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Alignment and the Wind in Southern ,

CHESTER B. BEATY Department of Geography, University of , Lethbridge, Alberta, Canada T1K 3M4

ABSTRACT GEOLOGIC AND PHYSIOGRAPHIC be little detectable relation between these SETTING larger structural features and details of sur- Study of the distinctive pattern of align- face form. ment and geographical distribution of more The study includes all of the non- The Alberta of the Interior than 250 in the of southern mountainous part of Alberta south of lat Plains of is also Alberta shows that (1) the coulees in ques- 51° N., a region roughly 280 x 200 km, ex- part of the study area (Bostock, 1970). Ele- tion have a mean orientation of N. 70° E., tending from the vations decrease from >1,100 m in the west and (2) their spatial distribution is not east to the (Fig. 2). to <800 m in the east. The east to northeast ubiquitous but rather displays a concentra- The southern plains of the province are regional slope has controlled development tion in the area from Lethbridge west to the underlain predominantly by horizontal or of the preglacial and postglacial master . near-horizontal sedimentary rocks of Late streams (Stalker, 1961; Farvolden, 1963). Several possible hypotheses of origin of Cretaceous age (Williams and Burk, 1964), Local relief in of the region is <40 m, the aligned coulees are considered, includ- generally veneered by glacial, interglacial, although the larger postglacial have ing subsurface structural control, the role of and postglacial deposits. Large subsurface incised segments of their courses to depths regional slope, the effect of lithologic dif- structures, which in places impart gentle >100 m, and Ridge, the Por- ferences, and wind action. easterly and westerly dips to the sandstone cupine Hills, and the (see Fig. Action of postglacial wind, operating to and shale strata, include the ex- 2 for locations) rise hundreds of meters initiate surface furrows (by wind-driven tension of the Sweetgrass Arch and, in the above the surface. The overall visual or ) that were enlarged by run- west, the Alberta Syncline. There appears to impression, however, is of a broadly rolling ning water, accounts for the three outstand- ing characteristics of the aligned coulees: (1) their preferred orientation of N. 70° E., which approximates the mean direction of the strongest chinook winds in the plains; (2) their geographical dis- tribution, which coincides with that part of the region experiencing the most pro- nounced chinooks; and (3) their almost ex- clusive location on windward topographic surfaces. No other hypothesis of origin is known that can satisfactorily explain all of these observed facts. Key words: geomor- phology, Holocene, drainage patterns, coulee origin, , wind action, Alberta.

INTRODUCTION

Casual examination of aerial photo- ^^fciS^^i graphs of the plains portion of southern Al- ~ "^HK^^^^B^l

berta revealed the existence of a series of ^ ( B^. short, narrow, parallel or subparallel J^^^H^^ -

coulees tributary to stream valleys (Fig. 1). ^ ^^ „, f lc Systematic examination of the photos indi- ^ jj^L^ * cated that the distribution of such coulees «^B 1 ^^ shows a decided concentration in the west- j,4 v ^^BL ^ ern half of the region. This study offers an explanation for the striking degree of paral- » lelism and pattern of geographical distribu- " tion of the aligned coulees based on the ac- Figure 1. Aligned coulees along of Lethbridge, Alberta; view toward north- tion of postglacial wind. east.

Geological Society of America Bulletin, v. 86, p. 119-128, 9 figs., 1975, Doc. no. 50116.

