In 1847, A.D. Bache set the U.S. Coast Survey markers for the ends of the Dauphin Island Base Line which were part of the EASTERN OBLIQUE ARC survey. One of these granite blocks that marked an end point on the line was found to be out of place in 1883. It now stands in the courtyard of Ft. Gaines, the oldest survey marker on the Gulf Coast, commemorating the work of the 1847 survey but serving no surveying purpose at all. I found the following article in a December 27, 1848 issue of Niles' Weekly Register out of Philadelphia. Back then there was still a 12 foot channel between D.I. and Pelican Island but in 1883 when the U.S. Coast and Geodetic Survey relocated the 1847 marker, PELICAN ISLAND HAD COMPLETELY DISAPPEARED. :

"FORMATION OF ISLANDS.–Prof. A. D. Bache, Superintendent of the Coast Survey, states some highly interesting facts in relation to the islands in Mobile Bay, &c.: Pelican Island in 1822, was 1723 yards long—in 1841 it was 2757, and in 1847 it had increased to 3457 yards, making an increase of 1735 yards in 25 years. The north end of this island has made a few yards further out in 1848. The shore of Dauphin Island, to the northward of Pelican, had cut out a few yards, so keeping the distance between them nearly the same. The distance between the north end of Pelican Island and Dauphin Island, in 1822, was 1957 yards—in 1841, it was 788, and in 1847, it was but 383 yards. The depth through this channel has remained the same since 1822, being 12 feet at mean low water... Sand Island, upon which stands the outer lighthouse, was in 1822 but 131 yards across, in 1841 it was 1542 in length -and in 1847 it had decreased to 908 yards. This Island is constantly undergoing changes, increasing or decreasing as the various causes."
http://www.holoscenes.com/cgi-bin/moin.cgi/EasternObliqueArc