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Backwash-and-swash-oriented current crescents: indicators of beach slope, current direction and environment
Sedimentary Geology, 84 (1993) 139-148
139
Elsevier Science Publishers B.V., Amsterdam
Backwash-and-swash-oriented current crescents: indicators of beach slope, current direction and environment Asokkumar Bhattacharya Department of Marine Science, Calcutta University, 35 Ballygunge Circular Road, Calcutta 700019, India (Received July 1, 1992; revised version accepted October 8, 1992)
ABSTRACT Backwash-and-swash-oriented current crescents, which are confined to the swash zone of beach environments, are reported, with their detailed morphology and formation mechanism, from the siliciclastic fine sandy beaches of Sagar Island and Lower Long Sand, two tidal islands of the Ganga estuary, northeast India. Morphologically, the swash-oriented arms of these current crescents are shorter and less prominent than the backwashoriented arms. The shorter arms open landward whereas the longer arms open seaward. They generally form around obstacles like armoured mud balls, polychaete tubes, tussocks etc. which are partially set in a scour implying greater anchoring or a semipermanent nature. These current crescents occur on fine to very find sandy beaches (Mz = 2.9 to 3.1 qS) with a beach slopes of 1:50 to 1:90 (1.4-0.7°). They are very commonly associated with current lineations. The arms of crescents parallel current lineations and are at right angles to the shoreline. In contrast, ordinary backwash-oriented current crescents form on steeper beach slopes, 1:9 to 1 : 25 (6-3°), in fine to medium sand(Mz = 1.8 to 2 ~5). They occur around tiny obstacles which are easily swept away by wave swash and backwash.
Introduction Current crescents or U-shaped furrows (Peabody, 1947) formed around permeable or impermeable bluff bodies occur as a physical surface structure in a wide range of aqueous or aeolian environments. The aqueous current crescents have been reported from fluviatile, shallow marine to deep marine environments. Aeolian current crescents, on the other hand, are known to occur in deserts, aeolian deposits of river flood plains, supratidal beaches and coastal fore dunes. Allen (1984), summarizing the large literature on current crescents, explained schematically the effects of bluff bodies on mean bed shear stress in different types of boundary-layer flow. The bluff bodies around which the current crescents occur include shells, mud balls, stranded wood, mineral and rock fragments, tussocks of algae, weed and other vegetation (Reineck and Singh, 1980; Allen, 1984). The semi-permanent tubes of polychaetes may stand in the path of wave swash and backwash to act as bluff bodies. In almost all cases, the current crescents are
preserved along with the obstacles around which they form. In certain situations, current crescents are formed lacking preservation of obstacles (Bhattacharya, 1992). In the list of the many surface physical features of the beach environment, current crescents are ubiquitous and occupy a definite position (Komar, 1976). Generally an obstacle placed on a sandy beach face and exposed in the path of wave swash and backwash deflects only the backwash flow producing a backwash-oriented current crescent (Komar, 1976; Allen, 1984). The U-shaped or V-shaped crescentic structure widens in the backwash flow direction. As a result current crescents on beaches so far received attention for their use in determining the direction of backwash or the offshore side of an ancient beach (Komar, 1976). The mechanism of formation of current crescents is attributed to the p h e n o m e n o n of flow separation out of vortices and velocity defects created during flow around obstacles (Sengupta, 1966; Karcz, 1968). Sengupta (1966) explained the mechanism of formation of current crescents in a
0037-0738/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
14(I
unidirectional sheet flow past a pebble in a shallow stream. The obstacle that lies in the path of downstream moving water deflects the flow and creates a velocity increase around the sides and upstream end of the obstacle. This excavates a semi-circular depression or moat on the upstream side of the pebbles with concurrent formation of two downstream oriented arms of the crescent marked by zones of fast deposition just exterior to the erosional arms. In the present p a p e r the writer describes the morphology and mechanism of formation of the backwash-and-swash-oriented current crescents created around single obstacles on siliciclastic tropical beaches of N E India. Contrary to the abundant occurrences of backwash-oriented current crescents on sandy beaches (Komar, 1976; Allen, 1984), these newly described backwashand-swash-oriented current crescents are less common or rare in occurrence. But whenever found, they are useful in environmental interpretation. The author is not aware of any previous literature on backwash-and-swash-oriented current crescent excepting only the record of traces of a landward tapering swash-formed crescent around a cockle shell resting on the fine sandy beach of Norfolk, England, where the backwashoriented current crescents around the same shell appear to be very prominent (Allen, 1984, fig. 5-17). In the above context, this study on the backwash-and-swash-oriented current crescents will be highly significant as a process-response structure of the beach environment. Further, their occurrence on well sorted, fine sandy silicielastic beaches with very gentle slope (particularly in areas of less than 1.5 °) suggests that this structure can be exploited sedimentologically to interpret the slope and character of ancient beaches. Study area
Current crescents, i.e. the obstacle marks of Dzulynski and Walton (1965), of various kinds and scales occur in the beach faces south of Sagar Island at the mouth of the Ganga estuary and that encircling the Lower Long S a n d - - a n offshore sandy shoal in the Bay of Bengal, northeast
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India (21°28'-22°15'N, 88°0'-88°40'E; Fig. 11. Being situated at the mouth of the Ganga estuary, the intertidal zones are characterised by tidal flat-related siliciclastic tropical beach faces that occur in a macrotidal setting (tidal range ttp to 6 m) having semi-diurnal tides. The wave energy at the coast is moderate to low excepting periods (:;f storm surge. Two dominant swell directions approach the coast throughout the year (Fig. 1). Beach character
The foreshore beach face ranges in width from 100 to 250 m with a seaward gradient ranging from 1 in 50 to 1 in 90 (1.4-0.7°). tn areas of ridge and runnel slopes the gradient is as steep as 1 in 9 (6°). The siliciclastic beach material is dominantly quartzo-feldspathic (95%) with about 5% of accessory biotite, muscovite, hornblende, and ilmenite. Texturally the beaches are composed of fine to very fine sand (Mz = 2.2 to 3.1 ~b) with good to very good sorting of sediments, (~rI = 0.18 to 0.57 ~b; Bhattacharya, 1987. 1988). There is in general, a negative correlation between grain size measured in phi, and wave energy or beach slope. Flemming and Fricke (1983) have shown that for any given grain size, the beach slope increases with decreasing wave energy. Alternatively, at similar wave energy levels,
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CRESCENTS
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a coarser sand always forms a steeper slope than a finer sand. In the present situation, measured values of beach slope correspond closely with computed values when plotted on the wave energy-grain size-beach slope diagram (Fig. 2). Further, beach composition has control over beach slopes. Guilcher and King (1961) recorded a gentler gradient of 1 in 71 to 1 in 89 for quartz sand beaches compared with a steeper slope of 1 in 16.9 to 1 in 23.4 for lower-density organic sand beaches of similar size and exposure. The slopes, 1 in 50 to 1 in 90 for the siliciclastic quartzo-feldspathic beaches under study, are consistent with that of Guilcher and King (1961). Geomorphologically, a series of narrow sandy longshore bars occurs discontinuously in the subtidal and in the lower intertidal zone of both the islands. As a result, some portions onshore, behind the bars, experience a barred tidal flat situation where the Ganga and Mooriganga riverborne suspended mud accumulates. These mud flats having a maximum thickness of 90 cm or more may continue parallel and perpendicular to the shore for some tens of metres.