119

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Longitudinal and Parabolic s«. Aligned Coulees

Kilometers

115°w Figure 2. Index map of southern Alberta showing rivers, outstanding topographic features, and settlements mentioned in text. General locations of aligned coulees and four areas of stabilized dunes are indicated.

till plain, interrupted by stream-cut valleys runoff route to the depression periods of copious snowmelt. Average di- and an occasional gentle topographic emi- had opened 8,000 to 6,000 yr ago, and it is mensions of a coulee in the region would nence. here assumed that the major rivers of the fall within the following ranges: length, 100 region were in, or close to, their contem- to 500 m; width, 50 to 200 m; depth at LATE-GLACIAL AND porary locations by that time. The precise mouth, 50 to 80 m. POSTGLACIAL HISTORY courses attained by late-glacial and postgla- of glacial and geomor- cial streams must have been conditioned by PATTERN OF ALIGNMENT phology do not agree on the number and at least three factors: (1) "initial" ir- Though most of the valleys in the plains age of Laurentide glaciations that took regularities and regional slope of the emer- of southern Alberta have short, steep place in the plains of western Canada (for gent surface; (2) relative resistance of earth tributaries, the pattern of alignment of the an instructive introduction to the problem, materials encountered; and (3) positions of coulees differs throughout the area. Most see Reeves, 1973). This stud)' will not at- the receding or stagnating ice front(s). As coulees east of Lethbridge are oriented ap- tempt to bring out of the conflicting the rivers became established in their pres- proximately normal to valley axes (Fig. 4). versions of the Pleistocene stratigraphy. ent courses and valley deepening pro- They appear to be consequent drainage The coulees are of late-glacial or postglacial ceeded, conditions favorable for coulee lines, cut by surface runoff taking the most age; thus, the late-glacial and postglacial formation came into being. direct route down the walls of the valleys. geomorphic history of the region is of In a zone extending roughly from primary interest. COULEES Lethbridge west to the mountains, how- Basing their estimates on physiographic General Morphology ever, a decidedly preferred orientation is evidence and a limited number of radiocar- evident in many clusters of coulees along bon dates, investigators have suggested that As defined for purposes of this study, a segments of the Oldman, St. Mary, Belly, the southern part of Alberta was ice free by "coulee" is a short, often straight, narrow, and Castle Rivers, as well as along several 15,000 to 12,000 yr ago (Gravenor and comparatively steep valley tributary to one smaller streams on the southern slopes of Bayrock, 1965; Westgate, 1968; Bryson of the perennial streams (Fig. 3). Such the Hills (Fig. 5). As determined and others, 1969; Prest, 196S1). Maps pre- coulees generally are dry valleys; they carry from measurements on aerial photographs, pared by Bryson and others (1969) and surficial runoff only during and shortly the mean trend of more than 250 such Prest (1969) indicate that an ice-free surface after heavy rain and, more rarely, during coulees is N. 70° E., with orientation vary-

