Sea ice pressure ridge study: An airphoto analysis

Photogrammetria - Elsevier Publishing Company, Amsterdam

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SEA ICE PRESSURE R I D G E STUDY: AN A I R P H O T O ANALYSIS V. H. A N D E R S O N l Photographic Interpretation Research Division, U.S. Army, Cold Regions Research and Engineering Laboratory, Hanover, N.H. (U.S.A.I (Received May 4, 1970) SUMMARY

Tested and proven techniques of terrain analysis using conventional aerial photography were applied to interpret the patterns associated with a sea ice environment. Ages and relative thicknesses of sea ice masses were determined from stereoscopic aerial photography. A classification scheme of sea ice pressure ridges is developed based upon their linear surface trace, their relative ages, their heights, widths and lengths, their location relative to recent ice movement, and the size of the material composing the ridges. The significance of sinuous ridges with respect to straight ridges is discussed relative to the forces involved in their formation. Estimates as to the underwater components of pressure ridges are deduced based upon elements of their surface configuration and pattern. INTRODUCTION

In response to a U.S. Coast Guard requirement to develop a program to study the sea ice environment off the coasts of Alaska, an initial effort was made to examine the extent of information that could be derived solely through an analysis of conventional aerial photography of sea ice in an attempt to develop a classification scheme for pressure ridges. The objectives of the remote sensing aspect of the pressure ridge program were: (1) to acquire stereographic aerial photography (conventional panchromatic) of an area of arctic sea ice north of Barrow, Alaska; (2) to analyze the aerial imagery using terrain analysis techniques developed by the Photographic Interpretation Research Division of U.S.A.C.R.R.E.L. (U.S. Army, Cold Regions Research and Engineering Laboratory) in order to delineate the areal distribution and orientation of pressure ridges, frozen leads and coherent ice masses; and (3) to develop a categorization of pressure ridges as viewed on aerial imagery which could be used as a guide in future field examinations. The manner and extent to which these objectives have been met is documented in this report. No "ground truth" data were acquired on the sea ice during the actual aerial sensing mission. All deductions and conclusions drawn concerning the physical properties and modes of origin of both the sea ice and its pressure ridges discussed herein were derived from the interpretation and analysis of the photol Present address: Photographic Interpretation Corporation, Box 868, Hanover, N.H. (U.S.A.).

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graphic imagery, and are based upon the author's personal experiences both on and above sea ice during numerous past programs of remote sensing in arctic environments. A I R P H O T O A N A L Y S I S O F T H E SI'A I C E IN T H E STUDY A R E A

A real distribution of the sea ice A reduced copy of the photo mosaic of the study area acquired on 14 May 1969 is presented in Fig. 1. An overlay is attached to this photo mosaic which serves to enhance the delineation of the various sizes and configurations of ice floc~ within the study area. Breakdown of ice patterns. The study area is comprised of the following sea ice features, some of which have been labeled or shaded as follows on the overlav: (l~ Relatively large, coherent, and generally rounded ice floes. (rod A series of rather well-rounded, medium-size ice floes. (bb) Areas of severely broken ice containing brash and block ice and small ice floes with newly forming ice acting as a bonding materi~l for the entire mass. (opaque) New, very thin ice in recent openings within the sea ice mass with some associated open water. Each of these features can be easily identified on the photo mosaic. Samples of each of these categories have been labeled on the overlay, while open water areas, or areas where new, very thin ice has just recently formed, have been made a solid tone. The fine interstitial brash and block ice between the identifiable rounded ice floes have been left clear. Discussion oJ the ice patterns. (1) The large, coherent and generally rounded ice floes labeled (//3. These ice floes have been grouped together under a single category because it was considered that, due to their size, they can be a destructive influence to each other and any smaller material if they come into contact with one another. Each is judged massive and coherent enough to cause alteration of equal or larger size ice floes and all smaller ice floes if and when they are put in a compressional situation. It can be observed on the mosaic (Fig. l) that many of these large floes are in compressional contact with each other and that large, sinuous pressure ridges have been formed along their common boundaries. The sinuous ridging between the large floes, 1 and 2 as identified on the overlay of Fig. l. represent this type of mutual edge destruction, (2) Well-rounded, medium-size floes labeled (roD. A category of medium floe (mr) has been created to include ice floes which position themselves like "'ball bearings" between larger ice floes and through which primarily rotational forces are evidenced. The rate of rotation of the medium-size floes is quite a bit faster than that of the large-size floes. These "'medium floes" are generally interspersed Photogrammetria,

