1:100,000 base maps, digitized by McMaster University and tiled together by Canadian Geographic, originally produced by the British Ordnance Survey. They were created at the beginning of war, 1912-1914.
Shows contour lines.
Maps were not made at a detailed enough scale to be useful other than planning the logistics of things like supply and communication on very large areas.
Check out all of McMaster's online map collection on World War 1.