 It was established in the Anglo-Saxon period from the territory of the Middle Saxons, and existed as an official administrative unit until 1965.The county is bounded to the south by the River Thames, and has the rivers Colne and Lea and a ridge of hills forming its other boundaries.The largely low-lying county, dominated by clay in its north and alluvium on gravel in its south, was the second smallest by area in 1831.