Fig. 3. Backwash-and-swash-oriented current crescents formed by wave swash and backwash around a Nereis tube resting on a sandy beach of Lower Long Sand Island in the Bay of Bengal. Landward-pointing swash-oriented current crescent remains preserved. Seaward toward left of photograph. E a s t - w e s t aligned current lineations clearly visible. Pen 14 cm long.
142
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Fig. 4. Conical scour pits shaped by wave action around polychaete tubes in the sandy beach of Sagar Island, NE India. A ccntJa! burrow within the tube can be seen. Backwash-oriented current crescents present. Seaward toward left ~f photograph Pcr~ 14 ci~
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Fig. 5. Backwash-and-swash-oriented current crescents formed by wave swash and backwash around armoured mud-ball~, resling ¢,r~ the sandy beach of Sagar Island, NE India. Seaward toward right of photograph. Scale 30 cm long,
BACKWASH-AND-SWASH-ORIENTEDCURRENT CRESCENTS
Tools for current crescents
The sun-cracked mud flat materials of the foreshore undergo transportation by wave action and are strewn all over the beach. Subsequent rolling induces a spindle or cylindrical shape to these initially columnar mud blocks which may be armoured with quartz sand or shell fragments. The long axes of the mud balls range from 5 to 10 cm with maximum cross-sectional diameters of 2 to 5 cm. Being situated in the path of wave swash and backwash flows these mud balls (with the long axes parallel to shore) act as obstacles or tools for the current crescents. Apart from mud balls, tussocks of roots, particularly of Suaeda maritima and some exotic rounded stones of about 10 cm diameter act as tools for the current crescents. The role of polychaete tubes as tools for current crescents is no less important. Many species of polychaetes, such as Nereis sp., Perinereis sp. and Sabellaria sp. inhabit semi-permanent tubes of variable length (up to 30 cm) and diameter (5 mm to 5 cm) in the lower foreshore beaches. Generally these tubular semi-permanent obsta-
143
cles are made up of fine sand, silt and clay sized particles with small pieces of shells cemented together with mucus material (Marshall and Williums, 1979). The mud balls, tussocks and stones, having diameters of over 5 cm, and the semi-permanent polychaete tubes of 5 mm to 5 cm in diameter, have a greater ability to anchor in the loose beach sand and act as tools for the backwash-andswash-oriented current crescents (Figs. 3-5). In contrast, very small mud pellets, shells and shell fragments ( < 1 cm in diameter) which are drifted easily by swash and backwash, act as tools when at rest for the common backwash-oriented current crescents (Fig. 6). Mechanism of formation of backwash-andswash-oriented current crescents
Unlike unidirectional flows in rivers, swashbackwash effects impart a bidirectional flow pattern on intertidal beach faces. Being influenced by downstream flow, current crescents in shallow river beds always point to the downcurrent direction (Peabody, 1947; Sengupta, 1966; Karcz, 1968;
Fig. 6. Closely spaced backwash-oriented current crescents formed by wave backwash around small mud pellets resting on fine sand. Beach of Lower Long Sand Island in the Bay of Bengal. Intersecting arms of current crescents resemble rhomboid marks. Seaward toward left of photograph. Pen 14 cm long.
