Much of Ethiopia is a vast hilly plateau of eroded volcanic rock, rising occasionally to groups of higher mountains, some with snow-capped peaks. The plateau extends to the Red Sea in the North, and is surrounded on all other sides by desert. The plateau is broken into two sections by the Great Rift Valley, which cuts southwest across the entire width of the country from the Red Sea to the Lake Region of Kenya. The highest mountains are in the northern section, spread across an area 400 miles wide, rising to over 15,000 feet in the Simen Mountains. From the surrounding desert, the outer escarpments of the plateau rise abruptly. Densely forested river gorges tumble over sheer cliffs and steep terraces thousands of feet high. Since so much of Ethiopia is mountainous, it is not surprising that the main population centers, such as the capital, Addis Ababa, are at elevations between 5,500 and 8,000 feet.