ordnung wie die K6rner hatte. Anastomosierende Kanllle, wahre 'Pendante" und 'Sammelhalbr6hren" ents tanden auf diese Art. Die Resultate zeigen, dass 1) ...
Proceedings of "the 8th International Speleological Congress, Bowling Green, Western: Kentucky University USA, July 18 to 24, 1981. pp 407-9. S~ulat~on
Qt
RQck rendants - Small Scale Experiements on Plaster Models
Stein-Erik Lauritzen Department of Chemistry, University of Oslo, P.O. Box 1033, N-Blindern, Oslo 3. Norway Abstract The classical theory of Bretz (1942) on the development of roof or rock pendants is widely accepted,
appearing in many textbooks in karst geomorphology. However, this genetic theory has never been checked
by experiments or direct process observations. This problem arose during the course of a study of some
karst caves in Norway and experiments were conducted. It was found that sub-watertable channels between
a sand fill and the roof of an inclined tube can produce a sinuos or even a braided pattern of flow. It
was further observed that, without sediment fill, only a scalloped surface was formed on the roof and
walls of plaster models. However, in contact with granular material, an uneven surface was formed,
having indentations of the same order of magnitude as the grains. Anastomosing channels, "true" pendants
and trunk half tubes were produced in this way. These results indicate that: 1. A corrosion mechanism
controlled by the rate of flow is necessary. 2. A granular material in contact with the limestone
surface seems necessary to produce half tubes and pendants. Except for the problems of scaling-up, these
results are in agreement with the theories of Bretz (1942) and Renault (1967/68).
~saJllftenfassung
Die klassische Theorie von Bretz Uber die Entwicklung von Deckenzapfen oder "rock pendants~ ist
weit verbreitet, akzeptiert und erscheint in vie len LehrbUchern Uber Karstgeomorphologie. Diese gen
etische Theorie aber ist noch nie durch Exper1mente oder durch direkte Observierung des Prozesses
geprUft worden. Dieses Problem entstand im Laufe von Untersuchungen einiger Karsth6hlen in Norwegen
und Experimente warden deshalbs ausgefUhrt. Wir fanden, das Kan3le unter dem Wasserspiegel zwischen
Sandablagerungen und der Decke eines schr3ggestellten Rohrs sinusf6rm1ge oder sogar verzweigte Str6 hmungsmuster bilden k6nnen. Es warde weiterhin observiert, dass ohne Sedimentablagerung nur aine
"scalloped" Oberf13che an der Decke und dem W3nden des Gipsmodelles gebildet warde. 1m Kontakt mit
k6rnigem Material dagegenwurde eine rohe Oberfl3che geformt, die Vertiefungen der gleichen Gr6ssen
ordnung wie die K6rner hatte. Anastomosierende Kanllle, wahre 'Pendante" und 'Sammelhalbr6hren" ents
tanden auf diese Art. Die Resultate zeigen, dass 1) ein Korrosionsmechanismus, kontrolliert durch
die StrOhmungsgeschwindigkeit, n6tig ist und 2) ein k6rniges Matrial im Kontakt mit der Kalkgestein
soberf13che n6tig zu sein scheint, urn Halbrtlhre und Pendante zu bilden. Abgesehen von gewissen
Schwierigkeiten beim UeberfUhren zu gr6sseren Masstab stimmen diese Resultate mit den Theorien von
Bretz und Renault Uberein.
/ Introduction Rock pendants are bedrock bodies of moderate and equal dimensions that protrude in a regular pattern from the enveloping surfaces of cave passages. In textbooks, they are often described as "stalactite like forms in bedrock" (SWeeting 1972, Warwick 1962, 1976). Btlgli (1978) classifies them as related to bedding or joint plane anastomoses. The genetic theory of Bretz (1942) requires a sediment fill contemporary or SUbsequent to the formation of the main passage (paragenesis, Renault 1968), making ceiling half tubes to a closely related feature. In this context, pendants should rather be defined by their intermediate spaces than by the rock pro trusions themselves. From experience and literature, the term "pendant" seems to cover different and poly genetic forms. To COver all these forms, the follow ing classification is suggested: I.
Inherited Structures a). "Pendants" formed by differential corrosion (petromorphs, stylolites, fossils, schist clast inclusions (St. Pierre 1966». b). Joint-determined mazes, sometimes exposed by breakdown or vadose erosion.
