Trans.Inst.Indian Geographers ISSN 0970-9851 Excavation Features on Rock Coasts: A Study of Honeycombs, Taffoni and Caves from North Konkan Ajay Kamble and S.R. Jog, Pune - Maharashtra Abstract Excavation of Geo-materials on a rocky coast, causes formation of geomorphic features like Honeycombs, Taffoni and Caves. These features mainly account for the consumption of cliff and platform surfaces. These differ not only in their size, but also as per their processes of formation. The North Konkan region has several patches of rocky coasts where these landforms are observed at many locations. These locations were surveyed to obtain morphometric variables such as size, length, width, depth, height etc. Apart from visual interpretation, the rocks and material characteristics were analyzed using microscopic observation of thin sections. Field survey, morphometric analysis and material analysis show that honeycombs are primarily a result of erosion by salt spray on a relatively resistant, fresh rock surface. Taffonis, which are similar features, are formed by excavation of severely weathered material. Notches and caves are larger in size, and are associated with differential weathering and erosion of different Basalt flows. Keywords : Rocky Coasts, Honeycombs, Taffoni, Notches, Basalt, Konkan Introduction rock-mass, and hence, can be grouped On rocky coasts, the disintegration together. Honeycombs, taffonis, notches of rocks by mechanical or chemical and caves are such excavation features, processes would ultimately alter a rock- observed on most of the rocky shores. mass to become physically weak and Honeycombs and taffonis are micro- chemically unstable. The weathered and features, while notches and caves are loosened material is removed by various of much larger size, in increasing order. agents of erosion. Removal of rock- Among these, honeycombs and taffonis, mass, if it is site-specific, shall leave taffonis and caves and notches and behind its imprints in form of small and caves have a close association, and at large excavation features. Though being times, may share a cause-consequence varied in their size and morphology, relationship; i.e. honeycombs may give these features owe their origin to rise to taffonis and notches may be a common process, i.e. removal of enlarged to form caves. Transactions | Vol. 41, No. 2, 2019 | 185 The present paper attempts to rather convenient to accept, since many describe the characteristics of these workers define them in a genetic context, features and discuss the various resulting due to a certain process. processes involved in their formation, Honeycombs have been observed in along North Konkan Coast of all climatic regions; from equatorial Maharashtra. to mid-latitudes (Palmer and Powers 1935, McGreevy 1985), and even in Study Area Antarctica (Nichols 1960). However, The area under consideration for the honeycombing contributes significantly study is a 40 km stretch in Raigad district to the consumption of rocks and may of North Konkan. Extending from Dighi be considered as one of the primary in the north to Velas in the south, the area mechanisms of rock-disintegration lies between 17°55’ to 18°19’ latitude in coastal areas. The occurrence and and confined between 72°55’ and 73°E characteristics of honeycomb structures long (fig.1 See page 197). in the study area can be described as follows: The area for the present study is a part of North Konkan coast, in Raigad 1. Not all parts in the study area show District of Maharashtra. It is a strip of a developed honeycomb pattern. about 35-40km, extending from Murud Honeycombs are more commonly in the north, to Velas in the south. found in the southern part of the study area, at Hareshwar platforms, The coastal area is underlain by Shriwardhan point headland, and Deccan trap formation made up of Velas platform. Towards north, Basaltic flows. These rocks were Bharadkhol platform does not formed due to outpouring of lava during exhibit a developed honeycomb the Cretaceous and Eocene period pattern. The platform rock rather (Krishnan 1982). has small pittings of 1-2mm, Rocky shores are observed at occupied by mollusks. Small sized, several locations in the study area. poorly developed honeycombs Some of these were selected for a may be observed at Adgaon detailed study. Bharadkhol Headland platform, though they are larger near Shekhadi, Headland at Shrivardhan and pronounced on the Adgaon point, Harihareshwar and Velas have stack surface. The sheltered sites occurrence of honeycombs. taffoni, of Dighi are more or less devoid caves and notches. of honeycombs. However, between Sarva and Nanvel headland to the Honeycombs north of Velas Agar, they show an Also called as “Stone lattice” or excellent development on platform “Stone lace”, honeycombs are small, surface. The following table closely-spaced depressions, up to a provides a summary of location few centimeters in diameter and depth and general characteristics of the (Trenhaile 1987). This definition is honeycombs in the study area. 186 | Transactions | Vol. 41, No. 2, 2019 Table 1 : Description and characteristics of honeycombs Location Description of honeycomb characteristics Size and depth Found over platforms and cliff face. Regular 2-2.5 cm in spacing, dark gray colour, sometimes reddish-brown diameter. Up to Velas with luster. Hard sidewalls. Found up to 4.5m above 2cm deep. the platform on the cliff face. Found mainly on cliff face, along with taffonis. 