medium and light bombers, a third type of surfacing material known as pierced steel plank (PSP), or Marsden Matting was introduced on the Normandy bridgehead in July 1944. It consisted of , steel planks joined together and laid perpendicular to the line of flight. Long used in other theatres, PSP would have been ideal for all airfields on the continent, but its limited availability and greater weight made this impractical. Moreover, because of supply problems, construction of even a PSP fighter-bomber field could take a month or longer, while similar PBS and SMT fields could be constructed in two weeks and one week, respectively. In addition, Sod and Earth runways were built for Emergency Landing Strips (ELS) and Refuelling and Rearming Strips (R&R). Captured airfields contained a wide variety of runways, most commonly Asphalt; Concrete; Macadam or Tar-Penetrated Macadam. Airfield types There were five main types of airfields built by the USAAF combat engineers on the continent. These were: Emergency Landing Strips (ELS) Consisted of a rough, graded runway approximately 2000 feet long to provide a place for emergency belly-landings of damaged aircraft. Supply and Evacuation (S&E) Usually a rough graded runway near the front line or an airfield in the rear that was used by C-47s for transport of casualties to the rear, or delivery of supplies and munitions to the front line. Refueling and Rearming (R&R) Consisted of a runway and an aircraft marshalling area on each end of the runway. It was designed to provide an airfield near the front lines upon which aircraft based in rear areas could land, be refuelled and rearmed, and take off again on a mission without having to return to their home field in the rear. Also could be used for dispersal or for when services other than refuelling or rearming was required. These airfields could be expanded into advance landing grounds by the addition of dispersal and other station facilities. Generally if an R&R strip was built, it would be sited wherever possible with a view to further expand it later into an ALG. Advanced Landing Grounds (ALG) An advanced landing ground could be constructed as such from the beginning or by development from an R&R Strip by the addition of dispersal facilities, expansion of the road network and other additions to the station and technical area in order for it to be used over an extended period of time.