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unglaciated parts of , South Dakota, , Wyoming, and Ne- braska and discussed four possible hypoth- eses of origin (1929, p. 251): (1) influence of regional slope, (2) differential erosion of weak and resistant strata, (3) effect of struc- tural deformation, and (4) effect of wind. Russell concluded (1929, p. 255) that prevailing wind, acting to build longitudi- nal dunes with a northwest-southeast orien- tation, which would control the location and pattern of surface streams, was proba- bly responsible. As a result of later studies in South Dakota, Flint (1955, p. 156-160) and Crandell (1958, p. 45-50) reached comparable conclusions regarding the ef- fectiveness of wind, as have other inves- tigators of surficial geology (for example, Price, 1943). On the other hand, a large body of litera- ture suggests that subsurface structural fea- tures, principally faults and joints, are re- sponsible for surficial lineaments in parts of the earth's surface, including many of those in the Great Plains (Blanchet, 1957; Sikabonyi, 1957; Mollard, 1957a, 1957b; Kupsch and Wild, 1958; Lattman and Matzke, 1961; Barton, 1962; Robinson and others, 1969; Nielsen, 1971; Ozoray, 1972). Speculation about causes for linea- ments in southern Alberta favors the Figure 3. Cluster of aligned coulees along Castle River west of , Alberta; view to- hypothesis of subsurface control, the argu- ward northeast. Buildings of on river bottom give scale. ment being that "reflection" of faults or joints through unconsolidated overburden ing from N. 60° E. to N. 90° E.; 85 percent rotational slumping has modified some creates linear zones of potential weakness of the coulees in question have an align- coulee walls, especially those facing north, that can be exploited by surficial weather- ment within 2°ofN. 70° E. The distribution northeast, and east (Beaty, 1972). Finally, ing and gradation. of the coulees with preferred alignment is the floor of the average coulee tends to be Nonetheless, the action of postglacial not random with respect to surface mor- flat and smooth, varying in width from 1 m wind provides an explanation for both the phology; most are on valley walls or the to as much as 10 to 12 m. Some coulees preferred orientation and geographical dis- flanks of topographic eminences with a have actively incising channels on their tribution of aligned coulees that is in many southwest (or windward) exposure. One of floors, but most do not, which suggests that ways more satisfactory than that dependent the areas of spectacular development of enlargement has ceased or at least slowed on subsurface structural control. preferred orientation is along the Oldman considerably in many. Coulee Distribution and Wind. Figure 2 River near Lethbridge (Fig. 6), where the The coulees appear to have originated shows the location of aligned coulees and marked parallelism and the tendency to primarily by erosion from ephemeral sur- also the position and orientation of four occur on windward valley walls are well face runoff, coupled with mass movements areas of longitudinal and parabolic dunes, displayed. and other slope processes on their walls. most of which are stabilized by vegetation. If coulees with preferred orientation were The key event in the evolution of a given A line circumscribing the fields and widespread in the southern Alberta plains, coulee must have been initiation of relief coulees delineates that part of southern Al- a broadly regional causal factor could be development in the valley to which it would berta subject to the strongest, most persis- sought, but the restricted geographical dis- become tributary. As the late-glacial and tent west-southwest winds, the so-called tribution suggests that a more localized postglacial streams incised their present val- chinook (Longley, 1967). The area of control has been exerted in their formation. leys, the necessary relief was provided. prominent chinook winds is downwind of All of the coulees in southern Alberta are and in line with , a major ORIGIN OF PREFERRED believed to have been formed by the same topographic break in the southern Front COULEE ALIGNMENT general set of gradational processes. The Ranges of the Rockies. The reason for the striking pattern of alignment probable "funneling" effect of the pass on Coulee Formation in a restricted part of the region is much less wind velocity be sustained in the plains certain. by Milk River Ridge and the Porcupine Coulees in the study area have a number Hills, between which the strongest winds of morphologic characteristics suggestive of Causes of Preferred Alignment appear to blow. their mode of origin. First, the usually The only long-time wind records availa- smooth, long profile is concave upward. In One of the earlier accounts of drainage ble for the region are from Pincher Creek, cross section, most coulees resemble a alignment in the North American Great Lethbridge, and Hat (see Fig. 2 broadly flaring "V," with comparatively Plains was presented by Russell (1929). He for locations). However, examination of straight side slopes. However, large-scale described the phenomenon as it occurs in diagrams depicting directional per-

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centage frequency and average wind speeds at the three stations is potentially useful (Fig. 7); for purposes of comparison, the wind roses for are included. The Kilometers data clearly indicate that west to southwest winds dominate the yearly surficial air-flow patterns at all three of the southern M localities; the pattern at Calgary is notably HI ] > different. More importantly, mean ve- locities are higher at Lethbridge and Pincher Creek than at ; this relation prevails on both a yearly and month-to-month basis. In terms of maximum observed hourly speeds and

short-time recorded gusts, both Lethbridge 0sko/< and Pincher Creek have experienced sig- nificantly higher velocities than Medicine Oldr, \±L/ Hat (Table 1). Without exception, the strongest gusts and sustained winds at Lethbridge and Pincher Creek have been TrrrnN /7T\v westerly to southwesterly, occurring most often during the fall and following the passage of cold fronts, while the greatest velocities at Medicine Hat have been southeasterly; at Calgary, the strongest recorded winds have been north- Grassy /y Coulees erly (Department of Transport, 1968). Available meteorological records are compatible with personal observations, his- torical accounts, and testimony of residents Figure 4. Map showing coulee development along South below junction of in suggesting that the most persistent west Oldman and Bow Rivers east of Lethbridge, Alberta. Alignment of coulees normal to valley axis is to southwest winds blow generally in the evident. Traced from 197C aerial photograph. Compare with Figure 5.

