Great Britain was first divided into administrative areas by the Romans, most likely following major geographical features such as rivers. Before their arrival there were distinct tribal areas, but they were in a constant state of flux as territory was gained and lost. After the demise of Roman Britain around 410 these first divisions of land were generally abandoned, although traditional divisions taking the form of petty kingdoms such as Powys, Dumnonia and Elmet, remained in those areas which remained British, such as south west England. The areas that would later form the English counties started to take shape soon afterwards, with the Kingdom of Kent founded by settlers around 445. In southern England more widely, shires emerged from earlier sub-kingdoms as part of the administrative structure of Wessex, which then imposed its system of shires, boroughs (or "burhs") and "ealdormen" on Mercia after it came under West Saxon control during the 10th century. Once the Kingdom of England was united as a whole in 927 it became necessary to subdivide it for administrative convenience and to this end, earldoms were created out of the earlier kingdoms. The whole kingdom was divided into shires by the time of the Norman conquest. Robert of Gloucester accounts for thirty-five shires and William of Malmesbury thirty-two, Henry of Huntingdon, thirty-seven. In most cases the counties or shires in medieval times were administered by a sheriff (derived from "shire-reeve") on behalf of the monarch. After the Norman conquest the sheriff was replaced and the shires became counties, or "areas under the control of a count", in the French manner.