(This chapter was contributed by Prof. Horst Fölster, Göttingen)

• Stone line is indicating a lithological discontinuity in the soil, e.g. between saprolite and a slope sediment. They mark a former erosion surface which has been covered by a slope sediment. The term slope sediment shall stress the fact that slopes not only are the localities of erosion but also of sedimentation. It is a layer (a few cm thick) of angular gravel. The form of the stones are angular to rounded.
  

  Fig.1 Stoneline, Ghana

  ( Source: Prof. Horst Fölster, Göttingen.)

  Fig.2 Nigeria; “double-discontinuity” of hillwash over gravel over saprolith. The last boundary (Grenze) of gravel over saprolith is marked by a stoneline.

  ( Source: Prof. Horst Fölster, Göttingen.)

• The relation between erosion and sedimentation is shown in Graph A,B, C:
  

  Fig.3 Climatic phase changes and their geomorphic consequences

  ( Source: Prof. Horst Fölster, Göttingen.)

• Erosion occurs when run off is high, e.g. on man-made disturbance (cultivation) but also under natural conditions during climatic phases of lower rainfall which renders the vegetation open. Such climatic phases of unstability have occurred again and again, usually separated by climatic phases of higher rainfall with stable, dense vegetation and low run off (stable phases).
  
• Surface run off concentrates, provokes rills (shallower or deeper gullies). The inclination of the rill bottoms is regulated by the amount of run off , the slope angle and the water level in the valley, and it decides on the level of the erosion surface.
  
• The old soil mantle above the new erosion surface is mobilised along the scarpes on the back and the sides of the rills. Elements too heavy to be transported away from the scarp, fall down and remain on the new erosion surface. This is the stone line in a narrower sense (sensu strictu). The remaining material may reach the valley but may also be deposited, like in Graph under A (in this case a ferruginous gravel). This is called intermittent transport, and is caused by varying rain intensities. Intermittent gravelly deposits are more frequently found in the soil mantle in regions with weakly inclined slopes because of the reduced transport power of the run off.
  
• When climate turns more humid, the vegetation denser, and the run off lower, only finer material can be mobilised and be transported (B in the graph). In this way two layers of slope sediment can develop (above the proper stone line) during the cause of one unstable phase.
  
• The situation in the soil mantle may become more complicated when a second unstable phase intervenes, like in C of the graph. In this case it was a minor instability which only eroded the valley part and sent a few rills upslope. They were later filled with a fine grained sandy deposit.