<P> The park contains a dozen large lakes and 700 smaller ones, but only 131 lakes have been named . Lake McDonald on the western side of the park is the longest at 9.4 miles (15.1 km), the largest in area at 6,823 acres (27.61 km), and the deepest at 464 feet (141 m). Numerous smaller lakes, known as tarns, are located in cirques formed by glacial erosion . Some of these lakes, like Avalanche Lake and Cracker Lake, are colored an opaque turquoise by suspended glacial silt, which also causes a number of streams to run milky white . The lakes of Glacier National Park remain cold year round, with temperatures rarely above 50 ° F (10 ° C) at their surface . Cold water lakes such as these support little plankton growth, ensuring that the lake waters are remarkably clear . The lack of plankton, however, lowers the rate of pollution filtration, so pollutants have a tendency to linger longer . Consequently, the lakes are considered environmental bellwethers as they can be quickly affected by even minor increases in pollutants . </P> <P> Two hundred waterfalls are scattered throughout the park . However, during drier times of the year, many of these are reduced to a trickle . The largest falls include those in the Two Medicine region, McDonald Falls in the McDonald Valley, and Swiftcurrent Falls in the Many Glacier area, which is easily observable and close to the Many Glacier Hotel . One of the tallest waterfalls is Bird Woman Falls, which drops 492 feet (150 m) from a hanging valley beneath the north slope of Mount Oberlin . </P> <P> The rocks found in the park are primarily sedimentary rocks of the Belt Supergroup . They were deposited in shallow seas over 1.6 billion to 800 million years ago . During the formation of the Rocky Mountains 170 million years ago, one region of rocks now known as the Lewis Overthrust was forced eastward 50 miles (80 km). This overthrust was several miles (kilometers) thick and hundreds of miles (kilometers) long . This resulted in older rocks being displaced over newer ones, so the overlying Proterozoic rocks are between 1.4 and 1.5 billion years older than Cretaceous age rocks they now rest on . </P> <P> One of the most dramatic evidences of this overthrust is visible in the form of Chief Mountain, an isolated peak on the edge of the eastern boundary of the park rising 2,500 feet (800 m) above the Great Plains . There are six mountains in the park over 10,000 feet (3,000 m) in elevation, with Mount Cleveland at 10,466 feet (3,190 m) being the tallest . Appropriately named Triple Divide Peak sends waters towards the Pacific Ocean, Hudson Bay, and Gulf of Mexico watersheds . This peak can effectively be considered to be the apex of the North American continent, although the mountain is only 8,020 feet (2,444 m) above sea level . </P>

From what time period are most of the rocks in glacier national park