morphometry of open rock basins, kananaskis area, canadian rocky mountains
TRANSCRIPT
Morphometry of open rock basins, Kananaskis area, Canadian Rocky Mountains
DAVID J. SAUCHYN AND JAMES S. GARDNER Department of Geography, Faculty of Environmental Studies, University of Waterloo, Waterloo, Ont., Canada N2L 3Gl
Received July 27, 1982 Revision accepted November 12, 1982
Digital elevation data have been obtained for 54 open rock basins from aerial diapositives using an analytical stereoplotter. The term "open rock basin" is used to distinguish these basins from the closed rock basins of classical glacial cirques. Whereas closed basin cirques have been subjected to considerable morphometric analysis, open rock basins, which are equally prevalent in mountain environments, have not. The morphometric analyses in this paper are based on nine indices: length, width, area, relief, lengthlwidth, lengthlrelief, widthlrelief, arealrelief, and compactness. Cluster analysis of compactness and areairelief data indicates five distinct clusters of open rock basins, which are attributed to variations in topoclimate and geologic structure. The three largest clusters are labelled rockfall chutes, rockfall funnels, and open cirques. The cirque cluster is compared with glacial cirques for which published morphometric data exist. Comparable lengthlwidth ratios suggest an equilibrium in rock basin planimetric shape. However, the considerably greater relief and lower arealrelief values of the open cirques suggest that glaciation of these rock basins has been limited to their upper reaches in the form of niche glaciers.
Des donnks numeriques d'altitude ont CtC obtenues pour 54 bassins rocheux avec des diapositives akriennes et au moyen d'un stCr6orestituteur analytique. L'expression de bassin rocheux ouvert permet une distinction d'avec les bassins dits fermCs de cirques glaciaires classiques. Les bassins fermCs de cirques furent I'objet de frkquentes analyses morphomCtriques, par contre les bassins rocheux ouverts qui prkvalent tout autant dans les milieux montagneux ne furent pas CtudiCs. Les analyses morphom~triques rapportCes ici reposent sur neuf indices: longueur, largeur, surface, relief, longeur/largeur, longueur/relief, largeurlrelief, surfacelrelief et densitC. L'analyse des donnkes group6es de la densit6 des bassins et du rapport surfacelrelief indiquent cinq groupements distincts de bassins rocheux ouverts attribuhs B des variations du topoclimat et de la structure gCologique. Les groupements les plus forts correspondent B des couloirs d'avalanche, h des entonnoirs d'avalanche et B des cirques ouverts. Les donnCes morphologiques du groupement de cirques sont comparCes avec celles dCjB publikes pour les cirques glaciaires. Les rapports longueur/largeur comparables suggkrent un Cquilibre dans la configuration planimCtrique de bassins rocheux. Cependant, pour les cirques ouverts le rapport surfacelrelief est considCrablement plus ClevC, ce qui rCvble que l'englaciation des bassins rocheux Ctait limitCe leurs altitudes sup6rieures formant des niches de glacier.
Can. 1. Earth Sci., 20, 409-419 (1983)
Introduction Small rock basins are a characteristic feature of
mountain landscapes. They are attributable to the heterogeneity of rock and, therefore, to differential weathering and erosion according to rock structure and lithology. The general term "rock basin" encompasses a range of features, including those referred to elsewhere as rockfall chutes, couloirs, rockfall funnels, mass- wasting hollows, nivation hollows, and cirques. Glacial cirques are the most often studied and described mountain rock basins. The basins examined here are superjacent to active fan-shaped colluvial deposits, and in this respect are distinct from the closed rock basins of classical glacial cirques wherein the cirque floor dips upslope to a threshold. The term "open rock basin" is proposed here for cirques or other rock basins that are superjacent to debris (talus) slopes or fans. From a systems perspective, the adjectives "open" and "closed refer to the transfer and a relative lack of transfer, respectively, of debris from the rock basin.
This paper is an initial assessment of the morphometry of 54 open rock basins in the Kananaskis region of the southern Canadian Rocky Mountains. The
[Traduit par le journal]
control of rock-wall morphology on the nature of episodic slope processes, and thereby on debris slope morphology and sedimentology, is frequently noted in papers on mountain debris slopes (see Luckman 1970; Musielewicz 1980). However, early descriptions of rock-wall morphology (Ahlmann 1919; Matthes 1930, 1938; Rapp 1960; Markgren 1964) have not, to our knowledge, been succeeded by quantitative analysis. In contrast, numerous quantitative, morphometric descrip- tions have been made of classical closed basin cirques in the Rockies and other mountain ranges (Aniya and Welch 1981; Gordon 1977; King 1974; Graf 1976; Trenhaile 1976).
