Rubble masonry or rubble stone is rough, uneven building stone not laid in regular courses.[1][2] It may fill the core of a wall which is faced with unit masonry such as brick or ashlar. Some medieval cathedral walls have outer shells of ashlar with an inner backfill of mortarless rubble and dirt.
^Fleming, Honour, & Pevsner. A Dictionary of Architecture.
^"Rubble masonry". Encyclopædia Britannica. Retrieved 23 March 2023.