= Solution
The <geomorphology> of active <normal faulting> records a competition between accumulated tectonic displacement and the ability of <erosion>, transport and deposition to modify it. A <hanging wall> moves down relative to a <footwall>, producing a <fault scarp>, mountain-front relief, and accommodation space for a <sedimentary basin>. Repeated earthquakes can build a large front even though each individual surface break is much smaller.<Normal-fault geomorphic features by United States Geological Survey>[The primary mapping report's table of geomorphic features lists scarps, faceted spurs, oversteepened mountain fronts and rejuvenated valley floors as common normal-slip indicators.]{ref}
\Image[/past-exam-of-the-year-2-of-the-earth-sciences-geology-course-of-the-university-of-oxford/2024/paper-3-q4-geomorphology.png]
{title=Original sketches of tectonic, erosional and depositional controls on normal-fault landforms}
{description=The panels distinguish cumulative block displacement, progressive scarp rounding, drainage and faceted mountain fronts, and burial of a scarp by sediment. Geometries and relative scales are illustrative.}
The principal controls and their observable effects are:
* \b[Displacement, rate and timing.] Larger single-event <fault slip> creates a higher fresh scarp; repeated slip builds cumulative offsets of fans and <river terraces>. The interval since the latest rupture controls degradation. For pure <dip-slip faulting> on a plane dipping $\delta$, vertical <fault throw> is $D\sin\delta$ and horizontal <fault heave> is $D\cos\delta$. Scarp height on an inclined eroded surface need not equal either the slip or the original vertical throw.
* \b[Geometry and structural history.] <Fault dip>, strike, curvature, segmentation, splays and along-strike variations in <fault slip> control scarp orientation and continuity. Linked faults can create a stepped front; synthetic and antithetic faults can create a <graben>. Cumulative deformation includes broad footwall uplift and hanging-wall tilting as well as a sharp break.
* \b[Lithology and cover.] Strong exposed <bedrock> can preserve steep scarps and fresh fault surfaces. Weak fractured <rock>, thick <regolith> and loose <alluvium> weather and slump more rapidly, spread deformation, and produce debris at the scarp base. Differential <erosion> can make a lithological front without recent tectonic movement, so a straight cliff alone is not sufficient evidence of activity.
* \b[Climate and surface transport.] Rainfall, runoff, frost action, vegetation and slope processes control scarp rounding and <colluvium> production. Steep new faces collapse towards stable slopes; later hillslope transport rounds the crest and fills the toe. A simple <scarp diffusion model> broadens a step over a length proportional to $\sqrt{\kappa t}$, but only if a suitable transport law and comparable material and climate apply.
* \b[Drainage and sediment supply.] Footwall uplift can incise channels and produce <river terraces>, knickpoints and triangular <faceted spurs>. Hanging-wall subsidence can pond drainage, deflect streams and promote <alluvial fans> and basin filling. Rapid <aggradation> can bury an active scarp, while stream incision can remove or exaggerate local relief.
* \b[Preservation and initial landscape.] Pre-existing slopes, relief, surface ages, glacial modification and human disturbance determine where offsets can be recognized. A fan abandoned long before the latest earthquake can accumulate more offset than a younger nearby surface. Compare like-age markers across a fault, and distinguish a mountain front's long history from a single event.
Useful observations therefore include scarp profiles, repeated offsets of dated surfaces, drainage organization, and continuity along strike. A high rugged front is not by itself evidence of a high present <fault slip rate>; conversely, a muted or buried scarp need not mark an inactive <fault>.
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