Robert F. Black

Periglacial features indicative of permafrost: Ice and soil wedges

Ice wedges are wedge-shaped masses of ice, oriented vertically with their apices downward, a few millimeters to many meters wide at the top, and generally less than 10 m vertically. Ice wedges grow in and are confined to humid permafrost regions. Snow, hoar frost, or freezing water partly fill winter contraction cracks outlining polygons, commonly 5–20 m in diameter, on the surface of the ground. Moisture comes from the atmosphere. Increments of ice, generally 0.1–2.0 mm, are added annually to wedges which squeeze enclosing permafrost aside and to the surface to produce striking surface patterns. Soil wedges are not confined to permafrost. One type, sand wedges, now grows in arid permafrost regions. Sand wedges are similar in dimensions, patterns, and growth rates to ice wedges. Drifting sand enters winter contraction cracks instead of ice. Fossil ice and sand wedges are the most diagnostic and widespread indicators of former permafrost, but identification is difficult. Any single wedge is untrustworthy. Evidence of fossil ice wedges includes: wedge forms with collapse structures from replacement of ice; polygonal patterns with dimensions comparable to active forms having similar coefficients of thermal expansion; fabrics in the host showing pressure effects; secondary deposits and fabric indicative of a permafrost table; and other evidence of former permafrost. Sand wedges lack open-wedge, collapse structures, but have complex, nearly vertical, crisscrossing narrow dikelets and fabric. Similar soil wedges are produced by wetting and drying, freezing and thawing, solution, faulting, and other mechanisms. Many forms are multigenetic. Many socalled ice-wedge casts are misidentified, and hence, permafrost along the late-Wisconsinan border in the United States was less extensive than has been proposed.

PERMAFROST: A REVIEW

Permafrost is estimated to occur in areas totaling about 26 per cent of the land area of the earth. Permafrost is as much as 2000 feet thick in Siberia and 1300 feet thick in Alaska, where its minimum temperature is −12°C and −10°C respectively. As it is defined on the basis of temperature, its composition varies over wide limits. Ice is one of the most important components, and large masses commonly are in the form of ice wedges. Structures in ice wedges are generally smaller and more complex than those in glacial ice. Dimensional and optic-axis lineations and foliations of inclusions of air bubbles, organic matter, and clastic material occur in all wedges. Not all structures and orientations of ice crystals in ice wedges can be explained. Permafrost results when the net heat balance of the surface of the earth over a period of several years produces a temperature continuously below 0°C. Although the general thesis of the problem is relatively simple, it is extremely complex in detail. Complete freezing of bedrock for long periods of time has little geologic aftereffect, except through control of movement of ground water. Freezing of mantle completely eliminates ground-water movement, preserves organic remains indefinitely, reduces or prevents mass movements within the frozen material, and promotes frost action in the overlying active layer. Most emphasis has been given the engineering aspects of permafrost. The volume of literature pertaining directly or indirectly to permafrost is becoming so great that individual competence cannot cover the various fields represented.

Pseudo-ice-wedge casts of Connecticut, northeastern United States

Since 1965, ice-wedge casts have been reported in deposits of sand and gravel in Connecticut. These are wedge forms up to 1.1 m wide and many meters high. Most are single forms, not in polygonal array. They are found in adjoining states as well. Their distribution, dimensions, structure, and fabric and an assessment of the former physical environment preclude their origin as permafrost features. They appear to be tension fractures produced by the loading of coarse clastics on fine clastics near and below the water table where sediments creep toward a stream or depression. Locally movement started with kettle formation during deglaciation. However, some wedges cut horizontal layers of iron-coated sand and gravel and must be younger than those distinctly postglacial phenomena. Moreover, modern B horizons of the overlying soil have moved down into some wedges more than 2 m, indicating that fracturing is still active today. Complex fracture fillings in bedrock also have been attributed to a permafrost origin, but this too seems unlikely.

Late-Quaternary sea level changes, Umnak Island, Aleutians—Their effects on ancient Aleuts and their causes

Abstract Late-Quaternary sea level changes in the eastern Aleutian Islands are of paramount importance in the reconstruction of the migrations and environment of the ancient Aleuts. A radiocarbon-dated ash stratigraphy provides the chronology into which geomorphic events can be fitted. These provide evidence for the sea level changes. Deployment of beach material and coastal configuration intimate that sea level was about 2–3 m above the present level about 8250 radiocarbon yr BP. Beach deposits suggest that sea level remained high until about 3000 radiocarbon y.a. when it gradually dropped to its present position. It is concluded that the ancient Aleuts that settled Anangula about 8400 y.a. used boats; all major passes in the eastern Aleutians were flooded, and did not have winter ice. Those ancient Aleuts did not have available the major year-around food resources of the present strandflats as they were cut during the high sea level stand 8250–3000 yr BP. The ancient Aleuts must have been marine oriented, for land-based food resources would have been limited. The cause of relative sea level changes on Umnak Island is considered indeterminate with present data. Eustatic, glacial isostatic, water isostatic, tectonic, and volcanic causes are considered the main possible controls in combinations such that a basic eustatic sea level curve and likely a glacial-water isostatic curve must be common to any solution. Representative solutions are given to illustrate some of the problems.

Valders–Two Creeks, Wisconsin, revisited: The Valders Till is most likely post-Twocreekan

The well-documented and dated post-Twocreekan drift (classical Valders Till) of the Green Bay–Fox River-Lake Winnebago lowland is traced to an interlobate junction with the Lake Michigan Lobe that overrode the Valders type locality. Additional support for the post-Twocreekan age of the surface drift between Valders and Two Creeks comes in part from geomorphic evidence of drainage from Later Lake Oshkosh into Glacial Lake Shoto between Manitowoc and Two Rivers. The key new data and reinterpretations include: (1) a radiocarbon-dated site demonstrating that the surface till of the Green Bay Lobe there is post-Twocreekan, (2) the correlation of that site to the Brillion esker only 500 m northeast and at the same elevation, (3) the correlation of the stratigraphy and morphologic forms of those two localities to an interlobate moraine, (4) the recognition of the interlobate character of the moraine by the amount and structure of lacustrine and fluvial sediments to the crest of that moraine which required ice on both sides simultaneously, (5) the relationships of the drainage of post-Twocreekan Later Lake Oshkosh to Twocreekan wood and to the upper level of Glacial Lake Shoto, (6) the equivalent level of post-Twocreekan Glacial Lake Shoto to the (1 believe) erroneously interpreted pre-Twocreekan Glen-wood Stage of Glacial Lake Chicago, and (7) other details of the stratigraphy and morphology of the drift that support the thesis outlined above.

Regional stagnation of ice in northeastern Connecticut: An alternative model of deglaciation for part of New England

Re-evaluation of published maps and reconnaissance of the rest of the Shetucket River basin lead to the conclusion that stagnation of the late Wisconsinan ice occurred simultaneously throughout the basin rather than in narrow zones during ice-margin recession as has been the accepted model for decades in New England. Evidence includes especially (1) ice-contact deposits laid down over large areas, from the highest hills to the valley bottoms, wherein ice to the south was required, and (2) a series of discontinuous ice-contact lake deposits extending tens of kilometres up all major valleys, which were graded to a major ice and drift barrier in the lower part of the basin. Neither of these situations is possible at the scale observed in ice-margin recession. Regional deglaciation suggests that previous estimates of the ground-water capacity of the valley fills and of the supply of coarse construction aggregates are too high.