Open rock basins are characterized by very steep gradients and, unlike glacial cirques, lack a characteris- tic low gradient floor. Because of the very steep gradients, field measurements are usually difficult, and because of their small size, the use of conventional topographic map-based data is not of great value. The morphometric analysis of the Kananaskis open rock basins is based on digital elevation data derived from aerial diapositives on an analytical stereoplotter. The analytical stereoplotter, which has been used exten-
Can
. J. E
arth
Sci
. Dow
nloa
ded
from
ww
w.n
rcre
sear
chpr
ess.
com
by
Mer
ced
(UC
M)
on 0
5/06
/14
For
pers
onal
use
onl
y.
410 CAN. J. EARTH
sively only since 1976 (Friedman 1980), provides estimation of coordinates with approximately 10 times greater accuracy than a conventional analogue stereoplotter (Friedman 1980).
In the absence of published data for other examples of open rock basins, published data on cirque morphom- etry provide a context for the morphometric analysis of the Kananaskis open rock basins. Open chutes and closed glacial cirques occupy opposite ends of a spectrum of rock basins, along which basins are increasingly large and arcuate. Where cirque glaciation is not a factor due, for example, to aspect or altitude, "mature" drainage basins might occupy the position of the glacial cirque on a similar spectrum. References to mass-wasting hollows, nivation cirques, and immature or secondary cirques in papers on cirque morphometry suggest that the samples of cirques and the open rock basins examined here are continuous or may overlap along a rock basin spectrum.
Study area The location of the study area and the open rock
basins is shown in Fig. 1. The Kananaskis area is located approximately 80 krn west-southwest of Calgary in the Front Ranges of the Canadian Rocky Mountains. The Front Ranges are a series of north-northwest- to south-southeast-trending ridges formed by Tertiary thrust faulting and folding of Paleozoic and Mesozoic strata (Price et al. 1972). The strata dip moderately to steeply westward. Paleozoic limestones and dolomites form the prominent ridges and peaks, and thus the open rock basins are formed in these units. Total relief in the study area is about 1800 m. Mount Rae is the highest peak at 3225 m. The area above timberline (2200 m) is characterized by extremely variable and relatively high rates of geomorphic activity (Gardner 1982). Glaciers exist in the north cirque of Mount Rae and along the continental divide (British Columbia - Alberta boundary).
Methods Digital elevation data for 54 open rock basins were
derived from aerial diapositives on an Autoplot analytical stereoplotter (Detwiler 1980) housed in the Division of Survey Engineering at the University of Calgary. The chosen basins are accessible, encompass a range of open rock basin morphologies, and are superjacent to mostly unvegetated debris fans that were previously or are concurrently under study. Twelve, three, and one stereo models were constructed using aerial diapositives at scales of 1: 15 840, 1 :30 000, and 1:21 120, respectively. The 1: 15 840 photography was acquired with a 12 in. (30.48 cm) focal length lens rather than the conventional 6 in. (15.24cm) lens, and therefore it exhibits negligible relief displacement.
SCI. VOL. 20, 1983
Absolute orientation of the stereo models was based on ground control data. The difference in elevation and the distance between features visible on the diapositives were calculated from field measurements. These features were distinct bedrock outcrops, large erratics, and rockfall remnants. Distances and the horizontal and vertical angles between the features were determined using an AutoRanger-S electronic distance meter and a Zuiho ZF- 1 transit, respectively.
The digital elevation data consisted of the coordinates of points arbitrarily chosen to best depict the highly irregular configuration of each rock basin. Thus, data were acquired at inflections on the perimeter, at breaks in slope, and along drainage divides and ridge lines. For each model, the x and y coordinates were expressed relative to a nearby ground control station. The elevation coordinates (2) were scaled with respect to the mouth of each basin. Figure 2 is a contour plot generated from the digital elevation data for one of the open rock basins. The basin perimeter and the distribution of data points are displayed.
Errors in the digital elevation data derive from errors in the scaling and orientation of the stereo model as a function of errors in the ground control data and the accuracy of the stereoplotter. The measured distances and vertical and horizontal angles were resolvable to 1 mm, 20 s, and 1 min, respectively. Thus, most of the ground control error results from imprecision in locating the ground control stations on the aerial photographs. This source of error has not been evaluated. However, since the morphometric analysis is based on relative rather than absolute elevations, the effect of scaling and levelling error is assumed to be small. Given a C factor (Combs 1980, p. 398) of 1525 for the Autoplot ana- lytical stereoplotter, the approximate "heighting" error of a single elevation value is H/7000, where H is the flying height. Therefore, the digital elevation data are accurate to within 0.5-0.7 m, discounting errors in the ground control.