II. Structures Formed Directly by Hydraulic Processes a). Bedding and joint plane anastomoses ex posed by breakdown. b). Upwards corrosion above a sediment fill, i.e. Classical "Bretzian" or paragenetic pendants. c). Possible stream "scour" forms (Jacuks 1977). Field· Observations The caves of Glomdalen near the Svartisen ice cap, North Norway are excavated in a very pure, homo geneous and coarsely crystalline calcite marble, Fig. 1. Pendants and half-tubes are abundantly develop ed, with all signs pointing towards a paragenetic origin (type lIb above) . . A general tendency found, was that larger "trunk" half-tubes branched into anastomoses· (pendants) and coalesced into trunk tubes again, Fig. 2. In some cases the larger half-tubes showed internal scalloping. which could be proved to have an up hill direction of flow (Lauritzen 1981). Laminated and sometimes cross-bedded sedi ments from phi -4 to +6, most probably of glacioflu vial origin, was in some places wedged in between pendants, leaving a tubular space along the ceiling, Fig. 3. The pendants and half-tubes can be described as "rejuvenated" anastomoses, developed along sur faces of higher hydraulic conductivity, i.e. the interface between the passage wall and a sediment fill. Half-tubes, which on this scale must be re
garded as master conduits, occur either along the shrinking or settling void along the ceiling, or along sediment beds of higher hydraulic conductivity (grain size) where they intersect the walls, or in some cases, even the floor. Pendants occur often where hydraulic gradients between previously unconnected master (half-tube) conduits occur, Fig. 4. Accord ing to the direction of flow, these trunk half-tubes are manifolds and collectors respectively, an analog to the concept of arterioles, capillary bed and venules in chordate anatomy (Romer 1970). As experimental verification of the paragenetic theory of pendant formation is not known, some pilot experiments were done. Experimental SuLaqu~tic
meandering A simple phreatic loop was cpmstructed from plex igla9s tubing, 10 em diameter, Fig. 5. Water and sus pended sand were led through it. A delta-type sedi mentation was easily obtained which subsequently filled up the passage completely. Atter a short period of in creased rate of flow, which opened up a straight channel along the ceiling, the discharge was slowed down and kept constant, while a small support of sand was added. An undulating channel soon developed which was stable for at least one night before it straightened up along the ceiling. Further experiments in tubes with flat ceiling indicate a greater stability, Fig. 7. The "sinuosity· of the thalweg in these channels changed with discharge, Fig. 6. Low discharge gave largest sinuosity, and also braided channels (ana stomoses). If the ceiling consists of a solOble mater ial, incised channels should be expected to be formed. Plaster Models Plaster blocks, either buried in the sand or cast onto the ceiling of the tube developed a rough surface, usually on their underside. Substitution of the sand with 2 mm glass beads gave a similar result. The rough surfaces resemble the branched half-tubes and pendants, Fig. 7. It should be noted that lime stone boulders supported in sandy streambeds often show pendants On their underside, but not elsewhere. Without the sediment fill, only a smooth surface was found. A plaster block that was protruding from the sand fill into the waterfilled space above, developed a distinct border between a smooth surface and "pen dants Plaster tubes filled with various grades of sed iment gave the same type of pattern. When silt was used, 1-2 mm wide half-tubes formed in some places, while the internal pores of the plaster was widened as well, indicating the limitation of using palster. A set-up with flat ceiling gave excellent results, Fig. 8. I ...
Another e~~eriment where a plaster block was placed i n sand with an inlet tube below it, developed a nicely radial pattern, similar to the ceiling of a cave passage.
Acknowledgements
Discussion
Financial support was provided by Fridtjo~ Nansen's and affiliated funds for the advancement of SC1ence and the humanities; the Department of Zoology, University of oslo provided the aquarium facilities necessary for the experiments.
Although carried out in a scale of 1/10 or less
of the natural, the experiments indicate that form ation of sinuous channels along the ceiling of sedi ment-filled tubes may take place in sub-watertable conditions. This is consistent with the requirements of Bretz (1942) who associated sinuous half-tubes with the aggradation of a subwatertable stream bed. The observation of half-tubes and pendants with up hill scalloping in steeply dipping passages also account for a phreatic development. The metastabil· ity of the sinuos channel seems related to the small dimensions used, and further experiments are needed with larger tubes. Plaster simulations are carried out by assuming a similarity in the dissolution kinetics of plaster and limestone. The formation of pendants seems to require a mechanism where the loca·l dissolution rate increases with the rate of flow. Another problem is the scale in hydrodynamic experiments, where the viscosity of water is exaggerated in small scales. The "pendants' formed in the experiments are about 1/100 to 1/10 of the corresponding forms in nature, and this is the strongest objection to the results. Further experiments on grain size dependence and a larger scale of the models are in progress.
Bretz, J.H. 1942. Vadose and phreatic features of lime stone caverns. Journ. Geol. 50, 675-811. ~gli, A. 1978. KarsthydrograpfiTe und physische Spel~ologie. Springer, Berlin, 292 pp. Jakucs, L. 1977. Morphogenetics of karst regions. Akademiai Kiado. Budapest, 284 pp. Lauritzen, S. E. 1977. Statistical symmetry analysis of scallops, a method for determination of flow dir ection in cave conduits. Bull. Nat.Speleol. Soc. in press. Renault, P. 1968. Contribution a l'etude des actions m~chaniques et sedimentologiques dans la speleogenese. Les facteurs sedimentologiques. Ann. Speleol. 23. Romer, A. S. 1970. The vertebrate ~. saunderS; Phil. 601 pp. St. Pierre, D. 1966. The caves of Gratadalen, Northern Norway. Trans. Cave. Res. Group. Great. Brit. 8(1) 1-64. Sweeting, M. M. 1972. Karst Landforms. MacMillan London. 362 pp. Warwick, G. T. 1962. The origin of limestone caves. pp. 55-82 in. Cullingford, C.H.D: British Caving, London.
References
1
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Fig. 1.
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The .location of marble and caves in Glomdalen, North Norway.
Fig. 2.
Scal·loped half tubes branching into pendant zones and coalescing into half tube.
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Fig. 4. a) Completely filled phreatic tube, half tube developed in zones of increased hydr aulic conductivity. b) Longitudinal section. c) Vertical distribution of flow velocity. Fig. 3.
with sedi
ment in situ. Laminated sediment
and t~spaces at different levels.
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Fig. 5. Experimental set-up of a phreatic loop made of Plexiglass tubing. 408
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Fig. 6.
Plot of "sinuosity· (amplitude/wave length) of sinuous channel against discharge. Slope of tube (~) 220. The grain size of the sand is shown.
Fig. 7.
Plaster model experiment. Winding and branching half-tube and pendants incised into the flat roof of a tuhe.
Fig. 8.
Flat plaster block immerced in sand. Braided channels and pendants.
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