Coming down out of the Chiltern Hills, the River Mimram runs in a comparatively narrow gorge in a S.E. direction, until it joins the Lea near Hertford. The last-named river pursues a parallel course to the westward, as a consequent stream, but on leaving the Chilterns it runs, still S.E., across a wide subsequent valley. The latter stretches, in a gentle curve, from Hertford on the E. to Rickmansworth, south of which place it debouches on the Thames valley. Down the huge subsequent valley ice must have pushed its way from the Bishop's Stortford and Hitchin gaps, reinforced by minor supplies from the Chilterns. There is boulder clay at Hertford and further west at Hatfield, within the valley. On reaching the southern side of the latter, in Hatfield Park, the Lea is suddenly deflected eastward. The way is barred by the high ground on that bank, capped by tertiary outliers. At Hertford it takes another and sharper turn, due south, to join the Thames at London.