Morphometric analyses of the open rock basins are based on four basin properties, length, width, relief, and area, and five form indices, lengthlwidth, length/relief, widhirelief, areairelief, and compact- ness. Length is the longest axis originating at the basin mouth. Width is the maximum dimension perpendicular to the long axis. Relief is the difference between the maximum and minimum elevation coordinates. Compact- ness (Blair and Bliss 1967) is the ratio of basin area to the sum of the moments of inertia of the infinitesimal area elements (? dA, where r is distance of an area element from the basin centroid) constituting the basin shape. Compactness is expressed relative to the compactness of a circle, and thus varies between 0 and 1. Length, width, area, relief, and compactness were computed directly from the digital elevation data using FORTRAN programs.
Can
. J. E
arth
Sci
. Dow
nloa
ded
from
ww
w.n
rcre
sear
chpr
ess.
com
by
Mer
ced
(UC
M)
on 0
5/06
/14
For
pers
onal
use
onl
y.
SAUCHYN AND GARDNER 41 1
ALBERTA
STUW f
, AREA a
FIG. 1 . Location of Kananaskis study area and open rock basin study sites.
Thus, many of the interpolation and operator errors morphometry; and (2) are frequently used in the analysis associated with the acquisition of map-derived data are of cirque morphometry, and thus facilitate comparison avoided. of the morphometry of open rock basins and closed
Ratios of the basic properties were used because they cirques. The ratio of headwall to floor gradients is also a (1) are readily computed and interpreted, and are thus common index of cirque morphometry. It was not appropriate for an initial examination of open rock basin applied here because most open rock basins do not
Can
. J. E
arth
Sci
. Dow
nloa
ded
from
ww
w.n
rcre
sear
chpr
ess.
com
by
Mer
ced
(UC
M)
on 0
5/06
/14
For
pers
onal
use
onl
y.
CAN. J . EARTH SCI. VOL. 20, 1983
FIG. 2. Contour plot of an open rock basin indicating the elevation data points and basin perimeter (maximum basin length: 980 m; contour interval: 20 m).
consist of distinct headwalls and floors. Since compactness is based on the distribution of basin area about the centroid, it is a more complete index of planimetric shape than lengthlwidth ratio. However, since cirque studies cite lengthlwidth values, they are computed here for comparative purposes. Basin gradient along the long axis is the arc tangent of the reciprocal of lengthlrelief.
Results and discussion Summary statistics for, and correlations between, the
morphometric variables for the 54 open rock basins are given in Tables 1 and 2, res ectively. The basins range S in area from 0.02 to 0.55 km . The largest basin exceeds 1 km in length. The mean lengthlrelief ratio cor- responds to a mean gradient of 43.3" with minimum and maximum gradients of 34.0 and 58.6". The
Can
. J. E
arth
Sci
. Dow
nloa
ded
from
ww
w.n
rcre
sear
chpr
ess.
com
by
Mer
ced
(UC
M)
on 0
5/06
/14
For
pers
onal
use
onl
y.
SAUCHYN AND GARDNER 413
TABLE 1. Summary statistics for morphometric variables (n = 54) -
Standard Coefficient Minimum Maximum Variable Mean deviation of variation value value Skewness Kurtosis
Length (m) Width (m) Relief (m) Area (m2)
Length /width Lengthlrelief Width/relief Area /relief Compactness
TABLE 2. Correlation matrix of morphometric variables (n = 54) - - - --
Log Log Log Log Log Compact- Length width Relief area lengthlwidth Length/relief widthlrelief arealrelief ness
Length 1.00 0.56* 0.82" 0.76% 0.08 0.45* 0.14 Log width 1.00 0.71* 0.91* -0.77* -0.14 0.82* Relief 1.00 0.83* -0.22 -0.13 0.19 Log area 1.00 -0.52* 0.01 0.61* Log lengthlwidth 1.00 0.50* -0.90* - Length/relief 1.00 -0.07 Log width/relief 1.00 Log area/relief Compactness
coefficients of variation indicate that, of the basin properties, relief displays the least variation, whereas width and, in turn, area are highly variable.
