Rain during the summer had turned soil to mud but the heavier autumn rains created a much longer-lasting variety, a mixture of soil and the chalky subsoil, which became "liquid, yellow-grey mud" which had extraordinary buoyancy and stuck to everything, covering men and jamming gun mechanisms and rifles. On roads, the mud stuck to wheels and caked the hooves of horses and mules. Men on foot were coated up to the knee and movement off the roads became impossible. Shell craters filled with a quicksand, which could drown soldiers and animals; a French writer called the Somme mud the worst on the Western Front. Engineers laboured all summer to keep roads open and to lay new ones, build corduroy roads of logs and railway sleepers and laid duckboard tracks as the front moved eastwards. The slow Anglo-French advance increased the distance that supplies had to be carried, from the intact road system in the rear to the front line on the far side of the beaten zone, which brought the transport system close to collapse whenever it rained. More vehicles on the roads accelerated their decrepitude and Bean wrote that many of the lorries broke down and were pushed aside.