Figure 5. Detailed map showing location of aligned coulees from Lethbridge west to Rocky Mountain Foothills. Coulee locations and orientations determined from 1970 aerial photographs. Compare with Figure 4.

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TABLE 1. SOUTHERN ALBERTA WIND VELOCITY STATISTICS*

Maximum Maximum hourly speeds gust speeds (mph) (mph)

Pincher Creek 85 1Ì0 Lethbridge 77 106 Medicine Hat 60 90 Calgary 65 79

* Source: Department of Transport, Meteorological Branch, 1968. Possible Structural Control. References to southern Alberta are not many in the lit- erature dealing with structural implications of surficial lineaments. Nielsen (1971, p. 23, 25) described and mapped a series of northwest- and northeast-trending linea- ments in the immediate Lethbridge area, the pattern of which, he suggests, resembles joint sets. Babcock (1973) discerned two major regional orthogonal joint systems in southern Alberta; that of interest to the present study has averaged sets striking N. 65° E. and N. 25° W. Employing spatial filtering techniques for purposes of struc- tural analysis, Robinson and others (1969) produced a series of maps depicting ortho- gonal intermediate-scale structural trends in the subsurface of the central and southern plains of Alberta, the most prominent of which are northeast and northwest; they suggest (1969, p. 2366) that the structures thus revealed are possibly basement blocks of low relief bounded by faults, and that these structures may have exerted control on, among other things, "... erosional pat- terns. . . ." Ozoray (1972) discussed possi- ble structural control of linear morphologic elements in the province and published a map (p. 36) showing what is termed the "Geometric Drainage Pattern ..." in Al- berta and the existence of what are labeled "NW-SE & SW-NE directed structural elements." Also, Westgate (1968, p. 13) mentioned the presence of surficial linea- ments in southeastern Alberta that, in his , cannot be explained by glacia- tion; he stated "It is believed that these lineaments represent fault or joint traces that have been reflected through the uncon- Figure 6. Vertical aerial photograph of Lethbridge area showing pattern of coulee alignment along solidated drift." Oldman River. North, top of photo. Concentration of aligned coulees on windward valley walls is conspicuous. Photo A21814-26, Natl. Aii Photo Library, Dept. Energy, Mines and Resources, Ot- Thus, evidence of a regional orthogonal tawa. pattern of subsurface joints or faults, trend- area from Lethbridge to the Rocky Moun- cial and bedrock geologic maps of southern ing roughly parallel and normal to the tain front; east of Lethbridge, the regularity Alberta disclose that the distribution of Rocky Mountain front, is not lacking in the and intensity of the chinooks declines con- aligned coulees displays no discernible rela- plains of southern Alberta. What is lacking spicuously. Furthermore, from Lethbridge tion with either bedrock type (mostly is evidence demonstrating an unequivocal to the mountains, both prevailing winds sandstone or shale) or surficial materials. genetic connection between a known sub- and the strongest gusts come from within a Coulees with preferred orientation are surface structural feature and a given linear degree or two of west-southwest. The close present on at least seven different forma- surface feature. What is further lacking is a parallelism between orientation of the tions ranging in age from Late Cretaceous satisfactory explanation of why subsurface aligned coulees (N. 70° E.) and mean direc- to Paleocene (Irish, 1968), with no detecta- structural control — if it really can be as ef- tion of prevailing and strongest winds ble preference for any particular one. Surfi- fective in determining location and align- (west-southwest: S. 67.5° W.) is apparent. cial materials into which coulees have been ment of surficial lineaments as has been Similarly, the geographical concentration of cut include loess, alluvium, lacustrine de- suggested — does not seem to have oper- such coulees in the zone of most intense posits, outwash, and till (Stalker, 1959, ated in the formation of aligned coulees chinooks is evident. 1962), and as is the case with bedrock, no throughout all of the region; the spatial dis- Earth Materials Involved in Coulee Mak- recognizable relation between coulee loca- tribution of such coulees suggests strongly ing. Field examination and study of surfi- tion and specific material is observable. that it hasn't.

