flanks rise to join the mountains. Based upon rugged folds of sedimentary rocks, rock beds, glaciation, and lava flows, the erosion of innumerable streams has made it relatively uniform. At its southern end are extended plains, reaching almost to the Columbia River. Puget Sound, for which the basin is the trough, is 80 miles long, 8 miles wide at the broadest point, and has depths of 900 feet: a body of water flanked by forested bluffs and low dikelands, with extensive bays, inlets, and passages between the 300 islands that lie within its shores. Of these, the 172 inhabitable islands of the San Juan group (see Island Tour 3) and Whidbey Island, second largest in continental United States, are most noteworthy.
Extending across the State from north to south at its approximate middle longitude is the great barrier of the Cascade Mountains-shaped somewhat like an hour-glass-a range, 100 miles wide at the Canadian and Oregon boundaries, and 50 miles at its middle. The numerous peaks average from 6,000 to 8,000 feet in elevation, while the volcanic cones of Mounts Rainier, St. Helens, Baker, Adams, and Glacier Peak rise much higher. Of these peaks, only one, Mount Adams, is on the range axis; the others, St. Helens, Baker, Glacier Peak, and Rainier are on the western flank. Because of their origin, the northern and southern Cascades are quite dissimilar. The rugged southern portion resulted from great igneous activity due to volcanoes, while the northern portion, seemingly a great raised plateau at one time, is more uniform; whatever ruggedness it possesses has resulted from erosion rather than from volcanic action. The streams of the range, as a whole, are strong and deeply bedded. Mountain valleys, once the beds of great glaciers, have been deeply eroded-leaving grand cirques and amphitheaters, such as the water-filled gorge of Lake Chelan, that are among the great attractions of the range. Only the Columbia River crosses the Cascade Mountains. Three tunnels—the Cascade, the Rockdale, and the Stampede—pierce the Cascades for rail transportation; and the Chinook, Stevens, and Snoqualmie passes make them surmountable by highways.
North of the "Big Bend" of the Columbia River and north of the Spokane River, and merging into the Cascades on the west and the Rockies on the east, are the Okanogan Highlands: beautifully rounded, broad, low hills sloping gently from watersheds to the river beds, with divides-often 6,000 feet in elevation-never sharp or abrupt. The Highlands, unlike the heavily wooded Cascades, are largely open and park-like, with a minimum of undergrowth.
South of the Okanogan Highlands almost to the Oregon boundary
