The Rock County Sheriff’s Office is investigating a fatal crash on Highway 81 west of Beloit.  Deputies report a car entered the ditch, rolled over and struck a tree between County Highway H and Smythe School Road sometime before 3:00 P.M. Sunday afternoon.  The driver and lone occupant was a 34-year-old woman from Beloit.  The State Patrol reports that section of the highway was closed for about two hours.  Authorities say speed appears to be a factor in the crash, and the victim was not wearing a seatbelt.