Islands in the Marsh – the Louisiana Cheniere Plain (1 of 3)

sunrise over Cameron Parish

Sunrise over the Hackberry marsh

From Vermilion Bay west to the Sabine River and south of the Intracoastal Water Way to the shores of the Gulf of Mexico is a stretch of land that’s generally more wet than it is solid.  Its residents’ primary occupations are shrimp and oil, it’s home to some nasty populations of biting insects and when the Gulf whips up a particularly nasty storm, the whole region can be completely covered in a salty tidal surge.
It’s not pretty, but it can be beautiful.
The vast majority of land remains untouched; a different kind of big sky country made of a marsh 20 miles tall and 100 miles wide.  It’s home to some unique specialist species and a stopover location for one of the most massive bird migrations on the planet but even in our modern day and age, it’s still a little too wild to be completely tamed by people except upon isolated slivers of high ground.  These landlocked former beach ridges are a fascinating geologic occurrence that makes up the Louisiana Cheniere Plain.


Chenier Plain

What is a cheniere?
I can only imagine the what the first settlers to attempt to land upon the shores in this land thought.  The silty beach dunes on the shore of the Gulf of Mexico are quickly replaced by an expanse of flat, wet, marsh as far as the eye can see.  The only realistic way to penetrate the solid wall of marsh vegetation would be to navigate a labyrinth of tidal creeks and bayous inhabited by alligators, snapping turtles and gar whose monstrous sizes weren’t yet diminished by human hunting.  Even then, more challenging aspects would present themselves in the form of swarms of stinging insects, lack of dry ground and the inescapable heat of the sub-tropical sun beating down in the treeless marsh.

It’s very unlikely people would have ever settled here at all if not for the presence of long, forested ridges that run horizontally across the entire region.  These ridges rise as high as ten feet above surrounding land, almost miraculously, out of the surrounding marsh, can run over a dozen miles long and a mile wide.  This elevation difference makes these ridges a natural trap for fresh water in otherwise brackish marsh and also gives an opportunity for upland plants whose roots would normally drown in the marsh a chance to grow in dry soil.  As a result, large stands of oak trees generally dominate the ridges which is why early French settlers called them “chenieres” which means “place of oaks”.

IMG_0081 - oaks on chenier

Oak Trees on a high chenier ridge in front of Rockefeller Wildlife Refuge in Grand Chenier, LA

Formation
“Cheniere” is a very Louisiana word, which is actually quite appropriate considering what a rare and unique geological feature the Cheniere Plain is.
Geologically speaking, a cheniere is a long, thin ridge of sand that generally runs parallel to the modern day shoreline.  If you were to dig into a ridge and examine the soil, you’d find coarse sand and seashells indicating that this was likely deposited on a seashore.  There’s just one problem… these ridges can be as far as 8 miles inland!
To understand how these features were formed, you have to remember that the shoreline of Louisiana is (geologically speaking) some of the youngest shoreline in North America.  As the Mississippi River system flooded, meandered and flowed into the Gulf, it has brought with it loads of sediments that have steadily been building Louisiana southward.  The river has never stayed in the same place for very long, its delta lobe constantly switching back and forth along with the sediment it dumped.

Mississippi_Delta_Lobes

Mississippi River Delta Lobes (via Wikipedia)

008.09 - Watermarks #38 - Cheniers and Ridges - formation clip

Formation of cheniers (Watermarks #38)

The coastline of the Cheniere Plain about 3,000 years ago was about eight miles inland from its current location. Where the Gulf of Mexico crashed against the shore, it piled sand high into dunes like any beach, but this coast was subject to the waxing and waning loads of sediment carried by the Mississippi River to the east.  At times while the delta lobe was to the south or west of its current position, the sediment load off the Cheniere Plain was high and instead of building dunes, accumulating silts and clay particles settled, advancing a marsh forward from the old coastline.  When the Mississippi River would switch eastward, the sediment load would fall and the Gulf would once again pound the shore into sandy dunes.  As this cycle of high and low sediment load shifted, the landscapes of southern Cameron and Vermilion Parishes as we know them today were formed.

Next time, we’ll set aside the science lesson and go on a road trip around the Cheniere Plain.