Review of TC-2023-73 Norris et al.

This is a well-presented study that compiles and synthesizes a large amount of geomorphic data from the southwestern sector of the Laurentide Ice Sheet, placing their geomorphic reconstruction in the context of recent advancements in our understanding of the chronology of regional ice sheet recession. In all, the manuscript is well structured and written and pulls together a wealth of data that is of interest to the wider community. I would recommend this for publication and list a few minor suggestions for improvement below:

Broader comments: 

The interpretation of a change in ice-marginal behaviour during BA to YD period connected to a change in underlying lithology is quite interesting but somewhat lacks explanation. The change in subglacial lithology will drive a change in friction at the ice sheet’s bed, but also potentially yields a change in water availability at the bed-ice interface (moving from more porous sedimentary rocks to less porous crystalline rocks). The observation is worth noting, I feel the interpretation requires a little bit more discussion. 

There may be an issue with figure numbering at the end of the manuscript, it appears figure 7 isn’t referenced in the text, though a non-existent figure 6a is referenced.  

Line by line notes: 

55. There is inconsistency in “Fig.” vs “Fig” in figure references (only an issue when one tries to search where a figure is referenced in the text).

63. I don’t think pre is needed here to describe existing chronologies. 

95. It’s not clear what “these” refers to in this sentence.

104. I’m not sure biases is the right word here. Perhaps use lag or lag in the timing to describe the temporal offset between deglaciation and biotic colonisation?

164. The numbers on the flowsets in the panels of Figure 4 are very difficult to read. This either requires having the google earth document open or a copy of Figure 3 which is more legible. 

250. It would be useful to note here that the large-scale reconstructions you are using (Dyke’s and Dalton’s) are based on radiocarbon only, while the data set you are using have data from multiple dating techniques. I don’t think there is an issue with this approach, given the scale and approach, but I do think the discrepancy should be pointed out to the reader. 

270. Any hypothesis for where these ice divides might have been, or how far away?

327. I’m not sure what is meant by “extensive ice thickness”?

358. Is it possible to quantify these retreat rates? Otherwise, it would be useful to make a comparison here, rapid retreat with respect to something (either a timing or specific region).

393. I think this is meant to be “Bordering” rather than “boarding”. 

400. Can you provide any locations here? The references are useful, but place names would help a reader who is unfamiliar with the literature base. 

454. Can you provide a bit more glaciologic reasoning for this? (see broader comment above)

471. Is this point still true, it sounds like this paper has at least in part clarified this issue. 

548. There is no Figure 6a, I think this is Figure 7a. 

644. The supplemental file wasn’t able to be opened without adding”.kmz” to the file name. 

Figure 4- The numeric labels for the flow sets is illegible at the current size/resolution. Either they can be omitted or should be made larger to be readable.

Overall comments

This is a well-written study that reconciles previously compiled flowsets with new mapping in a vast area of the SW LIS constrained by a recently updated deglacial chronology. The discussion is supported by a well-constructed schematic model and addresses relevant scientific questions within the scope of TC. Overall a very good contribution that warrants publication after minor revisions.  I recommend to clarify the interpretations of some flowsets and relative chronology where the data is poorly constrained, and indicate where the new mapping was completed. I also suggest improving several figures where it is laborious to see the flow directions and labels of the flowsets or the contrast in elevation between the uplands and the lowlands, important to appreciate the control (or not) on the ice streams during the various proposed phases. Also I question the use of Dalton’s ice margins on Figure 4 since a new deglacial chronology is available by the first author in this region, as well as the parallel flowset pattern over the Canadian Shield where the known record is complex and where ice streams have been suggested across and down-ice of the Athabasca Basin. Below are my detailed comments where suggested edits/revisions are recommended.

Detailed comments

- Line 61: Was the mapping entirely done from remote imagery interpretation?

- Lines 140-142: Considering the lack of striations on the Western Canadian Sedimentary Basin, were striae on boulder pavements considered to help define the relative chronology of the flowsets?

- Line 175: Clarify from which ice source is the SE to NW movement (CIS or LIS).

- Lines 199-200: Were the hummocky and stagnant ice terrains compiled from your recent mapping? This is the first time these are mentioned in the text. Or refer to previous work.