144
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Conybeare and Crook, 1968). Alternatively. current c r e s c e n t s o n b e a c h f a c e s a r e chiefly c o n -
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TABLE 1 A comparative table showing morphology and beach characteJ for the backwash-and-swash-oriented current c~e~
Backwash-and-swash-oriented current cres-
Backwash-oriented current cr,.~sccms
cents Morphology
Two backwash-oriented arms around an obstacle open seaward and two swash-oriented arms around the same obstacle open landward
Two backwash-oriented arms m,om~d an ob stacle open seaward
Backwash-oriented arms generally longer and more prominent than swash-oriented arms
Backwash-oriented arms quilc !~r,mmc,m
Nature of obstacle and scour
Permeable or impermeable bluff bodies of 5 to 10 cm in diameter set in a scour pit showing semi-permanent nature
Permeable of impermeable !flufi bodit:s. generally a few mm to less than I cm rest tm beach surface without any prtmHnenl ~cou~ pit
Beach slope
Gentler beach slope, 1:50 to 1:90 (1.4-0.7 °)
Steeper beach slope, 1:25 to I : ) !?..5 f~J
Grain size
Fine to very fine sand ( M z = 2.9 to 3.1 O)
Medium to fine sand ( M z = l.,~ i<, -'.2 ,h )
Associated structures
Current lineations parallel arms of crescents
Current lineations may or may ~o| bc pr~ sent
Rill marks rare
Rill marks common
A process-response structure of beach; typically signifies swash zone
Not a process-response structur,:., beach and varied environment
Environment
BACKWASH-AND-SWASH-ORIENTEDCURRENTED CRESCENTS
ments as tools, the backwash-oriented current crescents open their arms along the seaward slope. In this respect, these ordinary backwashoriented current crescents are not sensitive enough to be used as a process-response structure of the beach environment. Sometimes, intersecting arms of the closely spaced backwash-oriented current crescents may give rise to a pattern on the beach surface resembling rhomboid ripples (Fig. 6). The fact that current crescents look like rhomboid marks means that the vortices generated by the obstacles are constrained by surface shock waves on the free surface of the thin flow through which they penetrate. The small objects on the beach surface that lie in the path of wave swash and backwash, first move up the beach slope by uprush of waves and are then rolled down the slope for some distance in the direction of wave backwash. The small objects come to a position of rest when the backwash velocity is unable to move them downslope any further. Field measurements reveal that, where the beach material is medium to fine sand
145
and slope is between 1 : 25 and 1 : 9 (2.5-6°), tiny objects which have little ability to anchor on the sandy substrate act as obstacles in the path of backwash flows and consequently leave around them backwash-oriented current crescents. The crescents are preserved on the beaches when an extremely thin film of water is able to create a flow separation against these tiny obstacles. T h e b a c k w a s h - a n d - s w a s h - o r i e n t e d current crescents, on the other hand, are formed around relatively large objects on fine to very fine sand with beach slopes ranging between 1 : 50 and 1 : 90 (1.4-0.7°). Large (5-10 cm diam.) impermeable bluff bodies like mud balls, stones and polychaete tubes as well as permeable bluff bodies like tussocks, have a greater ability to anchor on the substrate, and generally act as obstacles for these current crescents (Fig. 7, Table 1). Flow separation and a concurrent velocity increase around these obstacles both during uprush and backwash flows create a circular or semicircular scour with time. The larger obstacles, which are well placed in the scour pits, have sufficient relief above beach surface to create flow separation for the
Fig. 8. Backwash-oriented current crescents formed by wave backwash around polychaete tubes on the sandy beach of Sagar Island, NE India, Scour pits around obstacle and rill marks on upcurrent side can be seen. East-west aligned current lineations clearly visible. Pen 14 cm long.
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to-and-fro moving water. The depth of the scours was almost one-half the equidimensional stony obstacles and somewhat higher than this ratio for cylindrical or spindle-shaped mud balls. These depths of scours are consistent with those deduced by H.N.C. Breusers (1965) and H.W. Shen et al. (1969), both in Allen (1984). Morphologically, these current crescents have shorter and less prominent swash-oriented arms compared to their backwash-oriented arms. The upslope movements of sediment, together with diminishing swash velocity due to bed friction, cause the shorter swash-oriented arms. The low relief difference between the upslope and downslope sides of the obstacles offers protection to these surface sculptures from wave erosion. As a result, these crescentic marks which form around one and the same obstacle are well retained with opening of arms in both the seaward and landward directions. The b a c k w a s h - a n d - s w a s h - o r i e n t e d current crescents form during both flood and ebb-flows of the neap and spring tidal cycles. They form during calm conditions; storms may completely oblit-
erate such low relief features on the beach, They are characteristic of the lower to middle foreshore zone and are thus located within 2 m elevation above mean sea level. The directions o| wave approach determine their orientation on the beach surface. Association of current lineations and backwashand-swash-oriented current crescents
The beach surfaces sculptured with backwashand-swash-oriented current crescents oltcn s h o a superimposition of current lineations i,c, segregation of finer- and coarser-grained striations (Figs. 3, 8). These current lineations are rather "streaming lineations" in the sense of Conybcarc and Crook (1968). The preferred sand-grain orienta .... tion in this structure with narrow elevations and depressions on the surface of the bed is chiefly controlled by backwash flows as revealed by the orientation of the striae parallel to tile arms of the current crescents. Curray ( t 9 5 6 ) o b s e r v e d current lineations on beach surfaces formed b3 backwash flows. Other examples of the association of current crescents and current tineations
Fig. 9. C u r r e n t c r e s c e n t s f o r m e d by b a c k w a s h a r o u n d r o u n d e d stone f r a g m e n t s ( 7 - 1 0 cm diam.) r e s t i n g on fine samL Beach o~ S a g a r Island, N E India. Scour pits a r o u n d obstacles and incised rill m a r k s on u p c u r r e n t side seen, S e a w a r d toward left o i p h o t o g r a p h . Pen 14 cm long.
147
BACKWASH A N D - S W A S H - O R I E N T E D C U R R E N T CRESCENTS
are obtained from Pettijohn and Potter (1964), from P. Cepek and H.E. Reineck (1970) in Reineck and Singh (1980), and from Allen (1966, 1984). Current lineations in sand occur in the plane bed phase of the upper flow regime (Allen, 1964; Reineck and Singh, 1980) and result from streamwise sand graining over a level bed (Sorby, 1908; Crowell, 1955). According to Allen (1984) current lineations are formed due to the action of boundaryqayer streaks, the transverse spacing of which is determined by flow properties. The association of backwash-and-swash-oriented current crescents and current lineations in the present study justifies the fact that the former takes place in areas of very gentle slope to flat beach faces. But unlike current lineations, the current crescents which are products of scour and deposition around obstacles, are a lower flow regime structure (McDonald and Banerjee, 1971). Association of rill marks and backwash-oriented current crescents: resemblance with backwashand-swash-oriented current crescents
In areas of considerably steeper beach slope ( 3 - 6 °) two small incised rill marks originating on the upslope side may converge at the scour pit of the obstacle with backwash-oriented current crescents (Figs. 8, 9). These low-relief rill marks are generated during sinking water level and resemble arms of the swash-oriented current crescents. Thus, a combination of backwash-oriented current crescents and a pair of low relief rill marks may be mistaken for backwash-and-swash-oriented current crescents. Careful field observations are needed to identify them from the recent beach deposits. Recognition of their ancient analogue from rock records will be more difficult because of post-depositional changes. Geological significance of backwash-and-swashoriented current crescents
(2) The longer arms of the crescents form out of backwash flows and so their directions of opening point seaward. The shorter arms of the crescents, on the other hand, are formed by wave swash and their arms open landward. (3) Backwash-and-swash-oriented current crescents form on very gentle beach slopes of 1 : 50 to 1:90 (1.4-0.7 °) composed of fine to very fine sand (Mz = 2.9 to 3.1 ~b). The backwash-oriented current crescents prefer a steeper beach slope of 1:25 to 1:9 (3-6 °) made up of medium to fine sand (Mz = 1.8 to 2.2 ~b). Thus current crescents can be utilized to interpret ancient beach slope. (4) The backwash-and-swash-oriented current crescents form around both permeable and impermeable obstacles which are semi-permanent and well anchored to the beach sand. In contrast, the objects responsible for backwash current crescent are easily moved by wave swash and backwash and are less able to anchor on the beach surface. (5) The formation of current crescents is related to normal calm situations of the beach. Storm situations may erode and obliterate these low-relief beach features. (6) The backwash-and-swash-oriented current crescents are often associated with current lincations. The former are lower flow regime structures whereas the latter indicate upper flow regime. (7) The features formed by convergence of two rill marks into the scour pit on the upslope side of backwash-oriented current crescents may be mistaken for backwash-and-swash-oriented current crescents. (8) The shoreline is always at right angles to the extension directions of the arms of the crescents. (9) The direction of wave swash and backwash on beach surfaces can be interpreted from the alignment of the arms of current crescents. Conclusion
(1) Backwash-and-swash-oriented current crescents act as a process-response structure of beaches. The direction of wave swash and backwash are marked by the alignment of the arms of the crescents.
