The ice age
During the last 2.5 million years, dramatic, quick changes in the global climate have taken place. Because of these rapid climate changes, large glaciers (ice sheets) of continental proportions repeatedly expanded and contracted across northern regions of North America, Europe and Asia. Glaciers form over time when snow continues to accumulate and gradually turns into ice. The ice mass then begins to flow outward and downward. Dating back to the Pre-Cambrian era, 600 million years ago, ice ages have occurred over geologic time at intervals of approximately 200 million years. Ice ages can last for millions of years, and are times in history where the entire Earth experiences dramatically colder climate conditions. Many people consider the most recent period of glaciation as the "Ice Age," which was at its height 20,000 years ago.
Walrus in the sea, Phippsoya 1996. Photo courtesy of Ólafur Ingólfsson Research indicates that the exact causes of ice ages and the glacial cycles that take place within them remain mysteries. They are most likely the result of a complicated dynamic interaction between solar output, distance from the Earth to the sun, position and height of the continents, ocean circulation and the composition of the atmosphere. During each glacial episode, sea level dropped due to great volumes of water being tied up in ice sheets. The weight of the thick continental ice sheets was so enormous that the underlying earth's surface (the crust) was depressed and later, after the ice melted, rose back to its former position. Due to the fluctuating land and sea levels, flooding occurred, which caused the submergence of many coastal areas, including southern Maine.