- Line 205: Indicate the location of the Churchill River lowlands on one of the figures.

- Line 229-230: Indicate the possibility of the SE flow in the Churchill River lowlands to be part of Phase 3 (if not crosscut by other flowsets).

- Line 231: Flow directions (“from southwest-west to the northeast, northwest-to-southeast and north-to-southeast flow”) are incomprehensible. Revise.

- Line 240: There are many streamlined sediment landforms on the Shield, in addition to lineated outcrops, particularly over the Athabasca sedimentary Basin (e.g. Schreiner, 1984; Campbell, 2007, 2009; Campbell et al., 2007).

- Line 243: Could some of the landforms on the uplands be older and preserved under cold-based thinner ice?

- Line 263: Consider if the Cameron Hills fragments could have been part of Phase 4 to the SW.

- Line 269: Ice divide or ice dome? Is there any evidence for an ice dome within your study area during LGM? Refer to previous work if necessary.

- Line 317: If there is no convergent flow in the CAIS/HPIS, why do you show a converging onset zone up-ice of the multiple flowsets on Fig. 4b (in light grey over the Shield boundary)?

- Line 363: Phase 3 is discussed in this section (with multiple references to Fig. 4c). Add Phase 3 in subtitle or in title of section 5.4.

- Lines 412-413: Indicate where the Peace River lowlands are on a figure; refer to Fig. 4d at end of sentence.

- Line 430: Indicate on a figure where the spillway is.

- Line 449: Refer to Fig. 4d.

- Line 452-454: There are also some Devonian lithologies (harder) at the Shield boundary. On the other hand, the Athabasca Basin sandstones form a large part of the Shield under your “Cree Lake ice flow” – with much softer rocks and an extensive sediment cover, up to 100 m+. So not entirely hard-bedded conditions over the Shield. Consider how this may affect your interpretations of a subglacial lithological control at the Shield boundary. 

- Line 551: Wrong reference to Fig. 3?

- Line 570: Refer to Fig. 7b?

- Line 576: Refer to Fig. 7c?

- Line 588: Refer to Fig. 7d?

- Line 600: Perhaps a good indicator of terrestrial ice sheet collapse but mainly pertinent to soft bedded areas?

Suggested edits on Figures

Fig. 1: Add description of inset map and source; it is very difficult to see the uplands/valleys and appreciate control (or not) on ice streams on main figure:  I suggest enhancing DEM colors and/or using hillshades (same comment applies for Figs 3, 4, 5 and 6).

Fig. 2c: It is too small to see anything; also, it does not correspond to same area as a-b-d-e (but it does to outline on Fig. 1).

Fig. 3: Which flowsets are new from this study? I recommend adding an inset map showing where the main areas of changes/additions are located; it is tedious to figure out the directions (arrows) of the ice flow within the flowsets: I suggest making lines thinner inside the flowset; previous mapping in the NE (area of parallel flowset #164) supported by field datasets shows much more complex ice flows, with some diverging and many x-cutting, and probable ice streams (soft-bedded and hard-bedded). Also the Maskwa system was depicted as reaching much further up-ice into the ice sheet by Ross et al (900 km long including 350 km on the Canadian Shield) based on mapping and till composition. Mention alternative interpretations to what you are proposing here for the Shield.

Fig. 4:  Add Phase # on each figure 4a,b,c,d,e; the ice margins as depicted appear to be those of Dalton et al? It is confusing since you constrain the ice retreat using your updated chronology. Can you use your modelled ice margin retreat from the 2022 paper? Add the ages on each isochrone; it is very difficult to see the numbers, writings, and different colors representing ages of ice flows - revise; add a scale on Figs. 4b-e.

Fig. 4a: The generalized ice flow legend symbol is too small and very difficult to follow on each map. Enhance colors and increase size.

Fig. 4b: Add names/numbers of the three corridors; what are the grey polygons surrounding the flowsets?; is the CIS/LIS saddle location well defined?

Fig. 4c: Show or number the six NW to SE ice flow corridors.

Fig. 4d: One flowset within IS 4/5 appears in Phase 2 as well.