From the July 14, 2016, Carolina Weekly News "The Rocks: A 19th Century Civil Engineering Landmark."
Thousands of North Carolinians travel annually past Wilmington, North Carolina, headed for vacation destinations like Carolina Beach, Kure Beach and the Fort Fisher area. Unless they go beyond these as well as the North Carolina Fort Fisher Aquarium to the Fort Fisher-Southport Ferry, they won't know about the huge civil engineering job that has been built at the terminus of US-421 where they will find what is commonly referred to as "The Rocks."
This is essentially what amounts to as a dam built across and closing what was called New Inlet.
The port of Wilmington was extremely important during the Civil War because of the double entrances to its Cape Fear River which proved to be a haven for blockade runners, especially at what was called New Inlet. This was the entrance/exit protected by Fort Fisher.
After the war, the American Society of Civil Engineers (the forerunner of the Army Corps of Engineers) recognized a need to keep the Cape fear River channel open and clear of uncharted sandbars and shoals decided to do something about these.