The current patterns in a beach, particularly in the swash zone, are essentially bipolar because of the rushing up of wave-swash and backwash. However, very few physical sedimentary struc-
148
tures are significant enough to be utilized for determining these bipolar current directions, excepting the rhomboid ripple marks in which the convex or broad side points downcurrent and the concave or pointed side points the upcurrent direction (Hoyt and Henry, 19631. The present study has confirmed that apart from rhomboid ripple marks, the backwash-andswash-oriented current crescents around an obstacle appear to be very diagnostic of bipolar currents in the swash zone. This structure also helps to interpret the beach slope, direction of the approaching waves and shoreline orientation. Although current crescents can appear in varied environments, the backwash-and-swash-oriented current crescents are confined to the swash zone of beaches. This interpretation gains additional support when they occur around mud balls armoured with shells of littoral zones a n d / o r around polychaete tubes that rest on lower foreshore beaches. Recognition of backwash-and-swash-oriented current crescents would add one more to the list of process-response structures of the beach environment. Fossilized current crescents plausibly of littoral origin are known from Hall (1843) and from W. Vortisch (1973) in Allen (1984). Records of such features from rocks will be of interest for interpreting ancient beach environments. References Allen, J.R.L., 1964. Primary current lineation in the Lower Old Red Sandstone (Devonian), Anglo-Welsh Basin. Sedimentology, 3: 98-108. Allen, J.R.L., 1966. On bed forms and palaeocurrents. Sedimentology, 6: 153-190. Allen, J . R i . , 1984. Sedimentary Structures--Their Character and Physical Basis. Elsevier, Amsterdam, Vol. 1, 593 pp., VoL II, 663 pp. Bhattacharya, A., 1987. Interpretation on the pattern of wind and water movement over Lower Long Sand from studies on texture and structure of the sediment. J. Mar. Biol. Assoc. India, 2 9 : 1 1 5 - 1 2 3 .
A. IBHA VI'ACI-tAR'YA Bhattacharya, A., 1988. Intertidal depositional character of the Hooghly tidal islands, West Bengal. Indian J. Geol., 60: 153-164. Bhanacharya, A., 1992. Current crescents formed by marinc algae (Valonia sp.): a new record of obstacle marks lacking preservation of obstacles. Sedimentology, 39:513 516. Conybeare, C.B.B. and Crook, K.A.W., 1968. Manual ol: sedimentary structures. Bureau of Mineral Resources, Geol ogy and Geophysics, Canberra, A.C.T., Bul!. /02. 327 pp. Crowell, J.C., 1955. Directional current structures from alpi~c flysch, Switzerland. Geol. Soc. Am. Bull., 66:1351:1384. Curray, J.R., 1956. Directional grain orientation studies on recent coastal sands. Bull. Am. Assoc. Pe|. Geol.. 40:: 2440-2456. Dzulynski, S. and Walton, E.K., 1965. Sedimentary Feature~ of Flysch and Graywackes. Elsevier, Amsterdam, 274 pp. Flemming, B.W. and Fricke, A.H., 1983. Beach and nearshore habitat as a function of internal geometry, primary sedi mentary structures and grain size. In: A. McLachtan and T. Erasmus (Editors), Sandy Beaches as Ecosystems~ I)l. W. Junk Publ., The Hague, 757 pp. Hall, J., 1843. Geology of New York, Part IV. Caroll and Cook Albany, N.Y., 683 pp. Hoyt, J.H. and Henry, V.J., 1963. Rhomboid !~ipple marks, indicator of current direction and environment. J, Sediment. Petrol., 33: 604-608. Karcz, 1., 1968. Fluviatile obstacle marks from the wadis o~ Neveg (Southern Israel). J. Sediment. Petr¢~L 38: 10001012. Komar, P.D., 1976. Beach Processes and Sedimentatkm. Prentice Hall, Englewood Cliffs, N.J., 429 pp. Marshall, A.J. and Williums, W.D. (Editors), 1979. Textbook of Invertebrate Zoology. Macmillan, New YorL 874 pp. McDonald, B.C. and Banerjee, I., 1971. Sediments and bedforms on a braided outwash plain, Can. J, Earth Sci., ;5: 1282-13(11. Peabody, F.E., 1947. Current crescents in Triassic Moenkopi formation. J. Sediment. Petrol., 17: 73-76. Pettijohn, F.J. and Potter, P.E., 1964. Atlas and Glossary o~ Primary Sedimentary Structures. Springer-Verlag, Berlin. 370 pp. Reineck, H.E. and Singh, I.B., 1980. Depositionat Sedimentary Environments. Springer-Verlag, Berlin, 439 pp. Sengupta, S., 1966. Studies on orientation and imbrication of pebbles with respect to cross-stratification, i. SedimenL Petrol., 40: 81-101. Sorby, H.C., 1908. On the application of quantitative methods to the study of the structure and history of rocks. Q.3 Geol. Soc. London, 64: 171-233.
