I guess I always thought it was more womb-like than phallic, with an entry to an interior chamber and all. Either way, it seems it could represent fertility or rebirth...

As far as I know, it is a representation of a woman, with four chambers to represent the limbs and one for the head. I read all this in a book - the title of which escapes me-which went into huge details about both the Barrow and Silbury Hill , exploring the idea of Silbury Hill being a womb, which I think is a rather appealing idea, although most people I have voiced this to go , 'What a complete pile of bollocks..' Pah! to them, I say.