In its day, this garden was an engineering marvel. Avery Island is an extruded “bubble” of a salt dome, formed from the salt of an ancient Jurassic sea, leaving a vast bed of salt far below the marshy surface here along the Louisiana coast. Over the millennia, the Mississippi River laid down layer after layer of sediment over this salt bed, the weight of which caused the more buoyant salt to rise up through unconsolidated places in the overlaying sediment – much like toothpaste being squeezed from a tube – thus creating domes of salt below the soft subsurface of the gulf coast.
The Avery Island salt dome has actually broken through the surface of the earth, rising up over 150 feet above sea level. The dome is estimated to be over 26,000 feet deep – nearly equal to the size of Mount Everest.
During the frequent rains here along the Louisiana coast, water runs off of the hills of Avery Island with enough volume and speed to erode the relatively thin layer of topsoil. Sunken Gardens represents McIlhenny’s attempt to slow the rush of rain waters during heavy rain events, and to create a quiet garden in the process.
McIlhenny converted this section, which would have probably been just another dreary gully, into a shady peaceful area highlighting natural foliage that today serves as a pathway to one of the entry points to his old nursery. One can still take a stroll through the site of McIlhenny’s old nursery and observe various camellias and azaleas that have thrived over the years in southern Louisiana’s mild winters and semitropical summers.