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Elsevier Science Publishers B.V., Amsterdam
Backwash-and-swash-oriented current crescents: indicators of beach slope, current direction and environment Asokkumar Bhattacharya Department of Marine Science, Calcutta University, 35 Ballygunge Circular Road, Calcutta 700019, India (Received July 1, 1992; revised version accepted October 8, 1992)
ABSTRACT Backwash-and-swash-oriented current crescents, which are confined to the swash zone of beach environments, are reported, with their detailed morphology and formation mechanism, from the siliciclastic fine sandy beaches of Sagar Island and Lower Long Sand, two tidal islands of the Ganga estuary, northeast India. Morphologically, the swash-oriented arms of these current crescents are shorter and less prominent than the backwashoriented arms. The shorter arms open landward whereas the longer arms open seaward. They generally form around obstacles like armoured mud balls, polychaete tubes, tussocks etc. which are partially set in a scour implying greater anchoring or a semipermanent nature. These current crescents occur on fine to very find sandy beaches (Mz = 2.9 to 3.1 qS) with a beach slopes of 1:50 to 1:90 (1.4-0.7°). They are very commonly associated with current lineations. The arms of crescents parallel current lineations and are at right angles to the shoreline. In contrast, ordinary backwash-oriented current crescents form on steeper beach slopes, 1:9 to 1 : 25 (6-3°), in fine to medium sand(Mz = 1.8 to 2 ~5). They occur around tiny obstacles which are easily swept away by wave swash and backwash.
Introduction Current crescents or U-shaped furrows (Peabody, 1947) formed around permeable or impermeable bluff bodies occur as a physical surface structure in a wide range of aqueous or aeolian environments. The aqueous current crescents have been reported from fluviatile, shallow marine to deep marine environments. Aeolian current crescents, on the other hand, are known to occur in deserts, aeolian deposits of river flood plains, supratidal beaches and coastal fore dunes. Allen (1984), summarizing the large literature on current crescents, explained schematically the effects of bluff bodies on mean bed shear stress in different types of boundary-layer flow. The bluff bodies around which the current crescents occur include shells, mud balls, stranded wood, mineral and rock fragments, tussocks of algae, weed and other vegetation (Reineck and Singh, 1980; Allen, 1984). The semi-permanent tubes of polychaetes may stand in the path of wave swash and backwash to act as bluff bodies. In almost all cases, the current crescents are
preserved along with the obstacles around which they form. In certain situations, current crescents are formed lacking preservation of obstacles (Bhattacharya, 1992). In the list of the many surface physical features of the beach environment, current crescents are ubiquitous and occupy a definite position (Komar, 1976). Generally an obstacle placed on a sandy beach face and exposed in the path of wave swash and backwash deflects only the backwash flow producing a backwash-oriented current crescent (Komar, 1976; Allen, 1984). The U-shaped or V-shaped crescentic structure widens in the backwash flow direction. As a result current crescents on beaches so far received attention for their use in determining the direction of backwash or the offshore side of an ancient beach (Komar, 1976). The mechanism of formation of current crescents is attributed to the p h e n o m e n o n of flow separation out of vortices and velocity defects created during flow around obstacles (Sengupta, 1966; Karcz, 1968). Sengupta (1966) explained the mechanism of formation of current crescents in a
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14(I
unidirectional sheet flow past a pebble in a shallow stream. The obstacle that lies in the path of downstream moving water deflects the flow and creates a velocity increase around the sides and upstream end of the obstacle. This excavates a semi-circular depression or moat on the upstream side of the pebbles with concurrent formation of two downstream oriented arms of the crescent marked by zones of fast deposition just exterior to the erosional arms. In the present p a p e r the writer describes the morphology and mechanism of formation of the backwash-and-swash-oriented current crescents created around single obstacles on siliciclastic tropical beaches of N E India. Contrary to the abundant occurrences of backwash-oriented current crescents on sandy beaches (Komar, 1976; Allen, 1984), these newly described backwashand-swash-oriented current crescents are less common or rare in occurrence. But whenever found, they are useful in environmental interpretation. The author is not aware of any previous literature on backwash-and-swash-oriented current crescent excepting only the record of traces of a landward tapering swash-formed crescent around a cockle shell resting on the fine sandy beach of Norfolk, England, where the backwashoriented current crescents around the same shell appear to be very prominent (Allen, 1984, fig. 5-17). In the above context, this study on the backwash-and-swash-oriented current crescents will be highly significant as a process-response structure of the beach environment. Further, their occurrence on well sorted, fine sandy silicielastic beaches with very gentle slope (particularly in areas of less than 1.5 °) suggests that this structure can be exploited sedimentologically to interpret the slope and character of ancient beaches. Study area
Current crescents, i.e. the obstacle marks of Dzulynski and Walton (1965), of various kinds and scales occur in the beach faces south of Sagar Island at the mouth of the Ganga estuary and that encircling the Lower Long S a n d - - a n offshore sandy shoal in the Bay of Bengal, northeast
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India (21°28'-22°15'N, 88°0'-88°40'E; Fig. 11. Being situated at the mouth of the Ganga estuary, the intertidal zones are characterised by tidal flat-related siliciclastic tropical beach faces that occur in a macrotidal setting (tidal range ttp to 6 m) having semi-diurnal tides. The wave energy at the coast is moderate to low excepting periods (:;f storm surge. Two dominant swell directions approach the coast throughout the year (Fig. 1). Beach character
The foreshore beach face ranges in width from 100 to 250 m with a seaward gradient ranging from 1 in 50 to 1 in 90 (1.4-0.7°). tn areas of ridge and runnel slopes the gradient is as steep as 1 in 9 (6°). The siliciclastic beach material is dominantly quartzo-feldspathic (95%) with about 5% of accessory biotite, muscovite, hornblende, and ilmenite. Texturally the beaches are composed of fine to very fine sand (Mz = 2.2 to 3.1 ~b) with good to very good sorting of sediments, (~rI = 0.18 to 0.57 ~b; Bhattacharya, 1987. 1988). There is in general, a negative correlation between grain size measured in phi, and wave energy or beach slope. Flemming and Fricke (1983) have shown that for any given grain size, the beach slope increases with decreasing wave energy. Alternatively, at similar wave energy levels,
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CRESCENTS
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a coarser sand always forms a steeper slope than a finer sand. In the present situation, measured values of beach slope correspond closely with computed values when plotted on the wave energy-grain size-beach slope diagram (Fig. 2). Further, beach composition has control over beach slopes. Guilcher and King (1961) recorded a gentler gradient of 1 in 71 to 1 in 89 for quartz sand beaches compared with a steeper slope of 1 in 16.9 to 1 in 23.4 for lower-density organic sand beaches of similar size and exposure. The slopes, 1 in 50 to 1 in 90 for the siliciclastic quartzo-feldspathic beaches under study, are consistent with that of Guilcher and King (1961). Geomorphologically, a series of narrow sandy longshore bars occurs discontinuously in the subtidal and in the lower intertidal zone of both the islands. As a result, some portions onshore, behind the bars, experience a barred tidal flat situation where the Ganga and Mooriganga riverborne suspended mud accumulates. These mud flats having a maximum thickness of 90 cm or more may continue parallel and perpendicular to the shore for some tens of metres.
