It is somewhat difficult to trace the source of The Lark but checking on the Ordnance Survey maps it would appear to start somewhere near Bradfield St. George, by Rushbrooke, through Bury St. Edmunds, via the Fornhams, through West Stow and via a series of gravel workings at Lackford. It then continues on to Icklingham and Mildenhall, departing from the county somewhere near Prickwillow.
The Lark discharges itself into The Great Ouse south of Littleport which then continues its journey to Kings Lynn and The Wash