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with the larger floes in a random pattern. The medium floes represent a stage of ice floe deterioration between that of the large floes where the rounding of a floe's perimeter begins and that of the block and brash ice which contains the disintegrated floe ice along with disturbed, freshly formed thin ice. (3) The areas of block and brash ice labeled (bb). The block and brash ice zones (bb) can be seen to form the matrix within which the ice floes are contained. The material is the ground-up residue of former thick ice floes and the thin new ice which is constantly being formed on open sea water during the cold season ot the year. Many of the very small ice floes within the block and brash ice areas arc seen to have raised, broken ice rims about their edges, which are caused by their continual abrasion with their counterparts in a highly mobile and rotational environment. New ice forming between these small blocks is constantly being broken up with each new period of ice movement in the area and all this thin, new ice is pushed about and ground up into brash and in some cases is shoved up on top of small ice blocks, forming raised rims. Block and brash ice, if allowed to remain undisturbed, will thicken through natural sea ice growth and bonding processes until it becomes thick enough to withstand ice motion pressures and will begin to behave once again as a coherent ice floe. Areas of reconsolidated block and brash ice can be noted throughout the right half of the photo mosaic of Fig. I, while the block and brash ice evident in the left half is much more mobile and unconsolidated in nature. (4) Open water or very thin, new ice (the opaque areas on the overlay). Finally, the opaque tone on the overlay of Fig.1 represents virtually open water areas. In some cases, there is evidence of very thin, new ice in these openings; however, they are treated as open water areas within the ice cover. It is interesting to note that in almost all cases, any brash or slush ice associated with these openings is packed toward the lower edge of these openings, thereby suggesting that the wind is currently active in packing this material in one general direction throughout the mosaic. Also, the smooth light tone of wind-packed slush ice can be identified in most instances as forming a boundary with the new, very thin. young ice. This slush ice pattern is formed by the wind breaking up newly formed ice in the open water and compacting it in concentric bands downwind from the open water along a resistant ice shore. These open water areas which are attempting to freeze over represent the most recent openings in the ice within the study area and currently are the obvious zones of weakness within the ice cover as a whole. A ge o / t h e sea ice in the study area The significance of the age of the ice contained within the study area lies in the fact that perennial sea ice in the Arctic is normally thicker than the thickest seasonal ice. Perennial sea ice can attain a thickness, due to normal growth, of 9 - 1 2 ft., whereas seasonal or winter ice in the Arctic is limited to a thickness of approximately 6 - 7 ft. Photogrammetria, 26 (1970) 20l-228

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Each of these two types of ice is distinctive and to a degree can be dif~ ferentiated on stereoscopic aerial photography. The basis of the different patterns created by these two types of sea ice lies in the fact that the perennial sea ice has undergone at least one cycle of erosion and has survived to continue its growth, whereas the seasonal ice has not yet completed this cycle. The marks of surface deterioration of an ice floe withstanding an arctic melting season remain on the floe and often are evident to aerial surveillance even under the following winter's snow cover. Ice surface deterioration effects caused by the ice having gone through a melt season consist of: (1) the weathering, rounding and subduing of normally sharp, high-standing pressure ridges; (2) the ablation of small pressure ridges through weathering creating isolated hummocks on the ice; (3) the creation of melt pools and their subsequent refreezing; and (4) the establishment in some cases of a subdued drainage pattern on the ice surface as melt water runs off into the ocean. As the ice grows older and thicker, withstanding many melt seasons, the smooth, level ice areas within a floe diminish and the ice floe attains a very rough surface texture. Old weathering features, as described above, were not noted on any of the ice present within the study area. Instead, the ice contains all the evidence of youth; fresh, crisp and tall pressure ridges composed of angular blocks of ice, snow drifts associated with all but the most current pressure ridges and even "barchane" type snow dunes deposited upon large expanses of level ice (such as indicated in the area marked C on the photo mosaic overlay to Fig.l). These features indicate sea ice of one season's growth and would imply that the maximum thickness of ice due to the natural growth process (not pressure ridging) would be 6 - 7 ft.

Indicators of actual ice thickness As one stereoscopically examines the aerial photography of the study area, numerous instances can be found where broken blocks and plates of sea ice are standing on end, presenting to the viewer an opportunity to accurately measure the ice thickness at various locations. Fig.2 is a stereoscopic example of two ages of pressured ice; each presenting a few of its broken, upheaved blocks in a position to be accurately measured. Based upon the focal length of the camera system and the recorded altitude of the aircraft, the computed scale of the photography comes out to be 1 : 4,364. At this scale, I m m is equal to about 14 ft. Utilizing a calibrated hand magnifier, the upthrusted ice floe at point A on Fig.2 measures almost 0.5 mm, or about 6.5-7 ft. thick. This represents a piece of freshly upheaved ice and can be considered as a representative thickness for much of the similar-looking ice nearby. Actually, it should be expected that there are a few inches of metamorphosed snow covering the top surface of most of the ice in the area which might reduce the estimated ice thickness to the 6-6.5 ft. bracket. Ice thicknesses were measured on the photos at points B and C which are Photogrammetria,

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Fig.2 Stereo pail showing upturned ice blocks which can be measured to determine ice thickness.

parts of old, snow-drifted pressure ridges. The thicknesses at these points arc about 4 ft. at point B and 3 - 4 ft. at point C. These figures are representative of the ice thicknesses at these points at the time the pressure ridges were formed. Since the ridging is considered fairly old due to the snow drifting associated with these pressure ridges, the unridged ice in the vicinity of points B and C should have increased in thickness through normal sea ice growth and is probably now closer to the measured 6 ft. of ice at point A. A second example of judging ice thickness from stereoscopic imagery is presented in Fig.3. In this figure a comparison can be made between a floe of heavily ridged ice and an adjacent one which is less severely ridged. Both of these floes lie adjacent to a rather fresh opening in the ice mass where open water is exposed. The relative heights of the ice surfaces of each of these floes above sea level can be compared, and, based upon the previous ice thickness calculation from Fig.2, an estimate of the thickness of each floe can be made. If we use the figure derived for the ice thickness at the fresh pressure ridge at point A in Fig.2 (6-6.5 ft. thick) and assume that floe A in Fig.3 is approximately the same, we can utilize the buoyancy factor for salt water ice (about 7 to l Photogrammetria, 26 119701 201 22,',

