• (n.) Masonry constructed of unsquared stones that are irregular in size and shape.
Rub'ble·work` noun Masonry constructed of unsquared stones that are irregular in size and shape. 
Rubblework or rough-waiting is a coarse type of masonry that is constructed of rough, unsquared stones that are not large, but are irregular in size and shape, and not so flat bedded as in rag-work. In some districts it is often formed of flints. In large buildings, in neighbourhoods where better materials can be obtained for the outer face of the ...
masonry built of rubble or roughly dressed stones.