The pond was lined with insulated board, partly to help with heat retention if we cover in winter and also to give a smooth finish to the walls.
The slightly unusual shape is more obvious now, This was a result of extending the existing pond without demolishing the entire structure, and also to keep the excavation away from the roots of an established magnolia grandiflora on the the left hand side.
The final finish will be black, but that wont be until later today.
Gaz