Sea levels in Nova Scotia and southern New Brunswick during the past 10,000 years. Deepest past at the top, the present at the bottom,

The major tale of the late Pliestocene has been the subsidence and uplift of land coupled with its submergence by waters from the Atlantic Ocean. Sea-level history is complicated because it involves not only world-wide changes in the level of the ocean due to melting ice sheets, but also more local effects. These include land-level changes produced by the down warping of the earth's crust under local ice sheets and the rebound of the land after melting. There were more extreme changes involving sea level during the earlier Tertiary Period when the land rose and subsided through a distance of several hundred metres but the smaller changes following glaciation were nevertheless worthy of note. The effect of vertical movements on the sides of existing faults has only lately been factored into the equation of sea level change. In the Bay of Fundy there has also been an increase in the range of the tides since Wisconsinan glaciation.

Land-level adjustments occurred soon after the last Ice Age during the Pleistocene, from about 14,000 to 10,000 years ago, and the Holocene, which represents the past 10,000 years. Global sea-level changes caused the major effect, but there is evidence that local earthquake activity and distortions of the land surface that are not accompanied by large earthquakes may continue to warp the coast . Studies, involving radiocarbon dating suggest that world-wide (eustatic) sea level was some 110 meters (360 feet) below the present level about 14,000 years before present. Because the weight of local ice sheets, which were at least 3300 feet thick in Maine and southern New Brunswick, had bowed down the Earth's crust, the level of the coast was then below local sea level. Eustatic sea level was low, but rising at this time. Thus,relative sea level was about 230 feet. above the present base line. As the ice retreated inland, the land rebounded, but the sea invaded in contact with the retreating ice front, creating a vast inland sea, the DeGeer Sea, in the depression which now contains the Gulf of Maine. This sea deposited the Presumpscot Formation (orange on above surficial map of Maine) , a distinctive bluish-gray marine mud, which now is found in many parts of the Passamaquoddy region

 

 

New Brunswick Nova Scotia and Prince Edward Island at the extreme high stand of sea level;. The DeGeer Sea reached more than 60 miles inland as far as Millinocket and Bingham Maine and up the St. John River well beyond Fredericton. Exposed deltas and shorelines are preserved above present sea level in interior all of the Gulf surround. We know the timing from radiocarbon dates on fossil shells and wood preserved in Presumpscot Formation sediments. There are raised beaches throughout the Fundy surround. There are some within the Minas Basin at up to 40 metres and one at Cape Split, measuring 25 metres vertical height above sea level. Others may be found in the Annapolis Basin and at St, Marys Bay measuring 6, 16 and 29 metres. D.R. Grant thinks most of the flooding was caused by land subsidence rather than sea level rise and notes that inundation was greatest around the Truro-Moncton region in the above map area. It was zero at St Andre in Quebec, 15 metres in northern New Brunswick, 30 metres at Sackville and 20 metres at Halifax. These differences are presumed caused by the fact that the area was not as deeply depressed to begin with or was slower in recovering as the ice disappeared.

The release of glacial weight eventually allowed rebound of the land (isostatic rebound) to out pace the rate of global sea-level rise. Thus, local relative sea level fell, at rates up to 1.7 inches per year, until it reached a local low stand 180 feet. below present sea level. This caused the coastline to move well offshore as evidenced by terraces, low stand deltas, and land erosional features and river deposition on the inner continental shelves. After this low stand, global sea-level rise rates again became important as the rate of isostatic rebound decreased. Local relative sea-level rose at rates up to 0.9 inches per year, until the rate of rise slowed again about 9000 years before the present.. This may have related to large-scale up warping of the crust at a considerable distance from the ice sheet. Sea-level rise resumed, and the Gulf of Maine attained its present level over the past 5000 years.

The balance of eustatic and isostatic changes at 16,000 years was such that Halifax Harbour was 40 metres lower than today. The situation was more extreme by 11,500 years, when the difference was 65 metres. Bedford Basin at the head of Halifax Harbour was a fresh-water lake until 9,000 years ago. The Reversing Falls at Saint John had no inflow until after that time and Prince Edward Island was cut off by rising sea levels a mere 7,000 years past. An eighteenth-century road across the Tantramar marshes is now well below the sea In the Gulf of Saint Lawrence. the 2 millimetre rise in sea level each year has created erosion of about a half metre of coastal Prince Edward Island each and every year.