Noun
Coal brought by sea; -- a name by which mineral coal was formerly designated in the south of England, in distinction from charcoal, which was brought by land.
Source: Webster's dictionarysea-coal
The coal was often called sea coal because it was washed up from undersea outcrops on the Northumbrian coast. Source: Internet
The term 'sea coal' likely originated from chunks of coal, found washed up on beaches, that wave action had broken from coastal outcroppings. Source: Internet