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Fig.3. Stereo pair showing differing ice thickness of heavily and lightly pressure ridged ice relative to sea level. and figure that the floe rises a maximum of about l ft. above the adjacent sea level. Comparing this height above sea level to the facing edge of floe B, which admittedly is composed partly of pressure ridged ice, we can estimate how many feet higher the facing edge of floe B is above the water than that of floe A. A rough estimate would be about 3 times as high for floe B, thereby concluding the thickness of the ice mass B to be around 25 ft. Granted, these are rather general estimates all based upon the computed scale of the photography. Before leaving Fig.3, it might be noted that the lettered floes (A, B, C and D) have all rather recently separated from one another. There is a good fit between facing edges of floes B and C, and a similar fit between floes C and A. Floe D has separated from floe A and moved toward the upper edge of the illustration about 215 ft. Both ice surface topography and floe cdge configuration can be matched remarkably well in this illustration.

Indicators of ice motion in the imagery Indications of differential ice motion have just been mentioned in discussing Photogrammctria, 26 (1970) 201-228

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Fig.4. Single aerial photograph showing ice motion.

the stereo pair in Fig.3. The sharpness and angularity of the ice floe edges and the presence of open water and relatively unconsolidated block and brash ice are all indications of recent differential ice motion. The roundness of many of the ice floes in the study area (photo mosaic of Fig. 1) is another strong indication of ice motion. The two extensive shear zones within the ice mass, labeled A and B on the overlay to Fig. 1, are indicators of a shearing motion between large segments of the sea ice complex. The tilted, upheaved and ridged ice along the edges of many of the ice floes in the study area are additional indicators of motion. The presence of Photogrammetria, 26 (1970) 201 22~

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open water areas, fresh cracks and splits within the ice mass also are caused by differential ice motion. A single photograph has been included in this discussion on ice motion as Fig.4 and within it one can observe some of the mechanisms causing ice motion. In this example, floes A and B are currently being spread apart by the force of floe C. The rapidly freezing, open water area between these two floes is the result of their separation. In fact, ice in this area, only inches thick, has been torn by the slight movement in the direction of the arrow on floe E. A very small dark tone can be observed adjacent to floe E's bottom edge, which is darker than the thin, newly formed ice. Thin ice having this very dark tone is only a few inches thick. When this young ice reaches a thickness above 7 or 8 inches, its surface becomes dry enough to support a pattern of dry snow on the ice. Up to this point, however, any snow blown out upon this thin ice will be incorporated into the ice surface itself in the form of water or, at most, a weak, transparent slush. The streakedness of some of this young ice is caused by the wind "slushing up" the very thin, newly formed ice and compacting it in bands against the downwind shore of the water opening. As floes A and B split wider and wider apart, floe B comes in compressional contact with floe D. Here a large, sinuous pressure ridge is being formed. Floe C is encroaching violently upon the upper end of floe A, and more very fresh pressure ridges are being created where they meet. The wetness upon the overridden ice in front of lobe 1 of floe C indicates how current the activity is. Finally, there is some very fresh ice breakage in the vicinity of number 2. Here a shearing motion has been set up, as is indicated by the small arrows, which has caused ridging and cracking along a rather straight line. A I R P H O T O ANALYSIS OF PRESSURE RIDGES IN THE STUDY AREA

Density and distribution oJ the larger pressure ridges Fig.5 is a reduced copy of the photomosaic of the study area to which has been attached an overlay tracing all of the large pressure ridges. When viewed alone, the overlay presents no outstanding pattern of pressure ridge alignment or distribution. However, there seem to be two general types of lines on the overlay; one predominantly straight, while the other is quite sinuous or wavey. A close examination of the photo mosaic will reveal large numbers of both types of ridges, as well as a large group which falls somewhere between, more straight than sinuous or more irregular than straight. Each of these types of ridge traces, or a combination of the two, reflects the forces involved in their creation. The long, straight pressure ridges are interpreted as being the result of almost pure shear forces between ice masses with just enough of a compressional vector to cause ridging along the rather straight line of ice fracture. The purely sinuous pressure ridges are believed to have been formed largely by compressional forces where the ice breakage, reflecting both the over and under thrusting of adjacent ice masses, Photogrammetria,

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causes ridging having a sinuous or interlocking pattern. Each of these types is dependent upon the thickness of the ice undergoing deformation. The factor of ice thickness will be treated as the stereoscopic examples of pressure ridge patterns and are discussed in the following pages. Toward the right end of the photo mosaic, the pressure ridges traced out on the overlay appear much shorter than elsewhere in the imagery. This is due to the presence of a very broken, small ice floe condition in this section of the mosaic as compared to the larger more coherent ice masses present elsewhere. The floes are not large enough for any of the traced ridges to have a length and continuity comparable to some of the ridges present on the large ice floes elsewhere in the mosaic. A final observation regarding the distribution of pressure ridges on the mosaic of Fig.5 is that the ridges traced on the overlay generally fall into two areas with regard to the individual ice floes. First, an almost continuous series of pressure ridges appear to be concentrated along the edges of all of the larger ice floes. These ridges, for the most part, represent the locations where current ice ridging and abrasion is taking place, for each of the floes in the study area is undergoing a rounding process as it abrades against its counterparts in a matrix of brash ice and open water. Secondly, a separate collection of pressure ridges exists within the boundaries of each of the larger ice floes. These are relatively old, snow drifted and inactive. Many of the interior ridges terminate at the present ice floe edges, having been sheared off by differential ice motion.

