Fig.1 Stone-line

( Source: USDA-NRCS, 1996.)

• The stone-line is a mark of lithologic discontinuity indicating that the material above the line was deposited or formed at a different period from the material below. Some soils have multiple stone-lines.
  
• It is a layer (a few cm thick) of angular gravel. It often forms the transient zone between ( saprolite) and surface mantle. It is composed of quartz and fragments of Fe-crusts and pisoliths (= accumulations of gravelly like Fe/Al-concretions). The form of the stones are angular to rounded; some stones have diameters of 10 cm. The thickness of the stone-line varies considerably and can be as thick as 1 m. The form of the stone-line also varies, and might be discontinuous or undulating.
  

Fig.2 Undulating stone-line

( Source: Prof. Horst Fölster, Göttingen.)

Fig.3 Thin stone-line

( Source: Prof. Horst Fölster, Göttingen.)

• Various forms of stone-lines exist: a) undulating (fig.3), b) thin-layered (fig.2)
  
• Several theories exist on the origin of stone-lines that separate saprolites from the surface mantle:
  
  1. Exposure and erosion
     
  2. Relocation of material
     
     • a) hill wash and hang sedimentation
       
     • b) in situ bioturbation through termites
       

1. Exposure and erosion:
   
   • When a stone-line separates the saprolite from the top horizons it is assumed that the saprolite was once exposed at the surface. Erosion removed all fine soil materials and left behind a gravelly lag deposit that was later covered by the surface mantle (van Wambeke, 1992, 2003).
     
2. Relocation of material
   
   • a) hill wash and hang sedimentation (Fölster…)
     
   • The relocation of material through hillwash and sedimentation includes the processes of erosion, transport and sedimentation. Important for this process is the quantity of water present, the length of the slope, the inclination of the slope and the stability of the surface soil (vegetation cover).
     

Fig.4 Climatic phase changes

( Source: Horst Fölster, Uni Göttingen.)

• During periods of drier climatic conditions the vegetation cover is scattered (uncovered soil surface) and rainfalls are intermittent but intense, leading to a maximal phase of disturbance (see A and B of fig.4). During this phase the material is relocated with the surface runoff. Finer material is transported further away, whereas coarser material is transported only very little and coarse material is not transported at all.
  
• When the climate becomes more humid again, a vegetation cover is established so that runoff processes decline (see C: stabilization phase of fig.4).
  
• During this phase the fine material is transported discontinuously whereas the coarse material is not transported and remains at the new surface. The sorting of the particles according to their size finally leads to the formation of a stone-line (Fölster....).

  Fig.5 Transport

  ( Source: Horst Fölster, Uni Göttingen.)

  
  

2. Relocation of material

b) in situ bioturbation through termites

Fig.6 Bioturbation

( Source: Prof. Horst Fölster, Göttingen.)

• Termite activity provides a reasonable explanation for stone-line formation by upward transport of fine soil material. Termites interfere with many soil-forming processes that take place in tropical regions through i) nest-building

  Fig.7 Termites

  ( Source: Prof. Horst Fölster, Göttingen.)

  ii) enhancing soil horizon formation by mixing or sorting soil constituents, and iii) decomposition of SOM (Nye, 1955; van Wambeke 1992, 2003).
  

Fig.8 Nest

( Source: Prof. Horst Fölster, Göttingen.)

• It is difficult to determine when the nests that contributed to surface soil accumulation were built. Most investigators date the construction to dry periods, 4000 to 8000 years ago.
  
• Termites bring fine soil material (0.5 bis 2 mm) to the surface together with moisture leading to the formation of a textural gradient. The diameter of the particles that termites transport is limited by the maximum size they can carry in their mandibles. Generally, no gravel size grain can be transported.
  
• Termites dig into subsoil horizons and constantly bring new materials to the surface. The amount of fine soil material (0.5 bis 2 mm) displaced from the sub-soil to the surface can be as high as 300 to 1000 kg/ha/year (corresponding to 0.02 to 0.1 mm/year; Nye, 1955). At the surface the impact of raindrops disrupts the aggregates and detaches clay from them. Clay is removed as a suspension by runoff water leaving behind the coarse particles that may form a stone-line.
  
• Nye (1955) calculated that the mounds of Macrotemes bellicosus on soils derived from gneiss near Ibadan, Nigeria could account for the accumulation of a 30 cm surface mantle above the stone-line. The time required for the accumulation was estimated at 12.000 years.
  
• A close correlation of the mineralogy of the sand fraction in the surface soil and underneath in the stone line would acknowledge this theory.