T tests of the difference between the skewness statistics (Table 1) and a zero skewness for a normal population indicate that the distributions of five of the nine variables are significantly (p < 0.01) right-skewed. The distribution of the lengthlwidth data exhibits the largest departure from normality among the un- transformed variables. Two basins with very high lengthlwidth ratios of 5.7 and 7.4 represent the upper extreme of basin elongation. However, the skewness of the log-transformed values of these five variables are not significantly different from zero (p > 0.05). Since the assignment of significance levels to correlation coefficients requires that the variables be normally distributed, the correlations in Table 2 are based on log-transformed values for width, area, lengthlwidth, widthlrelief, and arealrelief. Significant (p < 0.00 1) positive correlations exist between all the basic properties. Variations in basin width account for much of the variation in area ( r = 0.91) and the form indices: length/width ( r = -0.77), width/relief ( r = 0.82), areairelief ( r = 0.89), and compactness ( r = 0.66). The
high correlation between length and relief ( r = 0.82) occurs because longer basins encompass greater relief.
Figure 3 is a plot of the planimetic variable, com- pactness, against a hypsomemc variable, wealrelief. The wealrelief ratio is significantly (p < 0.001) correlated with seven of the eight other form variables and, as mentioned previously, compactness is a more comprehensive index of planimetic shape than a form ratio of basic dimensions. The dashed boundaries drawn in Fig. 3 are based on a cluster analysis (Helwig and Council 1979, p. 157) of the Euclidean distances between basins as computed from standardized compactness and wealrelief data. Ratios of the number of distances within clusters to the total number of distances less than the maximum cluster diameter suggested that five clusters were optimal. A Mann- Whitney U test indicated that clusters I, 11, III, and V are derived from different populations @ < 0.001). A non-parametric test was used because (1) according to D'Agostino's D test of normality, the samples in clusters I and 11 are not normally distributed (p < 0.01); and (2) clusters 111 and V were considered too small (n =. 6 and 9, respectively) for the testing of normality and equality of variances required for a parametic test.
Can
. J. E
arth
Sci
. Dow
nloa
ded
from
ww
w.n
rcre
sear
chpr
ess.
com
by
Mer
ced
(UC
M)
on 0
5/06
/14
For
pers
onal
use
onl
y.
414 CAN. J . EARTH SCI. VOL. 20, 1983
COMPACTNESS VERSUS AREA/RELIEF
1.00 I I
I I * I
0.95 I . I
. . I I
0. so I P 0.85
y) 0.80 ul I W Z . . I L 0.75 u I
0 0.70
0. 65
0 .60
0.55
0.50
20 40 60 80 100 120 140 160 110 200. 220. 240 280 280 300 320 340 360 S O 400 420 440 460 480 500 520 540 560 580 120
AREA 1 RELIEF
FIG. 3. Compactness versus arealrelief values indicating five distinct clusters produced by a cluster analysis.
The basins in the three largest clusters, numbers I, LI, and V, are labelled rockfall chutes, rockfall funnels (Rapp 1960), and open cirques, respectively. The chutes and funnels are of comparable size with respect to area (X = 0.067 versus 0.078 km2) and relief (X = 501 versus 510 m). However, the chutes are considerably more elongate (lengthlwidth ratio: X = 3.79 versus 2.13) and less compact ( 2 = 0.64 versus 0.85). The cirques are more compact than all the chutes and most of the funnels. With a mean area of 0.36 krn2 the cirques are considerably larger than the combined sample of chutes and funnels (X = 0.073km2). Most of this difference is attributable to differences in width ( 2 = 477 versus 202 m). Figures 4 and 5 give examples at the upper and lower ends, respectively, of the spectrum in compacmess and arealrelief values for open rock basins.
Bedrock geology and variations in the effectiveness of weathering and erosion with variations in topoclimate are the controls on open rock basin morphometry. Unlike the morphometric data, information on the rock basin geology and topoclimate is necessarily descriptive since geologic or climatic observations made on lower reaches of accessible rock basins are not representative of other basins or the higher reaches of the same basins.