Straight, predominantly shear-type pressure ridges Ridges of this type can be found along the edges of ice floes as well as within the body of the floe. Although one can generalize by calling them sheartype ridges, since the predominant motion creating this long, straight fracture appears to be of a shearing nature, a ridge would not be created if there were not some element of compression involved. Fig.6, the stereoscopic portion of two consecutive airphotos, shows one of these long, straight ridges on the edge of an ice floe adjacent to an area of unconsolidated brash ice. These photos come from the left-hand end of strip 13 on the photomosaic of Fig.5 and represent a sampling of the large shear zone in the ice at this end of the study area. The arrows superimposed upon the photo in Fig.6 represent the direction of relative ice motion. This was determined by the lateral displacement of small blocks within the brash ice with respect to the large ice floe in an area which was covered by photography on adjacent strips representing about 20 to 30 minutes in elapsed time between photos. The freshness of the large pressure ridge is indicated by the lack of drifted snow associated with the ridge and its position with respect to differential ice motion. Also, there are some fresh cracks along the edge of the large ice floe, just a few feet from the large ridge, which again indicate the current ice activity relative to the pressure ridge. Photogrammetria,

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Fig.6. Stereo pair showing an example of a fresh straight pressure ridge formed by , dominant shear force with only a slight degree of compression. Under close stereoscopic examination, it can be noted that the size of much of the material making up the long, straight pressure ridge appears much thinner and smaller than what might be expected if the edge of the floe itself were breaking up and piling on top of itself. This is the same floe which was measured in Fig.2 as being about 6 ft. thick. The ice forming the pressure ridge under discussion is much thinner than this for the most part. It is postulated that the ice forming the bulk of the long, straight ridge in Fig.6 has been formed predominantly by thin. young ice which has been growing within the brash ice zone during periods o[ subdued differential movement. As larger floes pass along this shear zone, crushing the newly-formed ice along with the enclosed brash and block, some of the crushed ice is forced up on top of the edges of the more coherent ice floes as pressure ridges. Since the new thin ice is forming on the open water which is only about 1 ft. below the upper surface of the heavy, 6 ft. thick ice floe, it is mechanically easier to lift the broken ice upward 1 ft. and deposit it than to submerge it 5 ft. to be deposited below the thick ice floe. This may be interpreted as meaning that long, straight ridges which have been formed in similar predominantly shear zone environments might be expected to have rather shallow and insignificant roots. Assuredly, some heavy blocks and pans of ice will probably find their way beneath the edge of a Photogrammetria, 26 (1970) 201 22~

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coherent ice floe, but not nearly as much as might be expected in the case when two massive ice floes come in direct contact under a largely compressive force. This second type of ice ridging, however, will leave something other than a straight trace upon the ice surface and therein lies the means of differentiation between pressure ridges which are suspected of having deep under ice extensions versus those which may not. Sinuous pressure ridges In contrast to the straight pressure ridges associated with a dominant shearing motion, the irregular, sinuous type of ridge is believed to be caused by a predominantly compressive force. As two massive ice floes are brought together under compressional stress, their points of contact cause mutual destruction of the floes' edges and this compression forms an irregular and often a multiple sinuous pressure ridge system containing large blocks of thick ice torn from the floes themselves. When thick ice is being deformed in this manner, a large amount of broken material is forced both upwards and beneath the floes in conflict. Fig.7 is a stereo pair of photographs of such a sinuous pressure ridge system extending from left to right across the image. The much straighter pressure ridge, extending almost from top to bottom of the figure, represents the trace of a shear-type ridge having a large compressional component, hence the buildup of broken thick ice. There is a triangular area of disturbed ice present in the area where the straight ridge joins the sinuous ridge. In stereo viewing, one can see the tilting of fractured ice floes in this area and thereby gain a feeling for the magnitude of the ice being broken in this particular area. As one examines the sinuous ridge stereoscopically, the freshness of portions of its development becomes apparent. Fresh angular blocks of ice are upthrusted on top of the ice surface, the overriding lobes of one ice floe cause a depressional force upon the other floe and a moist zone along this advancing edge is formed by sea water as the ice is depressed below sea level. Examples of this phenomenon are indicated at A, B and C. As long as the compressional forces are operative, the floes will continue to be crushed along this zone of fracturing with the bulk of the crushed and broken ice being pushed beneath the converging floes, forming a vast deposit of broken ice which, after healing and solidifying, will become quite an obstacle for a ship to penetrate. This sinuous ridge is believed to indicate much thicker and more massive deposits of broken ice beneath the fracture zone than the straight linear ridges caused mostly by shearing forces. As one looks over the photo mosaic included as either Fig.1 or Fig.5, points where two large floes are converging can be recognized, and if one were to project the former edges of each floe beyond the zone of contact, a feeling for the amount of ice already included within the compressional zone can be attained. In some instances within the imagery, only the last remnant of an ice floe can be seen protruding from under a much larger floe where a series of sinuous ridges occur Photogrammetria,

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at the zone of merger. It is conceivable to the author that entire ice floes are completely destroyed in a sustained compressional environment and go into the formation of a zone of sinuous pressure ridges having massive and deep roots.

