This map represents the town of Thiepval (France) during the Battle of the Somme in World War 1.
J.R.R. Tolkien was engaged in the conflict as a Lieutenant in Lancashire Fusiliers.
This experience was very important in his writer life, as revealed by Jonh Garth in his book Tolkien and the Great War, Boston, Houghton Mifflin (2003).
British army was located below the blue dashed line, and red lines represent German positions (principal trenches and redoubts).
Note that the vertical scale is deliberately exaggerated, two lines are vertically separated by 20 meters : the real battlefield is flatter than this representation. It is clear that the German troops are entranched in a favorable position compared to the British troops.
The contourlines, and the river course, were extracted from a trench map available at the following adress : http://maps.nls.uk/view/101723165.