The 54 open rock basins are distributed among the hanging walls of the Bourgeau, Sulpher Mountain, Lewis, Rundle, and Misty thrust faults (listed from west
to east) (Halladay and Mathewson 1971). Carbonate rocks predominate, in particular the Etherington, Mount Head, and Livingston Formations of the Mount Rundle Group. The limestones and dolomites of the Palliser and Banff Formations are exposed beneath the Rundle Group on the north to east facing scarp slopes. On the dip slopes, the upper part of the Rocky Mountain Group overlies the Rundle Group, and thus the lower portions of some rock basins on west to south facing slopes are composed of sandstone and quartzite. Field observa- tions (J. Desloges, personal communication, 198 1) suggest that variations in fracture density in the Rundle and Rocky Mountain Groups and in the Palliser and Banff Formations are greater within a formation than between them. Variations in fracture density measured at outcrops depend largely on the orientation of strata with respect to the free faces, since the joint sets are mostly perpendicular to bedding planes. The Paleozoic carbonates are, on the whole, resistant to mechanical weathering and erosion. Recessive strata such as the solution breccia of the Salter Member (Mount Head Formation), the calcareous shale of the upper part of the Opal Member (Mount Head Formation), and the bituminous, cherty shale of the Exshaw Formation (which occasionally outcrops beneath the Banff Formation) are characteristically thin. Hence their control on rock basin morphometry is limited to narrow chutes on steeply dipping strata. Thus the influence of
Can
. J. E
arth
Sci
. Dow
nloa
ded
from
ww
w.n
rcre
sear
chpr
ess.
com
by
Mer
ced
(UC
M)
on 0
5/06
/14
For
pers
onal
use
onl
y.
SAUCHYN AND GARDNER 415
FIG. 4. Chute type of open rock basin, northwest face of Mount Chester.
rock type and fracture density on open rock basin morphometry is primarily a function of the structural geology, that is, the moderate to steep west- southwestward dip of overlapping thrust sheets.
Given the systematic variation in geologic structure and topoclimate, basin aspect, which is the clockwise angle between north and the long axis of the basin, provides a means of categorizing the physiographic settings of the open rock basins. Basin aspects for the total sample and the clusters are given in Table 3. The distribution of aspects for the total sample reflects the predominance of east- and west-facing slopes on the north-northwest- to south-southeast-trending Front Ranges.
The chutes (cluster I) occupy south- and west-facing slopes with only two exceptions and represent 60% of all south-facing basins. Oblique to vertical strata outcrop on south-facing slopes. Weathering and erosion occur preferentially along the recessive strata, and more resistant strata form the perimeters of the open rock basins (Fig. 4). The west-facing dip slopes are generally characterized by weathering and erosion confined to relatively few major discontinuities, producing infre- quent chutes and flatiron topography. Cluster I1 (rockfall funnels) contains a much greater proportion of basins with north- and east-facing aspects. The similar distributions of aspect for cluster I1 and the total sample reflect the modal position of funnels along the spectrum of open rock basins. Intensive weathering and erosion on these north- and east-facing slopes may result from preferred snow deposition with prevailing southwesterly winds and a negative energy balance. Avalanche erosion and nivation are more important, and a greater frequency of rockfalls (Gardner 1980) reflects both the predominance of north- and east-facing rock walls and greater effectiveness of mechanical weathering on these slopes.
The same topoclimatic and structural factors account for the predominance of north- and particularly east-facing slopes in cluster V. Near-horizontal exposure of bedding planes on east-facing scarp slopes
TABLE 3. Distribution of basin aspects for total sample and clusters in degrees east of north
North East South West
Total sample 13.0 29.6 18.5 38.9 ( n = 54)
Cluster I 6.25/14.3* 6.2516.25 37.5/60.0 50.0/38.1 ( n = 16)
Cluster II 15.0142.8 35.0143.75 10.0/20.0 40.0138.1 ( n = 20)
Cluster IU 33.3128.6 - - 66.7119.0 ( n = 6 )
Cluster IV - 33.316.25 66.7120.0 - ( n = 3)
Cluster V 11.0/14.3 78.0143.75 - 11.014.8 ( n = 9)
*(Percentage of the cluster with this aspect)/(percentage of sample with this aspect).
Can
. J. E
arth
Sci
. Dow
nloa
ded
from
ww
w.n
rcre
sear
chpr
ess.
com
by
Mer
ced
(UC
M)
on 0
5/06
/14
For
pers
onal
use
onl
y.
