German barbed wire, near Beaumont Hamel, October 1916. Cutting the barbed wire was always a major artillery responsibility. Here, 'knife rests' (wooden x-forms wrapped with barbed wire) have been shelled. Knife rests were relatively easy targets, as they stood fairly high and could be seen; the wooden frames were also fragile. However, no matter how well the artillery did its job of cutting the barbed wire, the wet ground here was another enormous obstacle to the infantry advance, one that the artillery were powerless to address.
Source: IWM photo Q1547.