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The possibility that subsurface structures Annual Percentage Frequency Annual Average Wind Speed (mph) have controlled the location and pattern of % calm shown ;n inner circle surface lineaments also does not account for the fact that the most pronounced de- velopment of coulees with an average orien- tation of N. 70° E. has occurred on wind- ward slopes. If structural control were the sole or primary genetic factor, the distribu- tion of such coulees presumably should be random with respect to slope aspect vis-a-vis the wind. It isn't; the concentra- wsw- -ESE tion on windward topographic surfaces of coulees with preferred orientation is strik- ing (see Figs. 5 and 6). An example of lack of correlation be- Pincher Creek tween orientation of a known subsurface structural feature and alignment of coulees is present in the western part of the study area (Fig. 8). Here, bedrock is nonmarine sandstone and shale of the Paleocene Por- cupine Hills formation, and the dominant joints are organized into two well-defined sets that strike approximately N. 20° E. and N. 60° W. The coulees along the Oldman River and its northbank tributaries, how- ever, have a consistent mean trend of about N. 70° E. (Fig. 9). The similarity between the pattern of coulee alignment and the geometry of the wind roses for nearby Pincher Creek is notable. And, it should be emphasized, most of the aligned coulees are Lethbridge on windward slopes. It is therefore concluded that, while sub- surface structural elements m£.y play a role in the origin of surface lineaments in many parts of the earth, such control has not been strong in the formation of most aligned coulees in southern Alberta. Their restricted geographical distribution and tendency to occur on windward slopes would seem to require a mode of origin controlled by some other factor(s). Other Possible Controlling Factors. In addition to wind control and the effects of structural deformation, Russell (1929, p. 251) mentioned two other possible explan- Medicine Hat ations for the origin of aligned stream courses: (1) the influence of regional slope and (2) the differential erosion, of weak and resistant strata. As noted above, regional slope has prob- ably dictated the general direction of flow of the master streams of the A lberta plains. Modifications in drainage pattern undoubt- edly were brought about by glacial deposi- tion and the late-glacial and postglacial draining of a number of proglacial , but the gross pattern of postglacial river de- velopment must have been ir_ accord with the east to northeast regional slope. Because the tributary coulees formed concomitant Calgary with cutting of the river valleys, only in the most indirect way could their orientation Figure 7. Rose diagrams showing directional percentage frequency and average annual wind speed and geographical distribution be related to for weather stations at Pincher Creek, Lethbridge, Medicine Hat, and Calgary, Alberta. Source: Dept. regional slope. Other things being equal, Transport, Meteorological Branch, 1968.

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Figure 8. Detailed map showing coulee location and alignment along Oldman River and north-bank tributaries north of , Alberta. The joint rose, based on measurement of 154 joints in three adjoining legal sections centering near the "north" arrow, is from Babcock (1973). Discrepancy between pattern of coulee alignment and orientation of dominant joints is apparent. Traced from 1970 aerial photograph. most coulees should have been consequent, Concerning the effect of differential ero- There is no discernible relation between with orientation normal to valley axes. East sion of weak and resistent strata, there is lit- bedrock type and the location of coulees of Lethbridge this is generally the case, but tie evidence to suggest that this factor has with preferred alignment; they are found on west of it different conditions prevail, been of importance in coulee evolution. all formations between Lethbridge and the

