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Geography

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Geology

Geological History of the Meduxnekeag watershed

NOTE: The following interpretation is written by a geology STUDENT. There is no guarantee that all of the information is correct and it should not be used for further research or in any scientific setting. 

It is believed that the geology of the Meduxnekeag river is mostly that of stratified calcareous sedimentary rocks, formed during the late Silurian 416-422 Million years ago (Ma). Rock outcrops on the nature preserve show evidence of being deepwater marine clastics formed from the pelagic carbonate factory, intermixed in stratigraphic layers with silt and mud particles. It can be interpreted that the deepwater carbonate sediments from the edge of the Laurentian sea are interlayered with fine grained deposits typical with a low energy deep marine environment. 

 

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How the pelagic factory creates sediment. Source: James (1984)

What Laurentia (modern day North America) looked like during the depositional period.

Source: Torsvik and Cocks (2016)

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The Silurian period saw the introduction of jawed fish and the ozone layer. The oceans were filled with trilobites, coral, stromatoporoids and mollusks. 

Trilobite fossil. Source: Alex Strekeisen (website)

Gastropod fossil. Source: British Geological Survey (website)

The deepwater deposits that will become the outcrops seen today are submerged underground by the Acadian Orogeny (416-359Ma). This supplied the heat and pressure necessary to turn the rocks up on their sides. Today, the outcrops display precise uniformity in their 220/80 strike/dip line. 

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The Appalachians were then uplifted again during the Alleghenian Orogeny that formed Pangea. This uplifting formed the largest peak in the history of the Appalachians, rising about the same height as the alps or rockies (4500m), possibly as tall as the modern day Himalayas (~6000m).

Earth during the Alleghenian orogeny. Source: Miall (2019)

The continents split, the mountains eroded away and the uplifted, tilted rocks on the preserve today are left presenting at surface level after several thousand meters of rock eroded and left the Appalachians at their meager heights of the present day. 

About 100,000 years ago at the tail end of the Pleistocene epoch, the most recent ice age formed glaciers over New Brunswick. They ebbed and waned, but eventually dissipated between 11 and 12,000 years ago (marking the end of Pleistocene). The melting glaciers created a series of outwash channels that acted as a drainage system for the melting glaciers. These channels collect sediments caught in the glaciers and when the water reaches Earth, they are deposited into thin, tall standing ridges called eskers. The melting also reveals drumlins (large, lopsided hills made up glacial sediments pushed into up into a hill). Both drumlins and eskers can be seen across the Meduxnekeag watershed.

Glacial History of the Meduxnekeag watershed

 

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Source: Earth@Home (website)

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Topographic map of the Meduxnekeag watershed. Possible esker/ drumlins along red lines. Source: GeoNB map viewer

The thickest parts of the glaciers reached 2km thick. As they melted, the glaciers freed up huge volumes of water, causing sea levels to rise dramatically and partially flood the land. Sea water rushed into the Saint John River, flooding Fredericton, Woodstock and Hartland. This would have caused the Meduxnekeag river to reverse direction and begin to fill the Meduxnekeag lake in Maine. They would stay flooded for several thousand years before Isostatic rebound (The reaction of the continent weighing less due to the melted glacier, uplifting further out of the mantle) then caused the Saint John to reverse again, dumping sea water back into the Bay of Fundy.

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Shows demonstration of a meandering river similar to the Meduxnekeag. Source: FISRWG (1998)

At this time, the Meduxnekeag was likely a higher energy river with less bends and larger clasts. Since the glaciers settled out, it has turned into a meandering river. The river has gained larger bends over time, as the point bars amass new sediments and the cut banks erode away. This is the cause for oxbow lakes and some of the small islands seen throughout the Meduxnekeag- Eventually, the river finds a more efficient path than the previous sharp turn. Over time, this allows the river to reroot itself in completely new locations on similar floodplains. 

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