CAN. 1. EARTH SCI. VOL. 20, 1983
TABLE 4. Comparative morphometry of Kananaskis
Length (m) Width (m) Relief (m) Area (km2)
Mean SD Range Mean SD Range Mean SD Range Mean SD Range
Kananaskis open cirques (cluster V) 743 154 541- 952 553 94 389- 713 700
Aniya and Welch (1981), Antarctica 21 16 660-4584 1679 840-3240 515
Gordon (1977), simple cirques northwest Scotland 625 346 100-1840 586 296 130-2250 276
King (1974) Montana 1296 498 Utah 1261 475
Graf (1976), empty cirques, American Rockies Livingstone Range 958 304 200 Beartooth Range 869 370 321 Wind River Range 471 153 227
Trenhaile (1976), isolated cirques, southern Canadian Cordillera
Andrews and Dugdale (197 I), Baffin Island 1053t 220-2620 833t 290-2258 257t
*Ratio calculated from reported mean values. ?Median rather than mean.
may in part explain the 78% of cluster V basins in the east-facing category. "Undeqnining, backwearing rock- falls" (Rapp 1960, p. 43) promote the erosion of resistant strata, and thereby comparatively rapid and uniform rock-wall retreat. Since erosion is not confined to particular strata, basins are more compact and have greater areairelief ratios. The nine open rock basins in clusters I11 and IV are considered transitional features between funnels and cirques. However, these types of basins require further examination. No particular significance is attached to their structural or topoclimatic settings given the small sample sizes.
The morphology and orientation of the basins in cluster V suggest that these basins may either have been glaciated or embody conditions for cirque glaciation. The two basins with the highest areairelief ratios (upper right comer of Fig. 3) contain snow patches that persist until the late summer or early fall. One of these basins is shown in Fig. 5.
In Table 4, cirque morphometric data from published sources are listed beneath summary data for cluster V. Where the authors have stratified their samples, only data from subsamples most similar to the open rock basins were used. That is, the published data generally apply to closed basin cirques developed immediately beneath a summit or ridge crest. Although the glacial cirques are clearly much larger than the cirque-like open
rock basins in terms of planimetric variables, the ratios of lengthiwidth are of similar magnitude. However, the areairelief values for the glacial cirques are generally an order of magnitude larger than the mean value for the open cirques. These values imply an equilibrium planimetric shape that spans a considerable range of area and areairelief values. The lengthiwidth ratios for cluster V correspond to a mean basin gradient of 43' with maximum and minimum values of 38.2 and 50.6". The mean glacial cirque gradients range from 9.7" (Antarctic) to 24.3' (Scotland). The steepest basins among the Scottish cirques and cluster V have similar gradients (50.0 versus 50.6'). The mean relief for open rock basins is significantly greater than the published values for mean cirque relief. Whereas cirque glaciation is limited to the altitudinal zone above the perennial snowline, open rock basins evolve by periglacial weathering and erosion throughout the periglacial altitudinal zone. That is, greater relief is available for the development of periglacial open rock basins than for glacial cirques.
Although the open cirques are comparable plani- metrically to glacial cirques in Scotland, Baffin Island, Antarctica, and other parts of the Rockies, they generally have greater relief, and in the study area extend below the floors of glacial cirques. Thus, glaciation of these basins was probably restricted to niche or slab glaciers in
Can
. J. E
arth
Sci
. Dow
nloa
ded
from
ww
w.n
rcre
sear
chpr
ess.
com
by
Mer
ced
(UC
M)
on 0
5/06
/14
For
pers
onal
use
onl
y.
SAUCHYN AND GARDNER 417
open cirques and glacial cirques
Lengthlwidth Length/relief Widthlrelief healrelief
Mean SD Range Mean SD Range Mean SD Range Mean SD Range
FIG. 5. Northeast-facing open cirque, 1 km west of Upper Kananaskis Lake.
Can
. J. E
arth
Sci
. Dow
nloa
ded
from
ww
w.n
rcre
sear
chpr
ess.
com
by
Mer
ced
(UC
M)
on 0
5/06
/14
For
pers
onal
use
onl
y.
CAN. J. EARTH SCI. VOL. 20, 1983
their upper reaches. The niche glacier hollows in Vestspitsbergen described and illustrated by Groom (1959) strongly resemble the cirque type of open rock basin and larger rockfall funnels of the Kananaskis area. Where snow is redistributed by avalanching within a basin to below the regional snowline, an open rock basin may not be a suitable site for future cirque glaciation.
Rapp (1960) proposed a model whereby rockfall funnels evolve from the denudation of bedrock ridges - between adjacent rockfall chutes. Similarly, an evolutionary model of cirque basin form is often either implied, by reference to immature or mature stages of cirque form, or explicitly stated. King (1970) discussed the positive feedback that promotes the enlargement of cirques. The same positive feedback mechanism, in theory, characterizes the growth of and transition between chutes and funnels, since the dissection of mountain rock walls enhances the processes that produce and accentuate dissection: wind deposition of snow and in turn avalanche erosion and nivatcon, growth and preservation of ice in discontinuities, and concentra- tion of snowmelt and rainfall runoff. Therefore, some of the variation in open rock basin morphometry is due to stage of development. However, without the observa- tion of change in rockwall morphology, time series models of rock basin morphology are speculative and cannot be tested empirically.
