  The long-term cyclicity and temporal succession of glacial-periglacial (or
deglacial) periods or epochs are keynotes of Quaternary geology on Earth.
Relatively recent work has begun to explore the histories of the mid- to
higher-latitudinal terrain of Mars, especially in the northern hemisphere, for
evidence of similar cyclicity and succession in the Mid to Late Amazonian
Epoch. Here, we carry on with this work by focusing on Protonilus Mensae [PM]
(43-490 N, 37-590 E). More specifically, we discuss, describe and evaluate an
area within PM that straddles a geological contact between two ancient units:
[HNt], a Noachian-Hesperian Epoch transition unit; and [eHT] an early Hesperian
Epoch transition unit. Dark-toned terrain within the eHt unit (HiRISE image
ESP_028457_2255) shows continuous coverage by structures akin to
clastically-sorted circles [CSCs]. The latter are observed in permafrost
regions on Earth where the freeze-thaw cycling of surface and/or near-surface
water is commonplace and cryoturbation is not exceptional. The crater-size
frequency distribution of the dark-toned terrain suggests a minimum age of ~100
Ma and a maximum age of ~1 Ga. The age estimates of the candidate CSCs fall
within this dispersion. Geochronologically, this places the candidate CSCs
amongst the oldest periglacial landforms identified on Mars so far.