Ice rafting patterns Until this study, the author had held the general belief that rafting patterns were primarily related to thin ice and that as the ice thickened it would not normally raft under compressional forces, but would crush and crumble along a rather irregular line. From this study, however, there appears to be evidence of ice rafting ranging from material which was only inches thick to ice presently rafting which is about 6 ft. thick. It appears that, given the opportunity, ice floes up to at least 6 ft. thick will prefer to slip either beneath or above ice of equal thickness rather than to directly crush itself along the zone of contact. The development of this concept begins with the phenomenon of finger rafting associated with thin ice, as illustrated in Fig.8. There are no areas of new, thin ice in the study area which exhibit a classical example of what a "finger rafting" pattern looks like on aerial photography. Fig.8 is a stereo pair showing evidence of a "finger rafting" pattern which was created when the ice was only a few inches thick. The ice floe now is about 5 - 6 ft. thick and the small "finger rafting" pattern is barely discernible on the snow-drifted ice surface. To better illustrate the angularity of the surface trace of this feature the line trace of some of the raft patterns present in the stereo image has been attached to the figure as a sketch. The compressional forces creating this rafting pattern are indicated by the arrows in the sketch. The significance of this illustration is twofold. First, the preservation of this very fine raft pattern is an indication that the ice is still quite young and has not undergone a melt season, for these tiny features would lose their continuity through a season of ice surface degradation by melting. Secondly, the very angular corners of the rafted tongues of ice are indicators of ice thickness in that as ice rafting occurs in thicker ice, the corners become more and more rounded until they finally cease to exist and instead become rounded lobes of rafted ice. Since the "finger rafting" in Fig.8 has taken place, and all stresses upon the ice have been relieved, the ice has proceeded to thicken through natural growth processes through the winter season and the rafting pattern is no longer an indicator of present ice thickness. In Fig.9 one can observe a similar rafting pattern more clearly, in that it is a larger feature created out of thicker ice, possibly a foot or two in thickness. This estimated thickness is based upon the observed size of the material creating the rafting pattern. This rafting pattern has a slightly different surface trace than the rafting pattern created by ice only inches thick portrayed in Fig.8. Instead of the corners of the rectangular overthrust lobe being very sharp and angular, as was Fig.7. Stereo pair of an example of sinuous pressure ridges where highly compressional forces act to build massive ridges which are interpreted as having deep roots.

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Fig.9. Stereo pair showing a pronounced ice rafting pattern created by ice estimated to have been from 1 to 2 ft. thick. the case in the thin ice pattern, they are somewhat rounded, which is felt as being of significance in interpreting the thickness of ice at the time of ridge formation. As very thin ice rafts in its characteristic "'finger-like" pattern, there are no obstacles on the overridden ice to abrade the sharp, angular overthrust plate of ice. However, as the ice becomes thicker and can support a snow cover, a rafting plate must overcome an element of ice surface roughness which may account for the rounding of the leading corners of the overriding plate. Ice rafting patterns such as these exhibit both the shearing and compressional components of the stress field. The blunt, squarish front of the advancing lobe of ice is somewhat crushed by compressional forces as it moves across and depresses the ice before it. The long, parallel ridges completing the box-like pattern are shear zones with some compressional force acting to heave up the ice into a narrow linear ridge. The ice rafting pattern itself indicates at least a doubling of local ice thickness as one floe rafts across another. Large root systems are not expected to be associated with a pattern of ice rafting exhibiting a predominantly rectangular surface trace. A final example of ice rafting is contained in Fig.10, where the overriding ice mass has been measured at about 3 ft. in thickness. The pattern associated with the ice movement in the vicinity of A still reflects the tendency of ice this thick to

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raft with a subangular configuration. The rafting ice has assumed more of a curved outline than in the previously discussed thinner ice rafting patterns. The shearing stresses and the compressional stresses are similar to those present in the previous examples of ice rafting. Fig.10, however, depicts the actual displacement along the shear zone which can be measured through the matching up of separated pressure ridges along with the similar edge configuration of the ice along the shear zone. The displacement amounts to about 80 ft. at the moment of photography; however, it is interpreted that the motion is still in effect. If one looks closely along the shear zone of the rafted plate, it can be seen that the irregularities in the ice along the edge of fracturing are smoothing out as the ice edges pass each other. Newly formed piles and ridges of broken ice are being formed on top of the ice floe edges as this abrasive process takes place. In the area of B in Fig.10, a rounded ice rafting pattern can be easily recognized lying on top of the large ice floe occupying the left side of the image. It appears that a substantial portion of ice floe C has rafted upon the edge of floe B very recently and that floe C has been separated from its rafted element. The ice which has done the rafting in this instance is also about 3 ft. thick, as was the case with floe A. It is also interesting to note how completely the overthrust ice has broken apart on top of floe B and also how the rafted ice has depressed the edge of floe B beneath the surface of the sea, as is indicated by the dark tones between and surrounding all of the fragments of the rafted ice. These dark tones are indicative of ice surface flooding. In this particular instance, the flooding is directly connected to open water and brash ice between the separated parts of floe C. In evaluating the changing of the surface pattern of rafted ice from that of very angular and sharp cornered in thin ice to a more rounded design in thicker ice, it appears as though the rafting nature of the ice floes will continue to express itself in even thicker ice. It would follow that the surface traces of rafted ice would become more rounded as the ice becomes thicker, as has been demonstrated in Fig.8-10. If this view is accepted, it is then suggested that the sinuous pressure ridges that are so evident in the study area are the traces of thick sea ice rafting patterns where the dominant forces involved are compressional and very little shearing occurs. The sinuous pressure ridge pattern depicted in Fig.7 is considered an example of thick ice undergoing both crushing and rafting.

