POLICE are investigating the cause of a crash which left a 38-year-old mother with serious injuries.The woman, from Port Melbourne, was travelling south on the Calder Highway between Big Hill and Ravenswood when she ran off the road and hit a tree.The woman’s two children, aged five and two, were with her at the time of the crash. The children were taken to Bendigo Hospital for observation and were later collected by their grandparents.The crash happened about 2.45pm and police said the woman was airlifted to The Alfred hospital in a serious but stable condition.