Whorled ridges, spaced about 2–6 km and forming lobate patterns with lobe widths of about 150 km, occur at many locations in the northern plains of Mars, commonly in close association with sinuous troughs that contain medial ridges. These landforms resemble moraines, tunnel channels, and eskers found in terrestrial glacial terrains, such as the midcontinent of North America. Some Martian landscapes may have formed by disintegration of continental glaciers that covered much of the northern plains into the early Amazonian (i.e., late in Martian geologic history). Meltwater processes apparently were important in the collapse of these hypothesized ice sheets; hence, the glaciers apparently were wet based in part. Whereas striking similarities exist among areas of the northern plains and some glaciated Pleistocene terrains on Earth, there are also important differences; notably, drumlin fields, such as those in many glacial landscapes on Earth, are rare, absent, or not yet resolved in images of the Martian northern plains. Another major difference is that postglacial fluvial and other water‐related modifications (especially erosion) of Pleistocene terrains are substantial, but similar modifications are not observed in the northern plains; a virtually complete and sudden decline in the activity of liquid surface water following glaciation in the northern plains seems to be implied. The climatic implications of the hypothesized Martian glaciers and their decline are unclear. We investigate two possibilities, alternatively involving a relatively warm paleoclimate and the modern Martian climate. The hypothesized ice sheets in the basins within the northern plains (generally at elevations lower than −1 km) suggest a relationship of these frozen bodies of water with former regional lakes or seas, which may have formed in response to huge discharges of water from Martian outflow channels. This possible relationship has been modeled. Glaciers may have evolved from seas by their progressive freezing and then grounding and sublimational redistribution of sea ice. The transition to glaciation may have taken several million years if the climate was very cold, comparable to today's, or tens of thousands of years if the climate was as warm as modern Antarctica. A glacierized sea may have involved an extended period of glaciolacustrine and ice shelf processes.

Cryoplanation terraces are bedrock steps or terraces on ridge crests and hilltops. The tread or “flat” area is 10 to several hundred meters wide and long and slopes from 1 to 5° parallel to the ridge crests. Terrace scarps may be from 1 to 75 m high. Terraces are cut into all bedrock types and are best developed on closely jointed, fine-grained bedrock. The scarps and treads are covered with frost-rived rubble 1 to 2 m thick. The rubble on treads is perennially frozen at a depth of 1 to 2 m or less on sharp but inactive terraces in Alaska.Cryoplanation terraces exist in many parts of the world in present or past periglacial environments. They occur chiefly in nonglaciated regions and near the general altitude of snowline. Cryoplanation terraces form by scarp retreat as the result of nivation. Surficial debris is removed across the terrace tread by mass-wasting. Terrace morphology depends mainly upon climate, bedrock type, and terrace orientation.No climatic data are available from active terraces. Indirect evidence indicates that climatic requirements include low snowfall and cold summer temperatures. Shallow permafrost is necessary to provide moisture and a base for mass movement as well as a base for nivation.Hundreds of sharp but inactive terraces occur in some areas in Alaska where the summer temperature is colder than 10°C. When these terraces were active, temperatures were colder. Recent work in Alaska indicates that terraces were active in some areas when the mean July temperature was about 4°C. The mean annual air temperature probably was in the neighborhood of -12°C or colder.

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