Zones o/multiple pressure ridges Throughout the aerial photography of the study area there are numerous zones within ice floes or where ice floes are in direct contact with one another where a multiple series of roughly parallel sinuous pressure ridges occur. Fig.11 Fig.10. Stereo pair showing ice rafting patterns created by overriding ice floes about 3 ft. thick.

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Fig.ll. Stereo pair showing a series of sinuous pressure ridges having tightly compact and roughly parallel alignment.

is one such example where the ridging occurred when the ice was about 3-4 ft. thick. A direct measurement of the thickness of an upturned block of ice in this illustration was made in connection with the discussion associated with determining ice thickness in Fig.2. This multiple ridge pattern, being composed of the sinuous pressure ridges which are believed to indicate dominant compressional stresses, suggests a large mass of broken ice lying beneath the pattern. The pattern suggests a correlation with multiple thrust faulting features in the field of stuctural geology, where secondary thrust plates of rock are piled up behind the major thrust fault once the resistance to further movement along the primary fault plane is greater than the strength of the rock comprising the thrust plate. A series of roughly parallel thrust fault traces are in this way inscribed across the landscape. Similarly in sea ice, thrusting or rafting takes place and will continue to take place until the resistance to the movement of the overriding plate exceeds the strength of the plate itself. At this time, it is postulated, a second rupture zone will occur as the entire rafting phenomenon repeats itself. Each of the sinuous pressure ridges comprising this multiple ridge pattern in Fig. 11 can be interpreted as being Photogrammetria, 26 (1970) 201-228

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Fig.12. Stereo pair showing two distinctly different ages of pressure ridging. The ice which is rafting upon the large ice floe is about 5-6 ft. thick. to

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Fig.13. Stereo pair of a portion of the largest ice floe in the study area, showing the antiquity of the pressure ridges based upon the n u m b e r of directions of snow drifting.

the leading edge of an overthrust or rafted ice plate. There is no telling how much ice is contained in each of these rafted zones. Based upon the compact nature of the roughly parallel ridges and the observable evidence presented in Fig. 10, where over 80 ft. of ice had been rafted on an adjacent ice floe, it is suspected that a great quantity of ice lies beneath these severely disturbed zones. Relative age of pressure ridges Pressure ridges of two distinctly different ages can be easily compared in Fig.12. Under stereoscopic examination, the large ridge toward the bottom of the illustration can be judged to be very fresh and unweathered by noticing the following points: (1) sharp angular blocks of ice lying at various attitudes along the trace of the ridge; (2) fresh open cracks within the body of the overthrusted plate of ice; (3) the dark tone of wetness along the edge of the ridge where the underlying ice has been depressed below sea level; (4) the lack of any drifted snow asPhotogrammctria, 26 119701 20t 228

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sociated with this particular ridge; and (5) the proximity of this ridge to the edge of a large solid ice floe. Conversely, the long sinuous ridge trending toward the top of the illustration is an old ridge. The indicators are: (1) drifted snow tending to bury most of the ridge (There are at least two distinct orientations of the snow drifts which indicate either that the snow has been deposited during two separate storm periods of differing wind directions, or that the floe has rotated during a single storm period allowing two sets of drifts to develop.); (2) no ice surface wetting adjacent to the ridge; (3) no fresh cracks associated with the ice adjacent to the ridge; and (4) its location well within a large floe where there are no signs of local ice movement. The small sinuous ridge in the upper right-hand corner of photo 621 may be older than either of the other ridges. There are four distinct orientations of snow drifts associated with this ridge. Fig.13 is an example taken from the largest ice floe in the study area and shows some old pressure ridges which have undergone many storms and associated snow drifting periods. Here there exists almost a lattice-work of snow drifts associated with some of the ridges and there is absolutely no sign of recent differential ice movement within the limits of the illustration. There is a great variety of sea ice features in this particular illustration. Pressure ridges of various configurations intersecting one another; a 40 ft. wide refrozen, snow-covered lead in the ice almost parallel to the right-hand edge of the illustration; rippled ice in an embayment along one of the sinuous ridges near the center of each photo; subdued angular finger-rafting patterns scattered throughout the illustration. Most of the sea ice patterns reflect the youthfulness of the ice, leading to the conclusion that even the largest floe in the study area is composed of ice grown in a single season. Vertical stresses relative to a large pressure ridge In previous discussions, is has been pointed out how ice rafting and its associated pressure ridges tend to depress the ice upon which the thrust plates are being rafted. Indicative of this phenomenon is the wetness along the edges of a thrust plate where the sea water floods on top of the depressed ice along the perimeter of the overthrust plate. It seems reasonable to assume that once the stresses involved with pressure ridge building have been relieved and the ice mass is left in a static state for a time, the pressured ice, which has collected underwater in the vicinity of the surface trace of the ridge, will freeze together as a mass through the bonding action of freezing interstitial sea water. This should especially be expected during the winter season when very cold ice is depressed into the sea, thereby causing rather quick bonding of ice masses by the contact freezing of sea water. Once the pressure ridge is held immobile by the adjacent ice and the ridge beneath the water surface becomes stable and begins to freeze, there is no guarantee that the mass of the ridge and its root are in isostatic equilibrium as far