2-4 cm in Hareshwar Larger range of size. Dark colour, hard sidewalls, diameter, up to luster. Not much on platform surface. 4 cm deep Shrivardhan Mainly of cliff faces, very intricate, regular, deep. 3-5 cm in Point Max. depth observed at this site. Larger and irregular diameter, up to Headland near joints. No luster. Dark brown colour. 6 cm deep. Formed in reddish brown altered basalt, poorly Up to 2mm in Bharadkhol developed, not uniform lattice, in form of small diameter and Headland pitting. depth. 1.5-2cm on Small, irregular honeycombs on platform surface. Adgaon platform, 1cm Larger and deep on stack surface (3cm) deep. Very well developed lattice, spread over entire Between platform surface, and insitu boulders. Absent Up to 3 cm in Sarva and between inter-boulder spaces. Hard sidewalls, diameter, 2-2.5 Nanvel collapse of sidewalls and fusing of honeycombs, cm deep. Headland sharp pinnacles seen. Luster on sidewalls. Almost absent, small and irregular, only on platform Up to 1cm Dighi surface, formed in reddish altered basalt. diameter 2. It may be observed that development 3. Honeycombs do not show any of honeycombs falls more or less in affiliation to slope. Vertical, sloping, line with the exposure to the sea. as well as horizontal surfaces have Velas, Hareshwar and Shrivardhan developed honeycomb lattice, point headlands have larger, hence it is difficult to associate their regular and deeper honeycombs, presence with surfaces having a than Adgaon or Dighi, which are certain degree of slope. sheltered sites. Bharadkhol platform A strong relationship, however, may be cited as an exception to may be identified between this pattern, where honeycombs honeycomb development and show a poor development despite the type of rock. The best occupying a headland location. developed honeycombs were Transactions | Vol. 41, No. 2, 2019 | 187 observed at Velas, Hareshwar of honeycombs does not seem and Shrivardhan Point headland, to be guided by joints. They are sites which have a massive, present on surfaces with, or without compact basalt (Plate 1a See joints, with almost equal frequency. page 198). This material is However, honeycombs tend to be cohesive, fine grained, and has larger and irregular in shape, near little amygdales, vesicles and the joints. The ones, occupying secondary minerals. The thin homogenous, joint-free areas, are section of these rocks show that more circular in shape and smaller. the basalt has not been altered 6. The maximum depth of an individual much (Plate1b See page 198). honeycomb structure observed is On the other hand, coarser and about 6cm. Beyond these depths, the altered basalt in the northern honeycomb walls either collapse, parts, i.e. Bharadkhol and Dighi, leaving behind sharp pinnacles. do not exhibit appreciable Fusing of honeycombs forms larger, honeycomb development. irregular shapes, but does not cause Bharadkhol headland does have any increase in depth. harder, compact, vertically jointed basalt forming cliff and 7. A close observation of an individual stack-top, but no honeycombs honeycomb reveals that the are observed on these rocks. depression has vertical walls, and the mouth of honeycomb does not 4. Neither the host rock, nor show any overhang or lip (Fig.2). honeycombs exhibit presence of any weaker material. The honeycomb Taffonis surface is smooth, sidewalls are hard and have a metallic luster of Taffonis and honeycombs may often reddish-brown colour. No rock- be confused with one another, due to debris is found in the honeycomb similarity in form. Since there is no depressions, except precipitation of precise criterion for differentiation salt. between the two, taffonis may be taken as large honeycombs. Some authors 5. Within a given cliff-platform have also considered honeycombing as complex, honeycombs have taffonis developed under unfavourable certain preferred locations. They conditions, so that their full size could are rarely observed near the low- not be attained (Martini 1978). It seems tide cliff or at platform margin. It to be an accepted fact that the taffonis is rather the central and landward are hollows carved in the rocks, which portions of platforms which have a are larger than honeycombs. Occurrence concentration of honeycombs. The of taffonis is not restricted to coastal cliff face also has honeycombs up environments alone. They have been to a height of 3-4m. The existence reported even from deserts (Mabutt 188 | Transactions | Vol. 41, No. 2, 2019 1977) and over a variety of rock-types, 3. The sidewall taffonis and the ranging from Granites (Bradley Et ones occupying the cave ceilings, al 1978, 1979), Sandstones (Mustoe are markedly different in their 1983), Limestones (Kelletat 1980) and shapes.