Variations in geologic structure and topoclimate account for much of the variation in rock basin morphometry. Uniform structural control prevails on many east-facing scarp slopes, where topoclimate encourages intensive periglacial weathering and erosion. Thus, 87.5% of the basins with easterly aspects are classified as cirques or rockfall funnels. The other major class of open rock basins, rockfall chutes, encompasses 60% of the south-facing basins. The structural control by oblique strata dominates the morphologic effect of processes on south-facing slopes.
Conclusions Open rock basins on mountain slopes can be classified
on the basis of their morphome&c properties and distinguished from glacial cirques, which are the most frequently cited and researched rock basins in mountain morphology. The 54 open rock basins examined in this study are superjacent to active debris slopes. As such, they appear to be an integral part of an ongoing system of mountain slope gradation by physical processes, the rock basin being the area of degradation and the debris slope being the area of temporary aggradation.
Two thirds of the 54 open rock basins occupy the lower quarter of the tqtal range of arealrelief values (Fig. 3, clusters I and II). These basins vary considerably in length/width ratio and compactness values with a significant discontinuity permitting the
identification of two types of rock basins: chutes and funnels. The majority of the remaining rock basins are in the upper three quarters of the range of area/relief values and the upper third of the compactness values (Fig. 3, clusters 111 and V). Those with the highest area/relief values conform to the definition of cirques (Evans and Cox 1974) but differ in some morphometric properties (Table 4). Cluster I11 is transitional between funnels and open cirques. The few examples in cluster IV may be transitional between recognized morpholog- ical types and may represent special or unusual geological contexts.
From estimated rates of debris (talus) accumulation, Gray (1971) concluded that rates of rock-wall retreat (mountain slope degradation) are 30 times greater in open rock basins than on undissected mountain walls. In the absence of cirque glaciation and, possibly, coincident with it, open rock basins are a major focus of weathering and erosion in the mountain environment. Gross morphology of rock basins is related primarily to geological structure. At the same time, this morphology controls the effectiveness of various geomorphic processes and agents, and thereby the detailed aspects of rock basin morphology. This paper has presented a morphometric analysis of open rock basins comparable to those that have been completed for classical, closed basin cirques. Given that the open rock basins are part of an ongoing degradation-aggradation system on contem- porary mountain slopes in the Front Ranges of the Canadian Rocky Mountains, future research will focus on the apparent relationships between rock basin morphometry and the ongoing erosional processes.
Acknowledgements We are grateful to the Division of Survey Engineering,
University of Calgary, and particularly to Clive Fraser and Gary Martinsen for enabling the acquisition of digital elevation data. The Resource Evaluation Branch of Alberta Energy and Natural Resources provided the aerial diapositives. Field work was supported by grant A9152 from the Natural Sciences and Engineering Research Council of Canada. Computing was supported by a grant from the University of Waterloo. Jamie Steel assisted in acquiring the necessary ground data. Joe Desloges shared his observations on bedrock fracture density.
AHLMANN, H. 1919. Geomorphological studies in Norway. Geografiska Annaler, 1, pp. 1-242.
ANDREWS, J. T., and DUGDALE, R. E. 1971. Quaternary history of northern Cumberland Peninsula, Baffin Island, N. W.T. Part V: Factors affecting corrie glacierization in Okoa Bay. Quaternary Research, 1, pp. 532-551.
ANIYA, M., and WELCH, R. 198 1. Morphometric analyses of Antarctic cirques from photogrammetric measurements. Geografiska Annaler, 63A, pp. 41-53.
Can
. J. E
arth
Sci
. Dow
nloa
ded
from
ww
w.n
rcre
sear
chpr
ess.
com
by
Mer
ced
(UC
M)
on 0
5/06
/14
For
pers
onal
use
onl
y.
SAUCHYN AND GARDNER 419
BLAIR, D. J., and BLISS, T. H. 1967. The measurement of shape in geography. Department of Geography, Notting- ham University, Nottingham, England, Quantitative Bulletin 11.
COMBS, J. E. 1980. Planning and executing the photogram- metric project. In Manual of photogrammetry. Edited by C. C. Slama, C. Theurer, and S. Henriksen. American Society of Photogrammetry, Falls Church, VA, pp. 367-412.
