Grande-Île, the main island, is long and wide at its widest (approximately ), though this is just the tip of a substantial and complex archipelago which is exposed at low tide. The archipelago comprises 365 islands at low tide, compared to only 52 islands at high tide. From a few dozen hectares of ground above the high tide line, the archipelago increases to around at low water, within an area roughly . The tidal range is one of the largest in Europe, with up to difference between low and high tide. The islands consist of a granitic geological formation, which has been subjected to erosion by sea and wind. Sandbars connect several parts of Chausey.