On the other hand, in the chalk hills near Hitchin we have a watershed of first class, or primary rank, for it divides the Thames catchment-area from that of the Great or Bedfordshire Ouse, which flows into the North Sea miles away from the estuary of the Thames. Only four comparatively small streams flowing into the Ouse basin lie for part or the whole of their course within the limits of the county. The first of these is the Pirre, or Purwell, a small brook which rises in the parish of Ippolits, and, after passing Much Wymondley, flows into the Hiz near Ickleford. The Hiz itself rises in a spring at Wellhead, a short distance southward of Hitchin (formerly also called Hiz), and after receiving the Purwell, flows to Ickleford, where it leaves the county, being joined in Bedfordshire by the Ivel, which rises not far from Baldock, passing Biggleswade to join the Ouse. Below Biggleswade it flows through Tempsford, where it unites with the Ouse. The last of the four streams belonging to the Ouse system is the Rhee (a Saxon term signifying a water-course or river), which springs strongly from the chalk a short distance west of Ashwell, and after passing Accrington Bridge and crossing the Ermine Street, eventually falls into the Cam. The whole of the remaining rivers of the county belong to the Thames catchment-area. With the exception of the Thame, to be mentioned immediately in a separate paragraph, these form two main systems, namely that of the Colne draining the western and that of the Lea the northern and eastern part of the county; the watershed between these two systems running in a north-westerly and south-easterly direction between St Albans and Hatfield, and thence to the north of Chipping Barnet. The Thame is almost entirely a Buckinghamshire river, but it rises in our county and runs on the north side of the great watershed formed by the continuation of the Chiltern Hills; this watershed constituting a broad, nearly waterless belt separating the catchment-area of the upper Thame from that of the Lea. The Thame itself springs from three heads in the parish of Tring; the first of these rising near the vicarage, the second at a spot called Dundell, and the third in a spring known as Bulbourne. The three become united at New Mill, whence, after passing through Puttenham, the Thame flows by Aylesbury, in Buckinghamshire, and from there continues its course till it eventually joins the Thames near the village of Dorchester, a considerable distance below Oxford.