Fig. 1. (a) Context MOLA image of the Athabasca Valles region, with 50 m contours. The black asterisk in the center-right of the image indicates the center location of (b). (b) MOC image R01-00745, centered near 9.67° N 203.95° W. Boxes show approximate locations of subsequent figures as labeled. Streamlined form, dune field, and fissure are also labeled. Black dotted and then dashed line indicates the continuation of the fissure across the dune field and then the channel floor as inferred from the faint break in density of the mounds and rimmed forms on the channel floor (see also Fig. 12). Illumination is from the left. (Comparison with MOC image E10-01384 shows that the two data drop-outs across the middle of the image cover an insignificant area.) Formation of the dune field during diluvial (flood) sedimentation is discussed in Burr et al. (2004).

Fig. 2. Examples of the features as discussed in the text. The black arrow points to a positive relief mound; the gray arrow points to a conical form with a summit depression; the white arrow points to a flatter, more irregularly shaped form.

Fig. 3. Three features chosen to represent the suggested evolution of form from taller circular mounds to flatter more irregular features. We suggest these three stages loosely correlate to pingos (a), collapsing pingos (b), and pingo scars (c).

Fig. 4. Graphs of circularity vs area and circularity vs perimeter for Athabascan forms. Circularity is defined as 4π×area/(perimeter²), with a perfect circle having a value of 1. The Pearson product-moment correlation coefficient for a group of randomly sized, nearly circular features would be nearly zero.

Fig. 5. (a) Incipient hydrostatic pingo in the town of Tuktoyaktuk. Person at left is 1.9 m tall. (b) Hydrostatic pingo, Tuktoyaktuk Peninsula. Person on top is 1.9 m tall. (c) Pingo showing signs of initial collapse, as indicated by summit crater. Probable dilation cracks (outlined by snow) suggest this collapse was initiated through exposure and melting of the ice core. Pingo is roughly 20 m high. (d) Collapsed pingos (in foreground, and in back left), Tuktoyaktuk Peninsula, photographed from atop a neighboring pingo. Person on back rim in image center is 1.9 m tall.

Fig. 6. (a) Aerial photo of Ibyuk Pingo, Tuktoyaktuk Peninsula, showing dilational cracking and a summit depression. Illumination from the right. (b) Conical features in Athabasca Valles, with interpreted cracking and collapse, similar to (a). Illumination from the left.

Fig. 7. (a) Oblique aerial photograph of collapsed pingo, Tuktoyaktuk Peninsula. Few-meter-tall pine trees to left of form suggest scale. (b) Closed rimmed feature in Athabasca Valles, with interpreted collapse similar to the collapsed feature in (a).

Fig. 8. (a) Image of two of the flatter more irregular Athabascan features. This contiguous and conjoining structure is similar to that of many terrestrial pingo scars. (b) Drawing of Welsh hydraulic pingo scars from Watson, 1971, Fig. 2. (Copyright John Wiley & Sons Limited, reproduced with permission.)

Fig. 9. Closed ring feature (in the center) with a lower elevation annulus or moat surrounding it. Illumination is from the left. Shadowing indicates that this moat has a raised rim on its straight eastern side, but this raised rim is discontinuous, missing or ambiguous elsewhere around the form.

Fig. 10. A closed ring feature enclosing three smaller conical features with summit depressions. These interior cones range at their summit from 8 to 16 m in maximum diameter.

Fig. 11. A closed ring feature with a smaller conical or ring feature at its northwestern edge.

Fig. 12. (a) Visible image mosaic, (b) stretched day IR, (c) stretched night IR, and (d) sketch of the fissure and streamed form adjacent to the rimmed forms. Visible image mosaic includes MOC images E10-01384, R11-00065, and R13-03976, overlain on THEMIS day IR I08483034. Each figure image (a–d) is 7.3 km; see scale bar on (a).

Fig. 13. Rimmed form within the sedimentary dunes. The black lines highlight the dunes' linear NW–SE strike; the circular forms are left in plain view.

Fig. 14. High-centered polygonal terrain, which is interspersed with the rimmed features.

Fig. 15. Three ice wedges along the Tuktoyaktuk coast, in very close proximity to thermal contraction polygons. Three people at left provide scale.

Fig. 16. Aerial view of high-centered polygons, Tuktoyaktuk Peninsula.