He is referring to the physical mud that is present on and around the battlefield. It is also a representation of the overall demenor of the soldiers as they experience the harsh reality of war Explanation:
ANSWER: The poet is talking about different kinds of mud found on hills, valleys and war zones coming down to make more mud. Mud: with scraps of iron lying in it and the straggling fragments of a nation, lolling, hanging about in the mud on the edge of disaster.”
