Butler and Alsop provide a clear and thorough explanation as to why our suggestion (Simms & Ernstson 2019), that the Lairg Gravity Low might be a buried impact crater translocated from further east, is incompatible with what is known of the structure of northern Scotland, and I concede that in this particular scenario they are probably right. My original paper (Simms 2015) was based largely on field observations that, amongst other things, indicated that the Stac Fada Member impact ejecta preserved at outcrop had come from the east (despite claims to the contrary by others; Amor et al. 2008, 2019). When subsequently I became aware of the LGL I realized that its location and scale, and the similarity of its gravity signature to other buried impact craters, raised the possibility that it might represent the source of the SFM. Understandably, considering its context, previous authors had considered it a tectonic structure (Leslie et al. 2010) although acknowledging that there were issues with such an interpretation. My suggestion that it might be a buried impact crater was broadly consistent with field observations of the impact deposit but it was only one, albeit interesting, possibility hence the question mark in the title of my paper.

Kord Ernstson's re-evaluation of the gravity data suggested that the anomaly might be reinterpreted as just the central part of a complex crater much larger than my original 40 km estimate, in which case some mechanism of translocation was necessary to accommodate it within the space available. The resultant paper (Simms & Ernstson 2019) was my attempt to do this but, regardless of this interpretation, Kord's reanalysis of the gravity data may shed new light on aspects of the region's structure. It is important to read the full text of our paper to appreciate that we are making only a tentative suggestion, rather than a categorical statement, despite my ill-advised use of the word ‘propose' in the abstract.

It is unfortunate that the structural aspects of our paper were not addressed during the review stage, something I raised myself with the journal editor. Similarly, observational errors in the latest paper by Amor et al. (2019) also were not highlighted prior to publication. It would have been preferable to iron out these various issues – in both papers – during the review stage rather than pointing them out in public, and it perhaps highlights one of the weaknesses of the current peer review process adopted by many journals.

On the positive side, our paper has prompted Butler and Alsop to describe the relationship between the Moine Thrust and the LGL in a far more eloquent manner than hitherto although, like some previous authors (Rollin 2009; Leslie et al. 2010), I have niggling doubts that the gravity anomaly can be attributed simply to density differences associated with thickening of the Moine and/or buried granite plutons. Butler and Alsop may consider it improbable that the remnants of a translocated 100 km diameter crater exist beneath Lairg, but their statement that the LGL is only ‘highly unlikely' to be a crater gives me a modicum of hope that a more modest impact structure might still be discovered there. Perhaps such a possibility, however remote, might encourage renewed investigation of this enigmatic structure.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Scientific editing by Graham Shields