I don't think you're wrong to be confused Pictish there is a definite ambiguity in the way the term is used. The wikipedia entry http://en.wikipedia.org/wiki/Gully would have it that gullies are natural erosional features resembling large ditches or small valleys.
However, the term is used in the building trade to refer to artificial drainage features e.g. rainwater gullies. So clearly it can have other uses. It's use in archaeology seems (in my experience) to have developed to encompass small artificial drainage features such as the pennanular gullies you may find around roundhouses in the midlands. As ever it's all about context! The size distinction is reminiscent of the Bluffer's Guide definitions for Post Hole and Storage/Rubbish Pit. In my view a gully would be something that an average adult would have no trouble stepping across, anything larger is a ditch
Except of course when it's a Palaeochannel!
However, the term is used in the building trade to refer to artificial drainage features e.g. rainwater gullies. So clearly it can have other uses. It's use in archaeology seems (in my experience) to have developed to encompass small artificial drainage features such as the pennanular gullies you may find around roundhouses in the midlands. As ever it's all about context! The size distinction is reminiscent of the Bluffer's Guide definitions for Post Hole and Storage/Rubbish Pit. In my view a gully would be something that an average adult would have no trouble stepping across, anything larger is a ditch
Except of course when it's a Palaeochannel!