Larger rivers such as the Lea have influenced local geography in several ways. Firstly a river and its marshland formed a significant barrier to east–west movement - the Lea formed a natural boundary between the historic areas of Middlesex and Essex. Secondly the valley of the Lea formed a route - both the river and later Lee Navigation, and roads including the Roman Ermine Street, the Hertford Road (A1010) and the later Great Cambridge Road (A10) and A1055. The Lea Valley is also followed by two routes of what became the Great Eastern Railway and had important marshalling yards and locomotive works at Temple Mills. Thirdly the river provided power for numerous water mills - well known examples being the Royal Small Arms Factory at Enfield and nearby Waltham Abbey Royal Gunpowder Mills, Wright's Flour Mill (London's last surviving working mill) at Ponders End and the Three Mills at Stratford. In the 19th Century the lower Lea became an important area for the manufacture of chemicals, in part based on the supply of by-products such as sulphur and ammonia from the Gas Light and Coke Company's works at Bow Common. In the 20th Century the combination of transport, wide expanses of flat land and electricity from riverside and canalside plants such as Brimsdown, Hackney, Bow and West Ham led to expansion of industries including for example Enfield Rolling Mills and Enfield Cables, Thorn Electrical Industries, Belling, Glover and Main, MK Electric, Gestetner, JAP Industries, etc. Much industry has now gone, to be replaced by warehousing and retail parks.