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Wind‐etched rocks (ventifacts) at Gweebarra Bay, County Donegal, northwestern Ireland, are located

Ventifaction on the Tolman Ranch Terrace surface in northwest Wyoming is controlled by factors other than intensity and direction of regional winds. Disintegration of granitic boulders in the terrace gravel causes large, fine-grained mafic boulders to be preferentially concentrated on the terrrace tread. These boulders rise above a critical height needed for ventifaction. Quartz-rich sand introduced into the system by weathering is used in the abrasive process while it is simultaneously removed from the system by topographically enhanced winds. Pronounced ventifaction in this area occurs only where granite disintegration and topography are properly combined to maximize abrasive work. Ventifaction in this setting may have been continuous or intermittent since deposition of the gravel; thus, features produced have no paleoclimatic implications.

Rocks were abraded by a sand-laden air-jet in the laboratory to examine the quantitative relationships between wind abrasion and wind velocity, quantity and grain-size of windblown sand particles, and physical properties of the target rocks. The results of 101 runs are expressed by an experimental equation with dimensionless constant, which indicates that the rate of abrasion is directly related to the kinetic energy of the sand projectiles and inversely proportional to the compressive strength of the target rocks. A tentative extension of the experimental equation to field condition leads to an equation for the wind abrasion in fields. An application of the latter equation to the natural settings is preliminarily examined by comparing it with Sharp's (1964) field experiments, and a plausible result is obtained.

Fluorite, halite, periclase, and sylvite have been abraded in a wind tunnel by windblown dust from the "normal" air of the intake room plus or minus introduced silt-sized particles of calcite, barite, or snow. The results indicate that the effective kinetic energy of a projectile and the bond strength of the target-rather than hardness differences, per se-may be fundamental controls of windblast abrasion. It is concluded that, contrary to general belief, some windblast erosion features have probably been produced in response to impact by relatively soft and/or small missiles, such as snow and dust, moving at fairly low wind velocities.

Many stones in a supposed Kansan terminal moraine near Stevens Point, Wisconsin, appear to be ventifacts. Some of them show also varying degrees of wear during glaciofluvial transport following the sandblasting. The position of many of the ventifacts beneath undisturbed pseudostratification indicates that the wind-scouring preceded incorporation in the till. The probability that the wind-scour occurred during advance of the Kansan ice is discussed.

Information is presented concerning sixty-one ventifact localities in nineteen states, of which eight yield specimens believed to be of pre-Pleistocene origin. Southeastern Massachusetts, southern California, and Wyoming are the chief regions where ventifacts occur.

Ancient ventifacts are abundant on gravel-mantled erosion surfaces along the eastern base of the Big Horn Mountains. Stones of varied lithologic characteristics and from less than 1 inch to 6 feet in diameter have wind-cut surfaces displaying the pitting, fluting, grooving, and luster that is characteristic of ventifacts. Faces making angles greater than 550 with the wind are pitted, and faces at lower angles are grooved and fluted. As many as twenty separate faces have been cut on a single stone. In this area such multiple faces cannot be explained by wind-splitting or by variable winds, so they are attributed to changes in stone position caused largely by congeliturbation (frost action) and wind scour. Ventifacts rest on erosion surfaces at four levels, 25-325 feet above stream grade. On the lower surfaces wind-cut stones are abundant and fresh, but on the higher surfaces they are sparse and deeply weathered. At least four separate periods of wind-cutting are indicated by this evidence. Cutting is attri...

Owing to the surface configuration in this locality, the winds blow alternately from opposite directions, carving sand-blast facets on flat limestone fragments. The arrangement of the facets with regard to wind directions, shape and position of the fragments, and the attitude of the base is discussed, and an account given of the stages in the alteration of the shape of pebbles as the process continues. Two main types are discriminated: those with a broad base relative to the height, which tend to be reduced to flattisli or broadly convex plates; and those with a relatively small base, from which more irregular forms are produced. Wind-faceted stones are classified on this basis and a terminology provided.

Sharp edges of small, late Pleistocene, wind-eroded pebbles (or ventifacts) embedded in bedrock at Ocean Cove, California, have an average trend oblique to inferred Pleistocene wind direction. This and other features suggest that abrasion was produced chiefly by a stream of suspended or partly suspended particles following local deflections of the wind stream. That such material would be capable of producing ventifact shapes is suggested by previous studies on wind flow over obstacles. Consideration of wind-flow deflection suggests that currently accepted concepts of ventifact abrasion by saltating sand grains must be modified, and it further suggests that source and size distribution of wind-transported abrasive material, more than size of ventifacts, may determine whether abrasion will result largely from scour by suspended material or from impact by saltating grains. Occurrence of three- and four-edged ventifacts strengthens the view that orientation of faces may be partly controlled by original pebble...

The dominant transportation agent within the extramarginal zones to continental glaciers is for most of the time exclusively the fierce anticyclonic storm wind directed outward normal to the glacier front. Transportation by running water during the remainder of the year is almost exclusively by meltwater. The volume of this water over the outwash plain fluctuates between trickling braided streams and floods covering the plain on warm days, so that the boulders carried in small icebergs from the glacier front either are buried within the deposits or are left stranded upon the plain. During the longer period of wind dominance there result pebble bands (pavements), ventifacts, etched boulders, dunes, and loess deposits-all of them extramarginal to the glacier.

In a recent summary of ventifact localities in the United States, the omission of certain regions where wind abrasion is widespread and obvious indicates that the criteria of this process are often overlooked. Conspicuous examples of wind carving are to be found in the Dells region of Wisconsin and the upper Arkansas River valley and South Park in the Rocky Mountains of Colorado. Little wind work now occurs in these regions; most of it probably took place under periglacial conditions in the Pleistocene ice age.

Abrasion by eolian transport was studied in three different types of wind tunnel. Limestone, feldspar, and quartz were tested in the shape of cubes, crushed crystals, or natural grains. Abrasion increases with size, wind velocity, angularity and roughness of surface. Polished medium-sized quartz grains sustain no loss. Wind abrasion of quartz is 100 to 1,000 times more effective over the same distance than the mechanical action of a river. But eolian abrasion is slight for fine quartz sand and drops to zero at 0.05 mm. in diameter. No quartz particles are produced in the size range dominating in loess. Experimental abrasion of ventifacts indicates that these are cut slowly in scores of years by medium strong winds, but in a matter of days by storm winds, and in a matter of hours if the cutting sand is coarse and angular. These results must be looked upon as first approximations, for many factors in nature are either unknown or not reproduced in the experiments. Experiments on frosting and defrosting will ...