DETWILER, W. R. 1980. Autoplot-the analytical stereoplot- ter from Systemhouse. Proceedings, American Society of Photogrammetry, Analytical Plotter Symposium and Workshop, April 1980, Reston, VA, pp. 137-144.
EVANS, I. S., and Cox, N. 1974. Geomorphometry and the operational definition of cirques. Area, 6, pp. 150- 153.
FRIEDMAN, S. J. 1980. Automation of the photogrammetric process. In Manual of photogrammetry. Edited by C. C. Slama, C. Theurer, and S. W. Henriksen. American Society of Photogrammetry, Falls Church, VA, pp. 699-722.
GARDNER, J. S. 1980. Frequency, magnitude, and spatial distribution of mountain rockfalls and rockslides in the Highwood Pass area, Alberta, Canada. In Thresholds in geomorphology. Edited by D. R. Coates and J. D. Vitek. Allen and Unwin Inc., London, England, pp. 267-295.
1982. Alpine mass-wasting in contemporary time: some examples from the Canadian Rocky Mountains. In Space and time in geomorphology. Edited by C. E. Thorn. Allen and Unwin Inc., London, England, The Binghampton Series in Geomorphology: International Series, No. 12, pp. 171-192.
GORDON, J. E. 1977. Morphometry of cirques in the Kintail-Affric-Cannich area of north west Scotland. Geografiska Annaler, 59A, pp. 177- 194.
GRAF, W. L. 1976. Cirques as glacier locations. Arctic and Alpine Research, 8, pp. 79-90.
GRAY, J. T. 1971. Processes and rates of development of talus slopes and protalus rock glaciers in the Ogilvie and Wernecke Mountains, Yukon Temtory . Ph.D. thesis, McGill University, Montreal, P.Q.
GROOM, G. E. 1959. Niche glaciers in Bunsow Land, Vestspitsbergen. Journal of Glaciology, 3, pp. 368-376.
HALLADAY, I. A. R., and D. H. MATHEWSON. 1971. A guide to the geology of the eastern Cordillera along the Trans Canada Highway between Calgary, Alberta and Revel- stoke, British Columbia. Canadian Exploration Frontiers Symposium, Alberta Society of Petroleum Geologists, Banff, Alta.
HELWIG, J. T., and COUNCIL, K. A. 1979. s ~ s user's guide. 1979 ed. s ~ s Institute Inc., Raleigh, NC.
KING, C. A. M. 1970. Feedback relationships in geomor- phology. Geografiska Annaler, 52A, pp. 145-159.
1974. Morphometry in glacial geomorphology. In Glacial geomorphology. Edited by D. K . Coates. State University of New York, Binghampton, NY, pp. 147-162.
LUCKMAN, B. H. 1970. The nature and variability of size sorting on talus slopes. Proceedings, Canadian Association I of Geographers, Winnipeg, Man., pp. 213-220.
MARKGREN, M. 1964. Geomorphological studies in Fennos- candia 11. Chute slopes. B. Systematic studies. Lund Studies in Geography, Lund, Sweden, Series A, No. 28.
MATTHES, F. E. 1930. Geologic history of the Yose@te
I Valley. United States Geological Survey, Professional Paper 160.
1938. Avalanche sculpture in the Sierra Nevada of California. International Union of Geodesy and Geophysics Bulletin, 23, pp. 631-637.
MUSIELEWICZ, S. 1980. Talus cones in the Polish Tatra Mountains. Studia Geomorphologica Carpatho-Balcanica, XIV, pp. 64-75.
PRICE, R. A., BALKWILL, H. R., CHARLESWORTH, H. A. K., COOK, D. G., and SIMONY, P. S. 1972. The Canadian Rockies and tectonic evolution of the southeastern Canadian Cordillera. In Field excursion A15-C15 guidebook. 24th International Geological Congress, Montreal, P.Q.
! RAPP, A. 1960. Talus slopes and mountain walls at
Tempelfjorden, Spitsbergen. Norsk Polarinstitutt, Skrifter 119.
TRENHAILE, A. S. 1976. Cirque morphometry in the Canadian Cordillera. Annals, American Association of Geographers, 66, pp. 451-462.
Can
. J. E
arth
Sci
. Dow
nloa
ded
from
ww
w.n
rcre
sear
chpr
ess.
com
by
Mer
ced
(UC
M)
on 0
5/06
/14
For
pers
onal
use
onl
y.