Pulhamite

Although the house has long since been demolished, much of the original 19th-century Sarisbury Court estate boasted gardens originally laid out by Sir Joseph Paxton, which now,in part form the sprawling local nature reserve and Holy Hill Park, featuring scenic lakes and grottoes, with trails down to the River Hamble.   

Much of the stone work for the grotto's around around the ornamental lake was man made, the work of the Pulham brothers,built in Pulhamite a cement based rock, the recipe for which has disappeared.

 The texture and shape reminds me of that in Central Park New York albeit on a much smaller scale. Apparently the brothers also did work at Buckingham Palace.