I am very familar with the Allaire area, but not sure where you were. Was this on the south side of the Manasquan River? If not, where was it in reference to the village and river? It's very likely the dumping spot from an old farm in the area, of which this spot, may be the former boundary of such a farm. Although, oyster shells were often used in the furnaces, they were also eaten...a lot, so it may be just refuse shells.