Fig. 3. Backwash-and-swash-oriented current crescents formed by wave swash and backwash around a Nereis tube resting on a sandy beach of Lower Long Sand Island in the Bay of Bengal. Landward-pointing swash-oriented current crescent remains preserved. Seaward toward left of photograph. E a s t - w e s t aligned current lineations clearly visible. Pen 14 cm long.
142
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Fig. 4. Conical scour pits shaped by wave action around polychaete tubes in the sandy beach of Sagar Island, NE India. A ccntJa! burrow within the tube can be seen. Backwash-oriented current crescents present. Seaward toward left ~f photograph Pcr~ 14 ci~
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Fig. 5. Backwash-and-swash-oriented current crescents formed by wave swash and backwash around armoured mud-ball~, resling ¢,r~ the sandy beach of Sagar Island, NE India. Seaward toward right of photograph. Scale 30 cm long,
BACKWASH-AND-SWASH-ORIENTEDCURRENT CRESCENTS
Tools for current crescents
The sun-cracked mud flat materials of the foreshore undergo transportation by wave action and are strewn all over the beach. Subsequent rolling induces a spindle or cylindrical shape to these initially columnar mud blocks which may be armoured with quartz sand or shell fragments. The long axes of the mud balls range from 5 to 10 cm with maximum cross-sectional diameters of 2 to 5 cm. Being situated in the path of wave swash and backwash flows these mud balls (with the long axes parallel to shore) act as obstacles or tools for the current crescents. Apart from mud balls, tussocks of roots, particularly of Suaeda maritima and some exotic rounded stones of about 10 cm diameter act as tools for the current crescents. The role of polychaete tubes as tools for current crescents is no less important. Many species of polychaetes, such as Nereis sp., Perinereis sp. and Sabellaria sp. inhabit semi-permanent tubes of variable length (up to 30 cm) and diameter (5 mm to 5 cm) in the lower foreshore beaches. Generally these tubular semi-permanent obsta-
143
cles are made up of fine sand, silt and clay sized particles with small pieces of shells cemented together with mucus material (Marshall and Williums, 1979). The mud balls, tussocks and stones, having diameters of over 5 cm, and the semi-permanent polychaete tubes of 5 mm to 5 cm in diameter, have a greater ability to anchor in the loose beach sand and act as tools for the backwash-andswash-oriented current crescents (Figs. 3-5). In contrast, very small mud pellets, shells and shell fragments ( < 1 cm in diameter) which are drifted easily by swash and backwash, act as tools when at rest for the common backwash-oriented current crescents (Fig. 6). Mechanism of formation of backwash-andswash-oriented current crescents
Unlike unidirectional flows in rivers, swashbackwash effects impart a bidirectional flow pattern on intertidal beach faces. Being influenced by downstream flow, current crescents in shallow river beds always point to the downcurrent direction (Peabody, 1947; Sengupta, 1966; Karcz, 1968;
Fig. 6. Closely spaced backwash-oriented current crescents formed by wave backwash around small mud pellets resting on fine sand. Beach of Lower Long Sand Island in the Bay of Bengal. Intersecting arms of current crescents resemble rhomboid marks. Seaward toward left of photograph. Pen 14 cm long.
