Between 250 and 300 million years ago, freshwater streams flowing from ancient mountains brought silt, sand and gravel into what is now the Gulf of St. Lawrence. These sediments accumulated to form a sedimentary basin, and make up the island's bedrock. When the Pleistocene glaciers receded about 15,000 years ago, glacial debris such as till were left behind to cover most of the area that would become the island. This area was connected to the mainland by a strip of land, but when ocean levels rose as the glaciers melted, this land strip was flooded, forming the island. As the land rebounded from the weight of the ice, the island rose up to elevate it farther from the surrounding water. Most of the bedrock in Prince Edward Island is composed of red sandstone, part of the Permianage Pictou Group. Although commercial deposits of minerals have not been found, exploration in the 1940s for natural gas beneath the northeastern end of the province resulted in the discovery of an undisclosed quantity of gas. The Island was reported by government to have only 0.08 tcf of "technically recoverable" natural gas. Twenty exploration wells for hydrocarbon resources have been drilled on Prince Edward Island and offshore. The first reported well was Hillsborough No.#1, drilled in Charlottetown Harbour in 1944 (the world's first offshore well), and the most recent was New Harmony No.#1 in 2007. Since the resurgence of exploration in the mid-1990s, all wells that have shown promising gas deposits have been stimulated through hydraulic fracture or “fracking”. All oil and natural gas exploration and exploitation activities on the Island are governed by the "Oil and Natural Gas Act" R.S.P.E.I. 1988, Cap. 0-5 and its associated regulations and orders. The Province of Prince Edward Island is completely dependent on groundwater for its source of drinking water, with approximately 305 high capacity wells in use as of December 2018. As groundwater flows through an aquifer, it is naturally filtered. The water for the city of Charlottetown is extracted from thirteen wells in three wellfields and distributed to customers. The water removed is replenished by precipitation. Infrastructure in Charlottetown that was installed in 1888 is still in existence. With the age of the system in the older part of Charlottetown, concern has been raised regarding lead pipes. The Utility has been working with its residents on a lead-replacement program. A plebiscite in 1967 was held in Charlottetown over fluoridation, and residents voted in favour.