Many of the large evergreen trees in the gorges of Mohican State Park and Mohican Memorial State Forest are eastern hemlock (Tsuga canadensis), a foundation species that flourishes in Mohican's cool gorges and shapes the environment for other species through dense shade that moderates air and stream temperatures and through slowly decomposing needles and wood that create a thick organic layer on the forest floor. Eastern hemlock forests provide critical breeding and foraging habitat for many wildlife species, including some that are rare or threatened. Hemlocks can live more than 800 years under ideal conditions, and stands 300 to 400 years old are not uncommon, though most existing hemlocks here are much younger because of heavy timber harvesting after European settlement. These trees are now threatened by the hemlock woolly adelgid (HWA), a small invasive sap-sucking insect that has already caused widespread hemlock decline and mortality in the eastern United States, and the Ohio Division of Forestry actively manages infestations statewide through an integrated pest management program to slow its spread and protect high-priority hemlock forests. The Mohican and Clear Fork Rivers flow through forested valleys with exceptionally clean waters, and the Mohican River, together with the lower portion of Clear Fork, was designated a State Scenic River in 2006. Mohican State Park's terrain of sandstone, shale, and conglomerate formed 350 million years ago when this land lay south of the equator beneath a shallow sea, and riverbanks downstream from the covered bridge contain fossilized remains of ancient sea life such as gastropods, brachiopods, and trilobites.