis flies, mayflies, stoneflies, black flies, mites, beetles, butterflies, moths, and fire ants . Numerous species of spiders and several species of scorpions including the bark scorpion and the giant desert hairy scorpion inhabit the riparian zone . </P> <P> Eleven aquatic and 26 terrestrial species of mollusks have been identified in and around Grand Canyon National Park . Of the aquatic species, two are bivalves (clams) and nine are gastropods (snails). Twenty - six species of terrestrial gastropods have been identified, primarily land snails and slugs . </P>

Which type of rock is present at the grand canyon national park