144
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Conybeare and Crook, 1968). Alternatively. current c r e s c e n t s o n b e a c h f a c e s a r e chiefly c o n -
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TABLE 1 A comparative table showing morphology and beach characteJ for the backwash-and-swash-oriented current c~e~
Backwash-and-swash-oriented current cres-
Backwash-oriented current cr,.~sccms
cents Morphology
Two backwash-oriented arms around an obstacle open seaward and two swash-oriented arms around the same obstacle open landward
Two backwash-oriented arms m,om~d an ob stacle open seaward
Backwash-oriented arms generally longer and more prominent than swash-oriented arms
Backwash-oriented arms quilc !~r,mmc,m
Nature of obstacle and scour
Permeable or impermeable bluff bodies of 5 to 10 cm in diameter set in a scour pit showing semi-permanent nature
Permeable of impermeable !flufi bodit:s. generally a few mm to less than I cm rest tm beach surface without any prtmHnenl ~cou~ pit
Beach slope
Gentler beach slope, 1:50 to 1:90 (1.4-0.7 °)
Steeper beach slope, 1:25 to I : ) !?..5 f~J
Grain size
Fine to very fine sand ( M z = 2.9 to 3.1 O)
Medium to fine sand ( M z = l.,~ i<, -'.2 ,h )
Associated structures
Current lineations parallel arms of crescents
Current lineations may or may ~o| bc pr~ sent
Rill marks rare
Rill marks common
A process-response structure of beach; typically signifies swash zone
Not a process-response structur,:., beach and varied environment
Environment
BACKWASH-AND-SWASH-ORIENTEDCURRENTED CRESCENTS
ments as tools, the backwash-oriented current crescents open their arms along the seaward slope. In this respect, these ordinary backwashoriented current crescents are not sensitive enough to be used as a process-response structure of the beach environment. Sometimes, intersecting arms of the closely spaced backwash-oriented current crescents may give rise to a pattern on the beach surface resembling rhomboid ripples (Fig. 6). The fact that current crescents look like rhomboid marks means that the vortices generated by the obstacles are constrained by surface shock waves on the free surface of the thin flow through which they penetrate. The small objects on the beach surface that lie in the path of wave swash and backwash, first move up the beach slope by uprush of waves and are then rolled down the slope for some distance in the direction of wave backwash. The small objects come to a position of rest when the backwash velocity is unable to move them downslope any further. Field measurements reveal that, where the beach material is medium to fine sand
145
and slope is between 1 : 25 and 1 : 9 (2.5-6°), tiny objects which have little ability to anchor on the sandy substrate act as obstacles in the path of backwash flows and consequently leave around them backwash-oriented current crescents. The crescents are preserved on the beaches when an extremely thin film of water is able to create a flow separation against these tiny obstacles. T h e b a c k w a s h - a n d - s w a s h - o r i e n t e d current crescents, on the other hand, are formed around relatively large objects on fine to very fine sand with beach slopes ranging between 1 : 50 and 1 : 90 (1.4-0.7°). Large (5-10 cm diam.) impermeable bluff bodies like mud balls, stones and polychaete tubes as well as permeable bluff bodies like tussocks, have a greater ability to anchor on the substrate, and generally act as obstacles for these current crescents (Fig. 7, Table 1). Flow separation and a concurrent velocity increase around these obstacles both during uprush and backwash flows create a circular or semicircular scour with time. The larger obstacles, which are well placed in the scour pits, have sufficient relief above beach surface to create flow separation for the
Fig. 8. Backwash-oriented current crescents formed by wave backwash around polychaete tubes on the sandy beach of Sagar Island, NE India, Scour pits around obstacle and rill marks on upcurrent side can be seen. East-west aligned current lineations clearly visible. Pen 14 cm long.
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to-and-fro moving water. The depth of the scours was almost one-half the equidimensional stony obstacles and somewhat higher than this ratio for cylindrical or spindle-shaped mud balls. These depths of scours are consistent with those deduced by H.N.C. Breusers (1965) and H.W. Shen et al. (1969), both in Allen (1984). Morphologically, these current crescents have shorter and less prominent swash-oriented arms compared to their backwash-oriented arms. The upslope movements of sediment, together with diminishing swash velocity due to bed friction, cause the shorter swash-oriented arms. The low relief difference between the upslope and downslope sides of the obstacles offers protection to these surface sculptures from wave erosion. As a result, these crescentic marks which form around one and the same obstacle are well retained with opening of arms in both the seaward and landward directions. The b a c k w a s h - a n d - s w a s h - o r i e n t e d current crescents form during both flood and ebb-flows of the neap and spring tidal cycles. They form during calm conditions; storms may completely oblit-
erate such low relief features on the beach, They are characteristic of the lower to middle foreshore zone and are thus located within 2 m elevation above mean sea level. The directions o| wave approach determine their orientation on the beach surface. Association of current lineations and backwashand-swash-oriented current crescents
The beach surfaces sculptured with backwashand-swash-oriented current crescents oltcn s h o a superimposition of current lineations i,c, segregation of finer- and coarser-grained striations (Figs. 3, 8). These current lineations are rather "streaming lineations" in the sense of Conybcarc and Crook (1968). The preferred sand-grain orienta .... tion in this structure with narrow elevations and depressions on the surface of the bed is chiefly controlled by backwash flows as revealed by the orientation of the striae parallel to tile arms of the current crescents. Curray ( t 9 5 6 ) o b s e r v e d current lineations on beach surfaces formed b3 backwash flows. Other examples of the association of current crescents and current tineations
Fig. 9. C u r r e n t c r e s c e n t s f o r m e d by b a c k w a s h a r o u n d r o u n d e d stone f r a g m e n t s ( 7 - 1 0 cm diam.) r e s t i n g on fine samL Beach o~ S a g a r Island, N E India. Scour pits a r o u n d obstacles and incised rill m a r k s on u p c u r r e n t side seen, S e a w a r d toward left o i p h o t o g r a p h . Pen 14 cm long.
147
BACKWASH A N D - S W A S H - O R I E N T E D C U R R E N T CRESCENTS
are obtained from Pettijohn and Potter (1964), from P. Cepek and H.E. Reineck (1970) in Reineck and Singh (1980), and from Allen (1966, 1984). Current lineations in sand occur in the plane bed phase of the upper flow regime (Allen, 1964; Reineck and Singh, 1980) and result from streamwise sand graining over a level bed (Sorby, 1908; Crowell, 1955). According to Allen (1984) current lineations are formed due to the action of boundaryqayer streaks, the transverse spacing of which is determined by flow properties. The association of backwash-and-swash-oriented current crescents and current lineations in the present study justifies the fact that the former takes place in areas of very gentle slope to flat beach faces. But unlike current lineations, the current crescents which are products of scour and deposition around obstacles, are a lower flow regime structure (McDonald and Banerjee, 1971). Association of rill marks and backwash-oriented current crescents: resemblance with backwashand-swash-oriented current crescents
In areas of considerably steeper beach slope ( 3 - 6 °) two small incised rill marks originating on the upslope side may converge at the scour pit of the obstacle with backwash-oriented current crescents (Figs. 8, 9). These low-relief rill marks are generated during sinking water level and resemble arms of the swash-oriented current crescents. Thus, a combination of backwash-oriented current crescents and a pair of low relief rill marks may be mistaken for backwash-and-swash-oriented current crescents. Careful field observations are needed to identify them from the recent beach deposits. Recognition of their ancient analogue from rock records will be more difficult because of post-depositional changes. Geological significance of backwash-and-swashoriented current crescents
(2) The longer arms of the crescents form out of backwash flows and so their directions of opening point seaward. The shorter arms of the crescents, on the other hand, are formed by wave swash and their arms open landward. (3) Backwash-and-swash-oriented current crescents form on very gentle beach slopes of 1 : 50 to 1:90 (1.4-0.7 °) composed of fine to very fine sand (Mz = 2.9 to 3.1 ~b). The backwash-oriented current crescents prefer a steeper beach slope of 1:25 to 1:9 (3-6 °) made up of medium to fine sand (Mz = 1.8 to 2.2 ~b). Thus current crescents can be utilized to interpret ancient beach slope. (4) The backwash-and-swash-oriented current crescents form around both permeable and impermeable obstacles which are semi-permanent and well anchored to the beach sand. In contrast, the objects responsible for backwash current crescent are easily moved by wave swash and backwash and are less able to anchor on the beach surface. (5) The formation of current crescents is related to normal calm situations of the beach. Storm situations may erode and obliterate these low-relief beach features. (6) The backwash-and-swash-oriented current crescents are often associated with current lincations. The former are lower flow regime structures whereas the latter indicate upper flow regime. (7) The features formed by convergence of two rill marks into the scour pit on the upslope side of backwash-oriented current crescents may be mistaken for backwash-and-swash-oriented current crescents. (8) The shoreline is always at right angles to the extension directions of the arms of the crescents. (9) The direction of wave swash and backwash on beach surfaces can be interpreted from the alignment of the arms of current crescents. Conclusion
(1) Backwash-and-swash-oriented current crescents act as a process-response structure of beaches. The direction of wave swash and backwash are marked by the alignment of the arms of the crescents.
