The earthwork remains of four large irregular mounds seen within the marshes of North Level on the Isle of Grain. These are thought to be the remains of a medieval saltern mound and was mapped from aerial photographs as part of the English Heritage: Hoo Peninsula Landscape Project. The remains of Rosecourt Barn is located on the eastern most of the four mounds.These mounds are the result of large-scale salt manufacturing where brine was extracted from salt-rich sands and sediments, concentrated and evaporated using process known as sleeching. The discarded waste material from the process built up around the production area into a sizeable mound, often with a hollow in the centred where a hut stood. These medieval saltern mounds are typically described as floriate in form because of their irregular lobed formation of dumped waste. They often occur in clusters around former and surviving tidal water-courses within the marsh. There has been considerable reclamation and subsequent sea wall construction since the medieval period which has isolated these sites from the sea. Many of these mounds were subsequently utilised as sheepfolds, sheep washes and stock refuges in the post medieval period because of their slightly elevated position in the readily flooded marshes. Roman finds have been extracted from some of these sites, but are believed to be from the lower levels and not associated with the mounds. Post-Roman flooding and silt deposition has resulted in Roman sites lying typically several feet below the current land surface.