Raw salt rock material is brought by trucks or conveyor systems into the crusher for initial grinding and size reduction. A screen (grizzly) may be included below the crusher to sort the rocks so that the smaller pieces go through, and the larger pieces are caught for re-grinding. A minimal amount of material should be maintained on the screen to act as a buffer to protect the screen from the impact of larger rock. As the material is brought into the crusher, it moves quickly through the crushing process. The fast-moving level must be monitored to determine when the material reaches a minimum level so that the next load can be started. Level measurement is challenging because the material is irregularly shaped and very dusty, and the overall distance may be as far as 30 m (98 ft).