English: The upper River Lea, and rivers which now flow into it near Hertford, were once tributaries of the River Thames. The Anglian glaciation diverted the Thames southwards and led to the creation of the lower River Lea.
Base map from topographic-map.com, "Free topographic maps visualization and sharing."
to share – to copy, distribute and transmit the work
to remix – to adapt the work
Under the following conditions:
attribution – You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
share alike – If you remix, transform, or build upon the material, you must distribute your contributions under the same or compatible license as the original.