Loose material moves downhill from a fresh fault scarp, rounding it. What sets the smoothing speed?

Answer: Sediment transport efficiency

Scarp height alone — Height matters, but not by itself. In the textbook Culling model, the slope relaxes as sediment moves from high ground toward low ground, described by a transport coefficient K. A tall scarp gives more relief to smooth, yet the rate depends on how efficiently loose material is moved by creep, shallow slides, water, and weather.

Sediment transport efficiency ✓ — Right. The model treats scarp rounding almost like diffusion: sediment flux smooths sharp topography, and the coefficient K captures transport efficiency. That is the cognitive hook: a broken earthquake scarp can be read with mathematics similar to heat spreading out. K is not the scarp's height; it is how fast material is redistributed.

Rock density alone — Not enough. Density helps set loads in geophysics, but this smoothing question is about surface redistribution. In a diffusion-style scarp model, the same initial shape can relax at different rates if the transport coefficient K differs. That is why the correct answer is a process rate, not just a material property.