This paper describes the application of an extended, simple stream-power and hillslope erosion model to investigate the evolution and controls on triangular facets at range fronts. The results highlight specific relationships between vertical and horizontal normal fault displacement rates, and the dip angle and dimensions of facets.

The paper contains some useful insights and prompted some ideas for future application, so is worthy of publication. However I suggest (extensive) minor but important revisions, because aspects of the presentation make it harder to follow than it needs to be; several assumptions are simply asserted or referred to a recent paper by the same author, without restating the key justification for the assumption in the current paper; and a few existing relationships between ridge relief and drainage incision and drainage spacing would be helpful comparisons. Detailed suggestions and questions below by line number.

1. (Abstract) why is it important that they’ve been studied for a century? This first line would be better as a brief description of what they are and our most general understanding of them. This would help the reader understand the second sentence, which presumes a detailed understanding of the setting and geometry of these features, which many readers will not have.

11. what does “rather” polygonal mean? Approximately?

13. impressing -> impressive ?

13-15. Same issue as abstract: needs to define what facets are, how they relate to terrain and tectonics, and why we care about them, before jumping into details of geometry and research questions. Again, the length of time they’ve been studied isn’t really relevant as motivation for this study.

** To this end, I also suggest making Figure 4 the first figure, and adding to it 2-3 photos of different-looking range fronts with facets, so the reader can see clearly what geometry you’re referring to. ** 

34. Re-cite Tucker here at the end of this paragraph.

39. Why is the 15 years relevant?

42-43. This omits the very relevant work of Densmore et al from the late 1990s who numerically modeled these in the Basin and Range (US) and explicitly discussed the planarity and other geometric aspects of the facets.

- Ellis, Densmore, and Anderson, 1999, Development of mountainous topography in the Basin Ranges, USA: Basin Research, v. 11, p. 2141, doi:10.1046/j.1365-2117.1999.00087.x.

- Densmore, A.L., Ellis, M.A., and Anderson, R.S., 1998, Landsliding and the evolution of normal‐fault‐bounded mountains: Journal of Geophysical Research: Solid Earth (1978–2012), v. 103, p. 15203–15219, doi:10.1029/98jb00510.

54-58. Regarding nonlinear hillslope transport, I don’t think the slope needs to be as steep as the threshold for rock stability; it includes shallow slips in regolith, nonlocal transport e.g. rocks going all the way down the hill, etc., and so planarity of slopes is approached well before the ultimate threshold slope is reached.

57. delete “then” at the end of the sentence

59-62. This doesn’t follow the discussion earlier in the paragraph, ad could be relocated down to the model description ~ line 105.

98. Justification for m=0? 

115.  “Some kind of” -> “a”; or perhaps the “only” preferred state

108-116. Is there any empirical support for this model?

131. “sheared” -> “shared”

157. “Must be the same” must it? Fault movement is episodic and generates substantial and persistent transients (e.g. Yanites, B.J., Tucker, G.E., Mueller, K.J. and Chen, Y.G., 2010. How rivers react to large earthquakes: Evidence from central Taiwan. Geology, 38(7), pp.639-642.)

Section 4 - I really appreciate the inclusion of empirical constraint on n for this application, rather than simply asserting n=1 as many studies do.

195. The data set used here is limited, but surely there are slip rate constraints on many ranges in (for example) the US basin and range, and geometries can be measured from DEMs, if more constraint is helpful.

235. One mesh width? 60 km? Or one grid cell width, delta-x?

236. Plain -> plane

237. Evaluating in the middle of the displacement steps: did you do a sensitivity test on this? Why not at the end of the step when more time for adjustment has occurred since the last step?

Fig. 7 caption needs to explain that the lines are the vertical map projections of the facets.

Figure 8 caption or legend needs to indicate that the shaded areas are now the facets corresponding to the same colors/times as the drainage lines.

Section 5.2. I guess this is useful to compare to earlier models, but do vertically-slipping vertical-dipping faults exist in nature?

254. I’d be careful with the word “impossible” unless the scope is clearly defined. (i.e. under these model rules)

271-272. This is hard to follow. The fault to the profile - what profile? Where are the knickpoints arriving for both rivers and hillslopes?

283-286. Is it the original trace of the fault that is important, or the total vertical uplift so far? For incision = uplift, only after the total uplift is equal to the eventual steady height of the facets/ridgelines is the form fully adjusted to the new state.  (e.g. for SPIM, Whipple and Tucker, 1999; but also more generally)

294-298. How does the density of drainages at the range front under this model compare with e.g. Perron 2008 and its nonlinear-diffusion equivalent?

Fig 11, others - the 4My drainage networks are very disorganized and densified relative to typical network evolution simulations, is this an artifact of the channel-head identifying routine, or something dynamical related to normal faults and horizontal displacement?

310-315, as before, if you have the same vertical uplift rate in the horst block between simulations, the time to equilibration of the form will also be similar.

335. What is the horizontal width, rather than the baseline? The facet length measured perpendicular to the range? This will be controlled by the height and angle, and therefore more related to the vertical motion, see previous comment.

342. I -> It

343. “Some kind of” -> “a”

Finally, I was left wondering: What would nonlinear-flux hillslope transport predict for the steady relief of the ridge lines between drainages (e.g. Roering 1999/2001) and how does this relate to facet height?