The Rockies, the Alps, the Himalayas -something about these peaks draws us to them．Many of us are simply inspired by their beauty, and some even risk life and limb (生命) to climb them．What enormous forces could have shaped common rocks into such massive mountain ranges? Elsewhere, we can see rocks that Nook twisted and bent．What powerful forces could so dramatically distort such seemingly resistant material, shaking our common belief in the hardness of rock?
Around North America, we can visit young mountains, such as the Cascades of the Pacific Northwest, that continue to rise higher even today．We can also visit older mountains, such as the Appalachians of eastern North America, that may have been much loftier once but have long since reached their maximum heights and begun eroding (腐蚀) away．We can even visit the eroded cores of ancient, now-departed mountains, like those widely exposed in Minnesota, Wisconsin, and northern Michigan and throughout the provinces of eastern Canada.
We can even see future mountains growing．In the rocks along Sagami Bay near Yokohama, Japan, lives a colony of clams (蛤蜊) called Lithophaga, or 'rock eaters'．These creatures scoop (挖) out small shelters for themselves from the soft rocks at sea level, and wait there for high tide to flood their homes and bring their meals of marine algae (海藻)．At the moment of Japan's great earthquake of 1923, the land at Sagami Bay shifted upward, leaving rows of Lithophaga to starve 5 meters (16 feet) above sea level．Even higher rows of abandoned Lithophaga dwellings populate the cliffs at Sagami Bay, including one that correlates with the area's 1703 quake and another that correlates with its earthquake of 818．The rocks adjacent to the bay have risen roughly 15 meters (50 feet) during the past 2000 years.
What is the main idea of the second paragraph?