The British advance in the Fourth Army area reached the Somme rapidly from 17 to 20 March, with a continuous pursuit by vanguards and the main body moving forward by bounds between lines of resistance, up to the Somme river and Canal du Nord, which ran north-to-south from Offoy to Péronne, then paused while the river was bridged, with a priority of light bridges for infantry first, pontoon or trestle bridges for wagons and field artillery and then heavy bridges for mechanical transport and heavy artillery. The heavy steel bridges could be transported from a Base Park at Le Havre with notice. A bridge over the canal near Péronne was built by surveying the ground on the night of 15 March, towing pontoons up river the next night, building beginning at dawn on 17 March and the pontoon being ready by noon. Infantry of the 1/8th Royal Warwicks crossed that evening and were then ferried over the river beyond on rafts, to become the first Allied troops into Péronne. On the right flank, IV Corps had to advance about over cratered and blocked roads to reach the Somme but Corps Mounted Troops and cyclists arrived on 18 March to find German rearguards also mounted on bicycles. Infantry crossed the river on 20 March by when the mounted troops had reached Germaine and the Fourth Army infantry outposts were established on high ground east of the Somme. "Ward's Force" was formed with corps cavalry, cyclists and two batteries of field artillery, two sections of engineers, a battalion of infantry from the 48th Division on 22 March as a precaution after cavalry was forced out of Poeuilly and neighbouring villages by a counter-attack and the corps cavalry relieved by the 5th Cavalry Division. The villages were reoccupied next day. The German retirement from the had begun on 19 March when Nurlu and Bertincourt were occupied by the British after slight pressure. British infantry and cavalry were finding greater German resistance.