Instead of saying “ice age,” scientists now use the words “glacial period,” because they understand that the coming of glacier ice to cover much of the planet is a regular cycle, not a one-time event. Our current 10,000-year period of relatively good weather is an interglacial period. Based on the movement of the earth in space, the shortest glacial cycle lasts 21,000 years, so we’re about halfway to the next glacial period. When that period comes, colder temperatures and increased snow could bring glaciers back across North America.
The granite mountains of Acadia National Park were once taller and connected to each other, before glaciers a mile thick piled up behind and plowed over them. Signs of this grinding collision show up all over the landscape. The bald granite mountaintops were cut from bedrock, which has not gathered enough soil since then to fully cover the peaks with trees and plants. The mile-thick ice layer pushing across the rock also explains the mountains’ rounded shape. On top of where you can drive or hike, you can see the scratch marks that the glacier left on the rock.
A huge boulder brought from about 20 miles north sits where a glacier left it upon melting (such a boulder is called a “glacial erratic”). Between places like Cadillac Mountain, with the hardest rock, the glaciers carved out channels of softer rock where they could flow through more easily. These became the valleys that now hold Eagle Lake, Jordan Pond, Echo Lake, and Long Pond—notice on the Acadia map how all of these point north and south, the direction the glacier flowed. The largest of the valleys that the glacier gouged became Somes Sound, the long, narrow bay that splits Mount Desert Island in half. It’s the only true fiord, or glacier-carved bay, on the East Coast of the United States.
The rock that the glaciers ground away from Mount Desert Island ended up far to the south, at the southern end of the glacier. That pile of rock now is known as the Georges Banks. The world’s sea level was about 400 feet lower back when the glaciers were here, because so much water was frozen in glacier ice. Today the Georges Banks are an underwater shoal and fishing grounds.