Location and types of caves
Bermuda with its networks of caves is typical of the many regions around the world where limestone is the dominant bedrock. In Bermuda, the greatest concentration of caves occurs around Harrington Sound in some of the island’s oldest rocks, of the Walsingham Formation. On the southwestern side of the Sound, near Green Bay, there is an extensive network of submerged caves, which are accessible only to cave divers. On the northeastern side of the Sound, there are the spectacular air-filled “show-caves” such as Leamington Cave, Crystal Cave, Fantasy Cave (Figure 6a) and Admiral’s Cave.
Bermuda’s caves can be divided into “primary caves” and “secondary collapse caves”, based on how they were formed. Primary caves are the product of tunneling out of limestone by flowing water, whose erosive powers may be attributed to turbulence and/or chemical aggressiveness. Flooded primary caves, navigable only by scuba divers, are found in the Green Bay and Red Bay cave systems on the southwestern side of Harrington Sound. Formed by the progressive enlargement of flooded conduits, such caves can ultimately evolve into chambers of sufficient lateral span that they become vulnerable to collapse.
Collapse caves are, thus, formed when the roofs of laterally expansive primary caves fall in. Collapse caves occupy the space between the fallen debris and the new roof. They have vaulted ceilings, and floors which are buried in large fallen blocks of limestone known as “breakdown” (Figure 6b and 6d). Some of those which have entrances from the land surface have been fitted with flights of steps and lighting, and developed as “show caves”.
Oscillating sea levels of the Pleistocene Epoch (Chapter 8) caused ground water to alternately saturate Bermuda’s limestones and then drain away. Caves, correspondingly, varied from partly or completely water-filled to almost completely air-filled. The occurrence of prolonged periods of lower sea level is evident from cave formations, or speleothems, such as stalactites and stalagmites that formed in the air but which are now submerged (Figures 6d and 6e)
When sea-level was low, the buoyant support that had been provided to cave ceilings in completely flooded caves was withdrawn, and they became more vulnerable to collapse. At successive sea level oscillations the cycle was repeated. Collapse caves expanded by upward void migration, or stoping, associated with progressive ceiling collapse. Good examples of accessible collapse caves are located on the notheastern side of Harrington Sound, such as Leamington, Crystal, Fantasy (Wonderland) (Figure 6a) and Admiral’s caves. Exploration of the submerged extension of these caves by cave divers has demonstrated a high degree of connectivity among them (Figure 6c), as well as indirect submarine connections between Harrington Sound and the sea. Such inter-connection between collapse caves suggests control of their distribution by underlying primary caves which themselves were part of a network.
The Genesis of Caves
Primary caves are the precursors to secondary collapse caves which are the dominant accessible caves in Bermuda. There are two types of primary caves – vadose and phreatic. The first is formed by fast flowing turbulent water of underground streams. Such flowing water when combined with an abrasive “bedload” of sediment will enlarge voids and fissures by mechanical erosion. Narrow conduits are, thus, widened into tunnels and caves, by the same processes that surface streams carve valleys. The second type of primary cave – phreatic – is formed within limestone that is saturated with slow-moving ground water. Phreatic caves are the product of limestone dissolution (leaching) by chemically aggressive ground water. The cause of such aggressiveness in coastal, or island, aquifers has been attributed to the mixing of fresh ground water (originating as rain) and saline ground water (from the sea). The whole process of coastal phreatic cave formation has been expounded by Mylroie and Carew in their “flank margin” cave model (MY2).
Primary caves feature smooth or sponge-like walls and “blind” passageways (leading to dead-ends) which are diagnostic of dissolution, or leaching, by ground water. According to Mylroie and Mylroie (MY4), rock surfaces showing such effects of phreatic dissolution are “extremely rare” in Bermuda caves. However, it appears that they based this conclusion primarily on observations in air-filled caves, given that examples of phreatic dissolution features are not uncommon in Bermuda’s submerged caves. They have been described by Bermudian cave diver Bruce Williams on the southwest side of Harrington Sound in the Green Bay system, with its 2 km (1.25 mile) of surveyed passageways, as well as to the northwest of the Sound in the vicinity of Strawmarket Cave (Figure 6f). Bruce Williams has also reported “key hole” structures, which represent the modification of initially phreatic (fully submerged) caves by vadose streams. This transition from phreatic to partially air-filled caves can be attributed to the lowering of ground water levels associated with a drop in sea level.
Although primary dissolution features are not as rare as once thought, Bermuda’s caves for the most part can be described as secondary collapse chambers. They do not fit into the phreatic (primary) cave development model created for other limestone islands such as the Bahamas, despite having a similar geology (MY4) . Bermuda’s caves do have counterparts in other parts of the world, however, such as those of Malta where Gines and Gines (GI2) reported that 90% of caves explored are characterized by collapse chambers.
Secondary caves, as found in Bermuda, are the outcome of multiple phases of ceiling collapse. They expand upwards, episodically, until they break-through to the surface, at which point a cave entrance is created (Figure 6g). The progression of cave collapse is determined by planes of weakness in the bedrock created by bedding (stratification) and by fractures. Many caves in Bermuda have steeply sloping ceilings (Figures 6a) which have fallen away along bedding planes that coincided with the sloping slip-faces of ancient dunes.
