CSIR NET Dec 2015 Q110: Sedimentology

A typical weathering profile, from surface downward, consists of: soil/humus layer at the top, followed by a clay-rich (often mottled) horizon, then saprolite (chemically weathered rock that retains the original rock's texture and structure but is friable and altered in mineralogy), and finally unweathered (fresh) parent rock at the base. Saprolite represents an intermediate stage of in-situ chemical weathering, more altered than fresh bedrock but less altered/reworked than the overlying clay-rich or soil horizons, so it characteristically occurs directly above the unweathered rock, matching option 4.

Option 1 (below the layer of humus) is too shallow a position; saprolite lies well below the humus-rich topsoil, with intervening soil and clay-rich horizons in between. Option 2 (below the layer of soil) is directionally correct but imprecise, since saprolite's defining relationship is specifically with the unweathered bedrock beneath it, not simply being below soil in general (saprolite is also below the clay-rich horizon). Option 3 (above the clay-rich horizon) reverses the correct stratigraphic order; the clay-rich horizon (more intensely weathered) lies above the saprolite, not below it. Option 4 is correct: saprolite directly overlies the unweathered parent rock, marking the transition zone between fresh bedrock and more thoroughly weathered material above.