Cape Cod has a landscape dimpled with mostly nearly round holes, many of them filled with water, that geologists call kettles. These depressions formed more than 18,000 years ago when the gigantic Ice Age glaciers that covered this region began to melt. Salt Pond is unusual among the kettles of the outer Cape because the ocean has captured it through a tidal channel from Nauset Marsh that breached the opposite side of the kettle, allowing salt water to enter, and its water levels rise and fall twice daily with the tides.