The depth range of caves
The lower parts of collapse caves are now largely choked with “breakdown” blocks; so it has not been possible for cave divers to fully explore them and understand their genesis. The maximum depth to which cave divers have been able to penetrate into Bermuda’s cave systems is approximately 24 m (80 ft) below sea level. This contradicts a supposition that large caves formed in the depth range of 30 m to 45 m (90 ft and 150 ft) below sea level, at the contact between limestone and basalt (volcanic rock) (MY1, MY3). The dearth of deep caves has been confirmed through drilling into the volcanic rock at numerous locations as well as through inspection of the flanks of the seamount by submersibles and divers (IL1).
Indications are that the major primary caves of Bermuda formed in the range of approximately 15 m to 25 m (50 ft and 80 ft) below present sea level. It is, however, unlikely that caves are forming at this level today, while sea level is relatively high and much of the limestone cap is occupied by slow moving chemically stable sea water (Figure 6h).
The best opportunity, if not the only opportunity, for large primary caves to form at depth within the limestone deposits of Bermuda was when sea level was approximately 20 m to 30 m lower than today as shown in figure 6i. It was then that the land area of Bermuda had increased in size by more than a factor of ten and a large body of fresh ground water, or a lens, would have accumulated. Hydrological processes would, accordingly, have operated on an incomparably larger scale than those of the present day. Ground water under-saturated with respect to calcium carbonate would have been generated by mixing at the water table – where infiltrating rain water met with ground water – as well as at the margins of the lens – where fresh ground water met with sea water. Hydrological conditions were optimum for formation of caves.
The Bermuda cave formation debate
The “flank margin” model (MY2) of cave formation is applicable to many coastal and island cave systems. It is based on the principle that chemically aggressive ground water is generated through the mixing of fresh and saline waters at the margin of a fresh water lens. However the circumstances by which dissolved limestone is transported to the sea on a sufficient scale to account for the creation of large caves remains the subject of debate, in Bermuda at least. Potential transportation pathways include: 1). Merging phreatic (submerged) passageways through which slow moving ground water carries dissolved calcium carbonate; 2). Vadose channels through which eroded limestone breakdown material is physically transported by streams; and 3). Phreatic passageways through which dissolved calcium carbonate is flushed, relatively rapidly, by strong tidal flows.
Bretz (BR1) first proposed that Bermuda caves, and limestone caves in general, “had their inception and largely their completion in continuously saturated rock below the water table”. He argued primary caves (as defined above) must have formed within a substantial volume of horizontally flowing fresh ground water which could only have accumulated at sea levels lower than that of the present day. Mylroie and Carew (MY2) similarly asserted, in their “flank margin” model, that some of the largest limestone caves which have formed within small islands, or near a coast, were phreatic in origin (Scenario 1, above). They attributed such formation to the action of chemically aggressive ground water created by the mixing of waters from different sources. Paradoxically, however, they argued that Bermuda’s primary caves were different, and were not the outcome of phreatic dissolution. Instead they concurred with Mylroie (MY1) that vadose streams flowing along the limestone/basalt contact were responsible for tunneling out of the limestone into large caves and for the physical transfer of eroded material and breakdown products to the sea (Scenario 2, above).
Gines and Gines (GI2) accepted that freshwater-seawater mixing causes under-saturation of ground water and dissolution of limestone, but questioned the process by which the dissolved material was flushed to the sea. They pointed out that glacio-eustatic sea level fluctuations and related rises and falls of mixed waters in caves represent a major speleogenitic mechanism which is often overlooked. To account for the removal of the enormous amount of dissolved material that is required for the creation of cave systems they invoked the effect of tidal flushing. (Scenario 3, above). Such flushing is the corollary of the twice-daily ebb and flow of large masses of ground water in coastal cave systems, driven by the pumping action of sea tides.
Deep caves and the survival of cave-adapted lifeforms
Returning to the hypothesis of cave formation at the limestone/basalt contact: it has been argued that the long-term survival of ancient cave-adapted aquatic crustaceans in Bermuda’s caves provides corroborative evidence for this hypothesis. It is asserted (IL1) that deep caves must exist at the base of the limestone cap, and even within the volcanic rock, in order for these species to have survived repeated lowering of sea level by more than 100 m (300+ ft) during the Pleistocene Epoch. The argument assumes that phreatic ground water completely drained away from the limestone cap at low sea levels, depriving cave-dwellers of an aquatic habitat in the cave systems as we know them today. However, such an assumption is inconsistent with some basic hydrogeological principles. A fall in sea level which exposed the entire Bermuda platform would create a catchment area of such an extent that even under lower rainfall conditions than today, there would have been substantial accumulation and retention of ground water within the porous limestone cap, even when sea level dropped below the top of the volcanic rock (Figure 6j) . Ironically it is the absence of caves at the base of the limestone cap that would have increased the likelihood of ground water retention.
It is argued here that the existence of “deep” flooded cave refuges need not be invoked to account for the survival of cave-dwelling organisms during Pleistocene glacial periods. Rather, it can be attributed to the transformation of the limestone cap into a fresh water aquifer at these times. This hypothesis assumes that these creatures could migrate between high porosity zones, fractures and caves as water levels changed and were able to tolerate, or adapt to, slowly fluctuating salinities.