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as sea level is concerned, for the confining ice floes may be exerting a lateral pressure, holding the ridge either above or below its plane of isostatic equilibrium. As time passes, the cold ice forming the root of the ridge will lose much of its residual cold to the sea water and ablation of the root will commence. It is conceivable then that the local isostatic equilibrium of the healed, coherent pressure ridge will become unbalanced with less and less underwater support for the mass of the ridge above sea level. Once the support of the adjacent ice floes is removed due to local sea ice fracturing, a situation similar to that in Fig.14 may result. In this figure one can see a large, rather old pressure ridge (note the associated snow drifts) located parallel to the edge of a large ice floe. Outward from the edge of the floe in this area is a field of mobile ice which is causing abrasion along the freshly developed floe edge. This situation is interpreted as being that period of time when the confining lateral support of rather thick ice on either side of the ridge has been removed by the fracturing of the ice mass along the very newly formed ice edge. The breakage of the ice floe along this line, lying some 20-50 ft. away from the pressure ridge, is no chance occurrence but happens to be a natural line of weakness in the sea ice at a point where it begins to flex and bend downward into the pressure ridge. Stereoscopic examination of Fig.14 makes this observation quite apparent. The line along which the ice parted had to be weaker than the pressure ridge itself or the ridge, an original line of weakness in the ice floe, would have split apart, This is the reasoning behind believing that the ridge, a consolidated ice mass, is now stronger than the adjacent ice. In the time since the breakage in the ice occurred along the present floe edge, a flooding of low spots along the right edge of the pressure ridge has occurred. This seems to indicate that once the lateral confining support by the adjacent ice has been removed, the loading of ice above sea level in the form of a large pressure ridge was too great for the present support of the ridge and, therefore, the ridge itself settled downward, increasing the tilt of the ice to the right of the ridge. The ice is so tilted that the edge of the floe rises an estimated 4 or 5 ft. above sea level, yet the thickness of this plate of ice should be, at the most, about 6 ft. This explanation is only a hypothesis; however, it has been presented to illustrate that it is not only possible to describe and detect lateral forces at work upon the ice mass through the analysis of stereoscopic aerial photography, but that by using this type of imagery in a manner where the vertical dimension can be evaluated, further interpretations regarding the sea ice environment can be made.

Fig.14. Stereo pair of a large pressure ridge along the edge of a large ice floe. The ice to the right of the floe edge is in the process of breaking up.

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SUGGESTED CLASSIFICATION OF PRESSURE RIDGES

Examples of some characteristics of pressure ridges which can be observed on stereoscopic aerial photography used in this study are: (1) the linear trace of a pressure ridge; (2) the relativc and sometimes measurable heights, lengths and widths of pressure ridges; (3) the relative age of pressure ridges; (4) the location of pressure ridges with respect to current ice motion; and (5) the relative size of the material contained in a pressure ridge. From these parameters, the author has chosen to develop a relationship between the surface trace of a pressure ridge and its expected under-ice component. The reaction of sea ice to shear and compressional stresses, and all combinations of the two, is responsible for all the varieties of pressure ridges occurring within the sea ice environment. The concept which has been developed through the preceding pages suggests that straight pressure ridges are created by predominantly shearing stresses, while the very sinuous, lobate pressure ridges reflect predominantly compressional forces. It is also suggested that because of the nature of the forces involved in creating these pressure ridges, it should be expected that sinuous ridges normally will have a more massive root than straight ridges. These relationships are offered as a simple classification system wherein remote sensing imagery, depicting the surface trace of pressure ridges, can be used to select field sampling sites in a program of pressure ridge research. Up to this point, however. the relationship of ice motion and breaking by shear and compressional stresses and the relationship of pressure ridge surface trace to its underwater component have been suggested through the interpretation and analysis of stcreographic aerial photography. These relationships must be checked and verified in the field. A second parameter listed above which the author feels is significant toward establishing the durability and strength of pressure ridges is point 3, the relative age of pressure ridges. Very simply, this means that recently-formed ridges may not have had time to heal and solidify and, therefore, are very apt to split apart if compressional forces are released. Conversely, an old ridge can be cxpected to be a solid, coherent ice mass because it has had ample time to heal. The indicators of youth and maturity with respect to pressure ridges have been discussed previously in the report. One of the problems in trying to ascertain whether a pressure ridge is more straight than sinuous (or the reverse) is physically determining the surface trace of a particular ridge when it occurs in a heavily pressure ridged area. The sinuous ridges are especially difficult to follow when they become intermingled with others of their kind. Stereoscopic study of aerial photography, such as was used in this report, is necessary to accurately delineate a particular ridge from among many which may display a similar pattern.

