A combine harvester including threshing and separating apparatus which are arranged to process a cut crop stream and each having a grate arrangement through which material separated from the crop stream falls under gravity. Grain conveyance means arranged below the threshing and separating apparatus catch the falling grain and material other than grain (MOG). A stratification pan is located below a front region of the threshing and separating apparatus and a return conveyor is located below a rear region of the threshing and separating apparatus, both conveyors being driven in a oscillating manner. The return conveyor conveys said grain and MOG in a forward direction to a front edge thereof which is located above the stratification pan. The stratification pan conveys collected material rearwardly to a rear edge from where the collected material falls under gravity into a cleaning unit. An accelerator roller is provided and is rotatably driven on a transverse axis in front of the front edge of the return conveyor. Material caught by the return conveyor is incident upon the accelerator roller and paddles provided on the accelerator roller serve to accelerate the material from the return conveyor forwardly.
