BRETON BAY

GEOLOGY

MARYLAND U.S.A

 

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LOWLAND DEPOSITS

Breton Bay lies within Pleistocene/Holocene (quaternary) Lowland Deposits consisting of Gravel, Sand, Silt, and clay. The base layers are medium/coarse-grained sand and gravel, along with cobbles and boulders. The whole 'formation' is 0-150 feet (0-45 meters) in thickness and is known to form cliffs along waterways. 

 

This 'formation' commonly contains reworked Eocene (56-33 million-year-old) glauconite which is an iron/potassium phyllosilicate associated with mica-based minerals, it is usually green in colour and has a very low weathering resistance hence why it's reworked.

 

Lignite Silty Clay and Siltstone come in various colours, the clay is most commonly visible in orange/rust and dark grey to grey colouring.

 

Clay in the cliffs of Breton Bay sometimes resembles lignite which is the lowest form of coal sometimes referred to as brown coal, its peat (partially carbonized plant mass) which has turned to rock and has a carbon content of 60-70%. 

Sometimes fossils deposited in an estuary (where a river meets the ocean) or marine setting are uncovered through the erosion of cliff structures along the bay. 

 

Images (Left): siltstone, (Middle, Right): Clay

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LANDSLIDES

With the amount of rain falling some of the clay/siltstone cliffs lining Breton Bay eventually failed, these sediments are very young and make up the mostly Holocene section of the Atlantic Coastal Plain Provance in Breton Bay. 

  

Since a lot of Breton Bay is comprised of clay, the added floodwaters tuned the oversaturated sediment into a slurry as clay spreads evenly through water. Cliff faces collapsed as the clay slid out of place and down towards the water's edge, this undercut parts of the above topsoil which in turn broke loose and fell towards the bay. 

 

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SINKHOLES

Tropical Storm Isaias triggered moderate to severe flooding events through Southern Maryland. The soil was already high in moisture as a strong line of storms crossed the basin several hours before at 3:20 pm. Isaias reached Southern Maryland at around 1:00 am, in the beginning, light rain was falling but the steady increase added to the unavoidable flooding risk. 

 

All the rainwater rapidly entered the unconfined aquifers below St. Mary's County, the water table was then forced to rise along with it.  

The below images represent surface water flowing downhill through a small lake and out into Breton Bay through a stormwater drainage pipe, this took lots of sediment out with it. This repeated along other properties facing the bay until the entirety of Breton Bay was the orange/brown colour of the eroding Lowland and Inland Deposits.  

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GROUNDWATEr

Groundwater moves in certain patterns underground and after a major rain event, there is a lot of it. With some layers of soil and rock beneath the surface being more porous than others rainwater can move through it. The movement is not completely straight down but horizontally as well since the water moves through permeable layers.

In Breton Bay, this is visible as very small streams were conjoining in the talus slope created by the landslides. patterns in the beach sand also show signs of water flow from the porous rock layers exposed by Tropical Storm Isaias.  

  

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REFEreNCES

Maryland Geologic Survey. St. Mary's County 1968. Geologic maps of Maryland.

Sinkholes. (n.d.). USGS. 

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