THE ROCKY MOUNTAINS are a chain of mountain ranges 3,000 mi (4,800 km) long and as wide as 350 mi (563 km) running predominantly north to south in the western part of the North American continent. The Rockies run through the U.S. states of New Mexico, Colorado, Utah, Wyoming, Idaho, Montana and Alaska, as well as the Canadian provinces of Alberta and British Columbia and the Yukon and Northwest Territories. They have been described poetically as the continent’s spine. The Rockies were originally formed approximately 100 million years ago, during the Cretaceous period, when dinosaurs still walked the Earth. There was a second period of uplift within the past 25 million years, and Pleistocene glaciers further reshaped many of the major valleys, widening and rounding their bottoms. Some small glaciers have survived high in the northern Rockies, remnants of the great Ice Age. The best known may be found in Glacier National Park in Montana on the Canadian border. Recently there has been concern that global warming is causing these glaciers to shrink and some of the smaller mountain glaciers may disappear altogether. The Rocky Mountains are rich in wildlife to match the rugged scenery. Piñon pine and juniper are common in the southern Rockies, while they give way to firs, pines, and spruces further north. Beyond the timber line (the altitude above which trees cannot grow), mountain goats and bighorn sheep browse in alpine meadows. The forests downslope are home to bears, deer, elk, hares, minks, cougars, porcupines, squirrels, and other woodland creatures. The rivers of the region are rich with fish such as rainbow trout and grayling. Within the Rocky Mountains lies the Continental Divide, which separates the waters that flow to the Pacific Ocean from those which will flow into the Atlantic. The Rocky Mountains contain the headwaters of such rivers as the Arkansas and Missouri (tributaries of the Mississippi), the Rio Grande (which forms the border between Texas and Mexico), the Colorado (which formed the Grand Canyon and ultimately flows into the Gulf of Cortez) and the Columbia (principal river of the U.S. northwest).