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mountains. Furthermore, the horizontal or very gently dipping attitudes of the sedimentary rocks of the Alberta plains as- sure that only rarely and locally are strata with significant differences in weathering or erosional resistance in juxtaposition at the surface. Regional slope and differential erosion of contrasting strata can thus be ruled out as significant contributing or controlling fac- tors in the origin of coulee alignment. Coulees and Wind. Evidence implying a genetic relation between wind and coulee alignment has been discussed above: mean alignment of the coulees, most of which are on windward slopes, corresponds closely with direction of strongest winds in that part of the province experiencing pro- nounced chinooks. Additionally, the pat- tern of alignment the orientation of stabilized dunes, suggesting that direc- tion of effective wind in dune building may also have been a factor in coulee formation. Finally, maximum gusts and hourly winds at Lethbridge and Pincher Creek are west- erly to southwesterly; whereas at Medicine Hat and Calgary outside the area of most intense chinooks, the greatest recorded ve- locities have been from the southeast and north, respectively. In short, the strongest sustained winds and gusts in the area of preferred coulee alignment come from a di- rection consistent with a wind-related model of origin. However, associating coulee alignment cial lake sediments and appear to little and deepening of the embryonic valleys and geographical distribution with the genetic relation to coulees with preferred created sufficient relief for the initiation of wind is one thing, specifying the details of alignment: they are in the wrong places to coulee development. In general, the mate- the association another. Four conceivable have had any effect on coulee evolution. rial in which coulee incision began was till, causes may be suggested: (1) The coulees Prairie uplands between adjacent aligned covered in places by lacustrine silts and eo- have been excavated primarily by deflation; coulees lack elongated masses of eolian sed- lian sediments, but homogeneous on a re- they would be, in effect, modified yardang iments, and while it is not beyond the realm gional scale. troughs (Blackwelder, 1934, p. 161). (2) of possibility that dunes may once have 2. Windward topographic surfaces Coulee formation has been controlled, in existed in such places, it seems unlikely that within the chinook belt are (and were) af- , by development of all traces of their formation would have fected in at least two ways by strong west longitudinal dunes. (3) The coulees were been erased in the short span of postglacial winds: (a) they retain less snow than east-, cut by snowmelt runoff originating from time. northeast-, and north-facing slopes; (b) elongated drifts. (4) The coulees have been The possible effects of linear snowdrifts their is considerably drier eroded by ephemeral surface runoff taking are demonstrated by narrow drifts that than that of the lee slopes, and the vegeta- advantage of furrows initiated by wind- build up downwind of surface obstacles at tion cover is sparser. As a result, such sur- driven snow or rain. times of strong chinook winds. There is faces are more susceptible to erosion. Deflation as a primary factor in coulee perhaps' a stronger tendency for wind- 3. Accordingly, on many of the south- formation seems unlikely for one basic blown snow to accumulate differentially on west-facing valley walls, wind-driven snow reason: almost without exception the north-, northeast-, and east-facing slopes, and rain differentially carved narrow, shal- coulees lack recognizable deposits of but northeast-southwest drifts are observa- low, elongated furrows that were later en- wind-blown materials on the downwind ble on occasion. Thus, availability of melt- larged by surface runoff;1 the orientation side (see Figs. 1, 3, 9). Although some mate- water from appropriately oriented linear of such furrows determined the rial could have been blown from walls and sources may have played a contributary pattern attained by the aligned coulees. The floors early in the history of coulee en- role in coulee origin. process is believed to have been most effec- largement, the almost universal lack of sig- However, the most probable wind- related cause of coulee alignment is pos- nificant wind-blown deposits in appro- 1 priate geographical proximity argues tulated to have been formation of surface That wind-driven snow is capable of corrasive ac- tion seems beyond dispute; reports of its effectiveness strongly against a deflative origin. furrows by wind-driven snow or rain, have come from subpolar localities in both hemispheres. The possibility that development of lon- which subsequently were enlarged by surfi- If wind-driven snow can polish and corrade bedrock gitudinal dunes has controlled coulee for- cial runoff. A feasible scenario follows: (see, for example, Teichert, 1939; Blackwelder, 1940; Fristrup, 1953, p. 56-57; Hamelin and Cook, 1967, p. 1. By 8,000 to 6,000 yr ago, the master mation also seems remote. The four dune 131), it is not unreasonable to speculate that under fields in the study area (see Fig. 2) are all streams of the southern Alberta plains were favorable circumstances it could also cut surficial fur- downwind from extensive areas of progla- in approximately their present locations, rows in unconsolidated glacial and postglacial deposits.

