The other part of Laurasia, North America, is narrower than its sibling, Eurasia, although it harbors the same range of climates, from rainforest to windswept desert to sprawling grassland. Two great mountain ranges flank the coasts of the continent---the ancient, weathered Appalachians to the East and the youthful, craggy Rockies to the West. Between these mountains was once solid forests (punctuated here and there by rain-shadow deserts) but the recent drying and cooling of the world has resulted in vast grasslands that stretch from the heart of Canada to the Gulf of Mexico. The richest land is to be found in central and southern North America, along the banks of the continent’s largest river, the Mississippi, where soil was dropped by melting glaciers at the end of the last the Ice Age.