I must throw my twopennorth in here to add weight to what Gibb and Malcolm have posted. None of the shanties extant today can be traced back to before c1830 as shanties and then most of them from about 1850-1880. Many people have put in a lot of time trying to trace back shanties before this and come up with only vague, brief descriptions in autobiographies and reminiscences. There are plenty of references to basic call and response chants, but precious little that could be described as a song. When people refer to A-Roving as coming from an earlier century they are referring to similar songs that the shanty derived from, not as such a shanty.