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five during fall and winter when (a) the approaching that of the strongest southwest effectively exerted in the formation of most strongest chinook winds occur, (b) many of winds. Departure from a direct of the aligned coulees in southern Alberta, the windward slopes are at times unfrozen down the slopes is a matter of only a few nor have regional slope, location of lon- and free of snow, and (c) the limited vegeta- degrees. The cuts are in till and were made gitudinal dunes, or differential erosion of tion cover is dead or dormant, thus in the of 1970; most are now vege- weak and resistant strata been of sig- minimal protection against surface distur- tated, and the surficial materials have fre- nificance. Postglacial winds, strongest from bance. Precisely where a given furrow quently been disturbed by frost action. the west-southwest, appear to have played a would have been formed must have, been Nevertheless, orientation of some of the direct role in coulee evolution. It is proba- dependent on impingement of strong sur- channels implies control by a factor other ble that the coulees have been excavated face wind against a bare, essentially unpro- than gravity, and the rough parallelism primarily by ephemeral surface runoff, but tected slope and would have been a matter with the pattern of strongest winds is their mean orientation of N. 70° E., their of chance. Once initiated, however, such suggestive if not overwhelming evidence limited rather than universal geographical furrows would have become the loci for that chinook winds have played a role in distribution in the region, and their location surficial runoff in their immediate vicinity. determination of channel alignment. predominantly on windward topographic 4. With occasional summer thun- Paleowind Directions. The proposed surfaces all point to wind as a major factor derstorms providing most of the runoff, model of coulee orientation is predicated on in their origin. erosional deepening and lengthening of the the assumption that contemporary wind furrows converted them from inconsequen- data are representative of surface flow pat- ACKNOWLEDGMENTS tial initial forms to recognizable, evolving terns in the region for the last 6,000 to drainage systems. 8,000 yr. Indirect evidence for persistence The manuscript was reviewed critically, 5. As the youthful coulees were enlarged of wind patterns during postglacial time is even ruthlessly, by A. E. Babcock, H.A.K. and thus became more effective snow traps, represented by the four dune fields men- Charlesworth, D. J. Easterbrook, and R. J. elongated drifts accumulated from time to tioned above. Parabolic and longitudinal Janda, and some of their suggestions were time on the upper parts of their northwest- dunes are also present in other parts of Al- incorporated. facing walls, as they do today. Melting of berta (Odynsky, 1958; Henderson, 1959; Tremblay, 1961). Those north of the study such drifts supplied a further source of REFERENCES CITED water for coulee growth. Some material area have northwest or west trends, while the orientation of dunes in the south indi- may have been deflated from floors and Babcock, A. E., 1973, Regional jointing in south- walls, but coulee enlargement was ac- cates effective west to southwest postglacial ern Alberta: Canadian Jour. Earth Sci., v. complished primarily by the erosional work winds (see Odynsky, 1958, map, p. 58). 10, p. 1769-1781. of running water, augmented by mass Obviously, such winds were active in Barton, R. H., 1962, Differential isostatic re- movements on the side slopes. southern Alberta for a sufficient time to bound — Possible mechanism for fault 6. Coulee alignment on many windward allow development of the dunes. It is not reflection through glacial drift: Am. Assoc. Geologists Bull., v. 46, p. slopes from Lethbridge west to the moun- known when the dunes were formed and the length of time required. 