The current patterns in a beach, particularly in the swash zone, are essentially bipolar because of the rushing up of wave-swash and backwash. However, very few physical sedimentary struc-
148
tures are significant enough to be utilized for determining these bipolar current directions, excepting the rhomboid ripple marks in which the convex or broad side points downcurrent and the concave or pointed side points the upcurrent direction (Hoyt and Henry, 19631. The present study has confirmed that apart from rhomboid ripple marks, the backwash-andswash-oriented current crescents around an obstacle appear to be very diagnostic of bipolar currents in the swash zone. This structure also helps to interpret the beach slope, direction of the approaching waves and shoreline orientation. Although current crescents can appear in varied environments, the backwash-and-swash-oriented current crescents are confined to the swash zone of beaches. This interpretation gains additional support when they occur around mud balls armoured with shells of littoral zones a n d / o r around polychaete tubes that rest on lower foreshore beaches. Recognition of backwash-and-swash-oriented current crescents would add one more to the list of process-response structures of the beach environment. Fossilized current crescents plausibly of littoral origin are known from Hall (1843) and from W. Vortisch (1973) in Allen (1984). Records of such features from rocks will be of interest for interpreting ancient beach environments. References Allen, J.R.L., 1964. Primary current lineation in the Lower Old Red Sandstone (Devonian), Anglo-Welsh Basin. Sedimentology, 3: 98-108. Allen, J.R.L., 1966. On bed forms and palaeocurrents. Sedimentology, 6: 153-190. Allen, J . R i . , 1984. Sedimentary Structures--Their Character and Physical Basis. Elsevier, Amsterdam, Vol. 1, 593 pp., VoL II, 663 pp. Bhattacharya, A., 1987. Interpretation on the pattern of wind and water movement over Lower Long Sand from studies on texture and structure of the sediment. J. Mar. Biol. Assoc. India, 2 9 : 1 1 5 - 1 2 3 .
A. IBHA VI'ACI-tAR'YA Bhattacharya, A., 1988. Intertidal depositional character of the Hooghly tidal islands, West Bengal. Indian J. Geol., 60: 153-164. Bhanacharya, A., 1992. Current crescents formed by marinc algae (Valonia sp.): a new record of obstacle marks lacking preservation of obstacles. Sedimentology, 39:513 516. Conybeare, C.B.B. and Crook, K.A.W., 1968. Manual ol: sedimentary structures. Bureau of Mineral Resources, Geol ogy and Geophysics, Canberra, A.C.T., Bul!. /02. 327 pp. Crowell, J.C., 1955. Directional current structures from alpi~c flysch, Switzerland. Geol. Soc. Am. Bull., 66:1351:1384. Curray, J.R., 1956. Directional grain orientation studies on recent coastal sands. Bull. Am. Assoc. Pe|. Geol.. 40:: 2440-2456. Dzulynski, S. and Walton, E.K., 1965. Sedimentary Feature~ of Flysch and Graywackes. Elsevier, Amsterdam, 274 pp. Flemming, B.W. and Fricke, A.H., 1983. Beach and nearshore habitat as a function of internal geometry, primary sedi mentary structures and grain size. In: A. McLachtan and T. Erasmus (Editors), Sandy Beaches as Ecosystems~ I)l. W. Junk Publ., The Hague, 757 pp. Hall, J., 1843. Geology of New York, Part IV. Caroll and Cook Albany, N.Y., 683 pp. Hoyt, J.H. and Henry, V.J., 1963. Rhomboid !~ipple marks, indicator of current direction and environment. J, Sediment. Petrol., 33: 604-608. Karcz, 1., 1968. Fluviatile obstacle marks from the wadis o~ Neveg (Southern Israel). J. Sediment. Petr¢~L 38: 10001012. Komar, P.D., 1976. Beach Processes and Sedimentatkm. Prentice Hall, Englewood Cliffs, N.J., 429 pp. Marshall, A.J. and Williums, W.D. (Editors), 1979. Textbook of Invertebrate Zoology. Macmillan, New YorL 874 pp. McDonald, B.C. and Banerjee, I., 1971. Sediments and bedforms on a braided outwash plain, Can. J, Earth Sci., ;5: 1282-13(11. Peabody, F.E., 1947. Current crescents in Triassic Moenkopi formation. J. Sediment. Petrol., 17: 73-76. Pettijohn, F.J. and Potter, P.E., 1964. Atlas and Glossary o~ Primary Sedimentary Structures. Springer-Verlag, Berlin. 370 pp. Reineck, H.E. and Singh, I.B., 1980. Depositionat Sedimentary Environments. Springer-Verlag, Berlin, 439 pp. Sengupta, S., 1966. Studies on orientation and imbrication of pebbles with respect to cross-stratification, i. SedimenL Petrol., 40: 81-101. Sorby, H.C., 1908. On the application of quantitative methods to the study of the structure and history of rocks. Q.3 Geol. Soc. London, 64: 171-233.