Flying in a Cessna airplane near Pass a Loutre was the first time I spotted one of these geologic absurdities. At first glance I thought the round and gray shape in the marsh was the lid of an old galvanized tank displaced from an oil and gas facility by a recent hurricane. 

I swooped down to a lower altitude to get a better look where I noticed a chimney shape that must have been nearly five feet tall with mud and water bubbling out the top. The ooze was startling. I thought I may have stumbled onto a leaky pipeline or a long forgotten facility deep in the marsh. 

Another few passes and my memory was jogged. I had read about mud lumps and their associated phenomenon at the mouth bars off the Mississippi River Delta. These geologic absurdities have been rare in recent decades. Mud lumps are formations of clay that pop up from below, a process caused by the enormous weight of new delta deposits settling atop more plastic clays.

The buried strata of clay were pushed to the surface like a hernia, overnight in some cases. These new islands varied in size and speed, with some popping up overnight and others appearing over a number of months but nearly all breaking the surface during high water season when new deposits are made. One old story shared had goats living on a particularly large one in Dixon Bay. 

Belching cool liquid mud squeezed out from deep below the Mississippi River Delta, the mud volcanos and vents look more like a geologic process from Yellowstone or Iceland than a river delta. This video shows one in action. “There are similar structures and systems to the mud volcanos in other places on the planet.” says Dr. Alex Kolker, a scientist specializing in coastal issues, “That said, these mud volcanos are very rare in the US, and this is certainly one of the few places that an American could see them.”

While the little mud volcano I stumbled upon isn’t a mud lump, the two are related. The Mississippi River Delta can sometimes seem geologically simple. Sand and clays flows downstream and stacks up when it slows down. These little mud volcanos were something different than normal, surface level deposits. However, the mud lumps are a unique occurrence here at the end of the Mississippi River. They’re so novel and unique, that they only occur at the end of certain passes (outlets) of the Mississippi River and found nowhere else on the planet. Pass a Loutre, Baptiste Collette, South Pass, and Southwest Pass all had mud lumps and the associated vents at one point in history. 

The observation of mud lumps are becoming more rare as sediment loads in the Mississippi River Delta have decreased and river management pushes the sediment load off the continental shelf instead of into the marshes and mouth bars.

Still, they pop up from time to time. I was able to observe one from an airplane off Southeast Pass just a few moths ago but did not want to risk flying far from shore near dark. What I could see from the Cessna were waves breaking on black earth nearly half way between the South Pass 70 platforms and Southeast Pass. The location was so far from shore I though I may have sighted whales, there was no way it was a simple sandbar. When I tried to investigate by boat nothing was found on site. A few storms later and it was gone.

Quite a few studies of the mud lumps were accomplished in the 20th century when there was an economic need for understanding the phenomenon due to the blocked shipping channels. Those first hand accounts are incredible. One ship became stuck, even after the pilot just traveled downstream in the same spot. After waiting overnight for tugs to arrive the next morning the bow was well out of the water.

You can visit the Mouth of the Mississippi River on one of our trips to see the places these mud lumps used to pop up. If you’re luck you may see one too.

-Richie