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CONCLUSIONS

The usefulness of the airphoto analysis approach (1) Existing techniques of stereoscopic airphoto analysis as a useful method of acquiring initial qualitative information about the sea ice environment. (2) The resultant analysis can be used to form hypotheses as to the mechanism and forces at work which are, and have been, active in creating the imaged pattern of sea ice. (3) Research program planning, field sampling point selections, and evaluation of accessibility can be determined from an initial phase of airphoto analysis of a proposed study area. (4) Measurements of lengths, breadths and, in some cases, heights of sea ice features, as well as measurements of ice thickness, can be made from stereoscopic aerial photography if the scale of the photography is known. ACKNOWLEDGEMENTS

The author is indebted to the members of the Photographic Interpretation Research Division of the U.S. Army Cold Regions Research and Engineering Laboratory and, in particular, to Miss Patricia A. Cook, Mr. Stephen B, McLaughlin and Mr. David B. Eaton for their assistance in the preparation of this report. Special appreciation is extended to Mr. Thomas L. Marlar, Scientific Photographer of the Division, for his personal efforts in acquiring some of the finest aerial photography of sea ice I have ever had the pleasure of working with. SELECTED BIBLIOGRAPHY ANDERSON, V. H., 1965. Radar imagery of Arctic pack i c e - - K a n e Basin to North Pole. U.S. Army, Cold Reg. Res. Eng. Lab., Spec. Rept., 94:5 pp. ANDERSON, V. H., 1966. High altitude side-looking radar images of sea ice in the Arctic. Syrup. Remote Sensing Environment, 4th, University o[ Michigan, Ann Arbor, Mich., 1966:845-857. AVGEVICH, V. I., 1963. Nekotoryie osobennosti morskikh l'dov, dighifviraiemyie po aerofoto shimkam (Aerial photo interpretation of certain aspects of sea ice). Okeany: Moria, Vopr. Geogr., 62:155-165. BUSHUEV, A. V. and LOSCmLOV, V. S.~ 1967. Accuracy in aerial observations and mapping of sea ice. Tr. Arktichesk. i Antarktichesk. Nauchn.--Issled. Inst., 257:84-92, DUNBAR, M., 1969. A glossary of ice terms (W.M.O. Terminology). In: The Ice Seminar~Can. Inst. Mining Met., Spec. Vol., 10:105-110. GONIN, G. B., 1960. Uchet torosistosti l'da po statishcheskoy obrabotke materialov aerofotos yemki (Calculation of the hummockiness of ice by statistical treatment of air photographic data). Probl. Arktiki i Antarktiki, 3 : 9 3 100. HARWOOD. T. A., 1969. Remote sensing of ice in navigable waters. In: The Ice Seminar--Can. Inst. Mining Met., Spec. Vol., 10: 95-104. KE'rCnUM, R. D. and WITTMANN,W. I., 1968. The role of drifting stations in sea ice prediction studies. In: J. E. SATER (Editor), Arctic Drifting Stations. Arctic Institute of North America, Washington, D.C., pp.167-179.

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KUDRITSKII, A. M., PoPow, I. V. and ROMANOVA, E. A., 1956. Osmovy Gidrogra[itsheskogo deshi[rovania aerofotosvismkov (Fundamentals of Hydrographic Aerial Photo-iltterpretation). Gidrometisdat, Leningrad, 343 pp. LOSHCHZI,OV, V. S., 1959. lspol'zovanie aerofotos emki pri ledovoi razvedke dlia opredeleniia srednei tol'shchiny lediango pokrova (The use of aerial photography in ice reconnaissance to determine the mean thickness of the ice cover). Probl. Arktiki i Antarktil, i, 1: 81-85. (Am. Met. Soc., T-R-306) PREOBRAZHENSKI. 1U. V. (Editor), 1960. Alhom aero[otosnimkov ledovykh obrazovunii ~ut moriakh (Album of Aerial Photographs of Ice Formations in Seas). Gidrometisdal, Leningrad, 221 pp. THOR~N, R., 1969. Picture Atlas of the Arctic. Elsevier, Amsterdam, 449 pp, VOLKOV, N. A. and VOROr~OV, P. S., 1967. Issledovaniya kriotektoniki morskogo l'da dlya tseley glyatsiologii i geologii (Studies of the cryotectonics of sea ice for glaciological and geological use). Probl. Arktiki i Antarktiki, 27: 152-168. WEEKS, W. F. and KovAcs. A.. 1970. On pressure ridges. U.S. Army, Cold Re,~,. Rex. En,e. Lab., Res. Rept., 70~-3. WITTMArCN, W. I. and SCHULE, J. J., 1966. Comments on the mass budget of Arctic pack ice. In: J. O. FLETCHER (Editor), Proceedings Symposium on the Arctic Heat Budget aml Atmospheric Circulation. R A N D Corporation (RM-5233-NSF), pp.217-246. ZuBov, N. N., 1945. L'dy Arktiki (Arctic Ice). lzd. Glavsevmorputi, Moscow, 360 pp.

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