It still kills me that you-all have what you call 'metalled' roads. ~ Jackie

I came across what could be the origin of that phrase. The Romans sensibly used local materials in their road construction. Where a road was passing through an iron making area they would use the slag left over from the iron working to mix in with their top layer, or wear coat. This would rust into hard coagulated chunks that proved highly efficacious and long lasting. Truly a metalled surface.