most of the burials here in Florida were water burials. In the archaic times they bundeled the dead and staked them to the bottom of the lakes and ponds. The windover site is the most famous one so far. Near Titusville. 8,000 year old burials...... Workers digging an old peat pond to make way for an exit ramp for I-95 disovered bones in the peat and called the cops, cops called the archs and they set up a drainage sytem and found over 100 burial so far, alot more since then, anyways the awesome thing about this site is the preservations of tissue. The peat allowed no oxygen and they found brains in alot of the skulls. They found fabric made from palmetto fibers, 26 strands to the sq.inch. This site is preserved and burials still undiscovered awaiting better technology in the future. Okeechobee had numerous burials exposed in the drought 2 years ago. I know of 3 lakes right now that have them. Our mounds came from the woodland period the safety harbor/weedon island culture/swift creek etc.
http://www.nbbd.com/godo/history/windover/
Most of our mounds have long since been destroyed by development or out of atate Archs and the goodies taken to out of state museums or even overseas museums. We have some of the best wet site archeaology anywhere. The Marco Island sight found by Frank Cushings( I think it was him) in the turn of the century unearthed wooden mask and wooden objects, paddles figurines stools , bowls etc. The masks had paint still on them, but when they were exposed to the air the quickly deteriorated. Check out wet site archeaology by Barbera Purdy.
C.B Moore was the main arch to destroy the state. He had a steam boat called the Gopher and took it up most of the rivers and dug the mess out of alot of mounds, he kept excellent records and his books are a must have for alot of Florida collectors. Copper, gold, mica and other fine materials have all been found in our mounds.Shell death masks or big Busycon shell cups that would have infant remains in them.
Even our shell mounds were quickly destoyed for road fill iin the late 1800`s. The university of Tampa had shell mounds dug up and brought in tp build the foundation of the college on the Hillsborough river. The thinking of the early settlers was that these mounds were natural and couldnt be man made. I check certain shell mound regularly after storms roll in. Some of these mounds were 3 stories high. We even have earthworks here also. Fort Center site in Okeechobee was one, the indians down in Naples dug canals inland from the bay for easier travels to their sites. I could go on and on but hopefully this answered some questions.
oh yeah , as for the depth, it would depend on where it was, you go 10 ft most places and your hitting water. Sand dunes on the east coast are a possibility. Here in the Tampa area we hit hardpan around 5 ft. A few single burials have been found in the past few years discovered during construction and they were only a few feet down.