2253-2254. tains was thus strongly conditioned by the Beaty, C. B., 1972, Geographical distribution of wind (see Fig. 5). In marked contrast, east A widespread search for ventifacts was postglacial slumping in southern Alberta: of the , where the intensity of chinook undertaken in the hope of finding further Canadian Geotech. Jour., v. 9, p. 219-224. winds declines, coulee development fol- evidence of paleowind directions. Much of Blackwelder, E., 1934, Yardangs: Geol. Soc. lowed a consequent pattern, as evidenced the southern Alberta prairie has been dis- America Bull., v. 45, p. 159-166. by the fact that most coulees there are turbed by man, so possible ventifact 1940, The hardness of ice: Am. Jour. Sci., v. oriented normal to valley axes (see Fig. 4). localities are limited mainly to the unculti- 238, p. 61-62. 7. The postulated process of coulee vated "breaks" along the larger valleys. Al- Blanchet, P. H., 1957, Development of fracture evolution is envisaged to have continued, though no "textbook" ventifacts were analysis as exploration method: Am. Assoc. Petroleum Geologists Bull., v. 41, p. intermittently, so long as valley deepening found, many of the larger undisturbed 1748-1759. rocks at the surface display features sugges- and related relief accentuation progressed. Bostock, H. A., 1970, Physiographic subdivi- tive of wind action. Often such rocks, most The developmental rate has probably de- sions of Canada, in Douglas, R.J.W., ed., creased as the master streams of the region of which are crystalline erratics from the Geology and economic minerals of Canada have approached or attained their pregla- , (1) are detectably (5th ed.): Canada Geol. Survey, Econ. cial depths of incision. smoother to the touch on the west and Geology Rept. no. 1, p. 9-30. The model of aligned coulee formation southwest surfaces, a condition possibly in- Bryson, R. A., Wendland, W. M., Ives, J. D., and outlined above would be strengthened by dicative of natural sandblast and (2) have Andrews, J. T., 1969, Radiocarbon iso- observational evidence of the supposed little or no lichen growth on windward sur- chrones on the disintegration of the Lauren- tide ice sheet: and Alpine Research, process of coulee initiation, namely, crea- faces, whereas their lee sides support lichen . v. 1, p. 1-14. tion of surficial furrows by wind-driven Crandell, D. R., 1958, Geology of the Pierre snow or rain. Laboratory attempts to dup- Orientation of aligned dune fields and the area, South Dakota: U.S. Geol. Survey Prof. licate the process with an electric fan and a condition of many boulders at the surface Paper 307, p. 45-50. variety of dry and moist earth materials thus add weight to the supposition that Department of Transport, Meteorological were generally unsuccessful. However, - postglacial prevailing and effective winds Branch, 1968, Climatic normals, Vol. 5, servation of development of shallow run- have had a strong west-southwest compo- Wind: , p. 22-29. nels on road cuts in the vicinity of the Uni- nent. Farvolden, R. N., 1963, Bedrock channels of versity of Lethbridge during the past four southern Alberta, in Early contributions to the of Alberta: Re- offers more promise. There is a SUMMARY search Council Alberta Bull. 12, p. 63-75. suggestion of wind action on some of the Flint, R. F., 1955, Pleistocene geology of eastern cuts because small channels eroded by Although subsurface structural charac- South Dakota: U.S. Geol. Survey Prof. ephemeral runoff do not cross contours at teristics may control development of linear Paper 262, p. 156-160. right angles but rather tend to be aligned morphologic features at the surface in many Fristrup, B., 1953, Wind erosion within the Arc- diagonally across the slopes in a direction parts of the earth, such control has not been tic deserts: Geog. Tidsskr., v. 52, p. 51-65.

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