The track surface consists of four layers of grading. The topmost of which must be of shale, granite, brick granules or other loose material of which no individual piece can be over in size. Competitors use this surface to slide their machines sideways (powersliding or broadsiding) into the bends using the rear wheel to scrub-off speed while still providing the drive to power the bike forward and around the bend. The skill of speedway lies in the overall ability of the rider to control his motorcycle when cornering and thus avoid losing places through deceleration. The use of asphalt, concrete and tarmac for any layer is prohibited. The top layer must be levelled or "graded" at intervals during an event by tractors towing specially adapted rakes to evenly re-distribute the surface. Tracks are watered before and, if needed, during meetings to prevent the surface becoming too dry and to protect the public and the riders from dust.