Such was one aspect, then, in the early days of English history, of the great plain that stretches from Cambridge to the sea. But our map-palimpsest has further physical facts to reveal which had an important influence on the civic and economic development of Cambridge. To the south-east of this great plain of low-lying fenlands rises the upland country of boulder clay, stretching in a line almost directly west and east from the downs at Royston, thirteen miles below Cambridge, to Sudbury-on-the-Stour. The whole of this ridge of high ground, which roughly corresponds with the present boundaries between Cambridgeshire and Suffolk and Essex, was in the early days covered with dense forest. Thus the Forest and the Fen between them formed a material barrier separating the kingdom of East Anglia from the rest of Britain. At one point only could an entrance be gained. Between the forest and the fen there runs a long belt of land, at its narrowest point not more than five miles wide, consisting partly of open pasture, partly of chalk down. In the neck, so to say, of this natural pass into East Anglia lies the town of Cambridge. A careful scrutiny of our map will show, on the under-text of our palimpsest, a remarkable series of British earthworks, all crossing in parallel lines this narrow belt of open land between the fen and the forest, marked on the map as Black Ditches, Devil's Dyke, the Fleam or Balsham Dyke, the Brent or Pampisford Ditch, and the Brand or Heydon Way. Of these the longest and most important is the well-known Devil's Dyke, near Newmarket. It is some eight miles long in all, and consists of a lofty bank twelve feet wide at the top, eighteen feet above the level of the country, and thirty feet above the bottom of the Ditch, which is itself some twenty feet wide. The ditch is on the western side of the bank, thus showing that it was used as a defence by the people on the east against those on the west. It was near this ditch that the defeat of the ancient British tribe of the Iceni by the Romans, as described by Tacitus ("Annals," xii. 31), took place in A.D. 50.[3] At Cambridge itself the ancient earthwork known as Castle Hill may belong to this British period, and have formed a valuable auxiliary to the line of dykes in defending the ford of the river and the pass behind; but upon this point authorities are divided.[4] Indeed, there is good ground for the opinion that the Castle Hill is a construction of the later Saxon period, and may, in fact, be referred to the time of the Danish incursions in the ninth century, during which time Cambridge is known to have been sacked more than once.