From the Quaternary Science Reviews, 2012:
Fig. 15. This map locates the sub-surface Walker Interglacial Swamp which is buried some 6 m below street level in downtown Washington D.C., only four blocks north of the White House (from Knox, 1966). Large well-preserved Bald Cypress logs and stumps have been encountered during construction projects, and are believed to have been buried by the Potomac River estuary during warm interglacial conditions and higher sea levels.