3. Dune Deposits

 The accumulation of sediment on the Bermuda platform

The skeletal remains of marine lifeforms – corals, molluscs, algae, forams, echinoderms etc – which live and die on the Bermuda platform are ultimately broken down, or comminuted,  into small fragments, by biological activity and wave erosion. Over millions of years, vast quantities of carbonate  sediment  have thus accumulated on the platform and spilled onto the flanks of the seamount. Ultimately, a portion of this sediment  was driven onto beaches and thence onto the land by wave action and by the wind.

Beaches are unstable features which typically receive sand from the sea in fair weather and give it up to the sea during storms. They are the source of wind-blown sand that is trapped in  vegetation to form coastal dunes. Present-day beaches and dunes  of Bermuda are of limited extent, relative to the length of rocky coastline. The beaches, which can be categorised as reflective,  are narrow and vulnerable to erosion; while the dunes are small, vegetated and inactive (Figure 3a). This contrasts with conditions which prevailed on many occasions in the past.

Warwick Long Bay annotated.png
Figure 3a. The beach and foredunes at Warwick Long Bay.  The beach at Warwick Long Bay features, from right to left: 1. A sloping wave-swept beach-face; 2. A ridge (just above the wrack line) known as the berm; and 3. A near-horizontal platform known variously as the upper beach, the backshore and the dry beach . 4. A slope representing the front of small vegetated wind-formed dunes known as foredunes. The character and size of these dunes has changed little in modern times but for occasional erosion by major hurricanes which is typically repaired by natural processes within one year.

The sand dunes that built Bermuda

An ample supply of un-stabilised sand and moderate to strong winds are prerequisites for the construction of large sand dunes. Notable environments where these conditions can exist are deserts (Figure 3b) and sandy coastlines (Figure 3c) (as well as the planet Mars).

Gobie dune
Figure 3b.  A Gobi Desert (Mongolia) dune.  This Gobi Desert dune is approximately 300 m (1000 ft) high. The barren rocky desert landscape (foreground) is the source of fine sand which is winnowed by the wind and transported to the dune-fields. Such dunes remain active over thousands of years shifting their shape in response to seasonal changes in wind direction. Note the steep dune “slip-faces”, down which sand avalanches from right to left  and towards the viewer. (Scale: telephone poles provide scale)
Oregon dune
Figure 3c. Coastal dune fields of Oregon, USA.  Oregon’s coastal dunes are able to exist despite the temperate climate which is favourable to vegetation growth. Strong winds and a plentiful supply of beach sand make this possible. The dune-field, shown here,  is fronted on the right side by a long sinuous slip-face down which sand avalanches. It is advancing landward into the forest (to the right) at approximately 1 m (3 ft) per year. Episodes of dune activity similar to this were occurring on Bermuda during the Pleistocene epoch. (photo by Charlie Bristow)

As early as 1837 Lieutenant Richard Nelson of the Royal Navy (NE1) established that the Bermuda islands are constructed principally  of hardened wind-blown sand dunes. Later, in 1931 American geologist Robert Sayles  coined the now widely-used term “eolianite”  based on his observations in Bermuda (SA1). He defined eolianite as cemented carbonate sand  which originally accumulated by action of the wind. .

The dunes that formed the limestone hills of Bermuda, although of a coastal variety,  extended inland well beyond the immediate vicinity of the beach. Their size (Figure 3d) and mobility were quite unlike anything observed in present-day Bermuda. Multiple dune formations attest to  numerous occasions in the geological past when a large supply of sand was accessible to the wind, on beaches of a width which exceeded the “critical fetch” (MO1). This is the minimum width of dry beach that wind must travel across in order to gather up, or entrain, sufficient sand for the growth of coastal dunes.

Khyber Pass Quarry dunes
Figure 3d. Quarry at Khyber Pass, Warwick Parish. The rock faces of this disused building-stone quarry expose large lithified dunes, or “eolianites”, which advanced to this hilltop position from beaches on the south shore approximately 120,000 years ago. The direction of advance – from left to right – is indicated by the inclination of  leeward slip face strata which dip down towards the north (right).

The fluctuation of sea levels associated with the cyclical advance and retreat of the continental ice sheets,  over the last one and half million years or so, were critical to dune building on Bermuda. The changing sea levels presented multiple opportunities for the development of wide wind-swept, dissapative, beaches along significant lengths of  Bermuda’ coastline. It was at these times that new ridges of dunes would have been be appended to the increasingly hilly islands (RO3).

Sand dune activity on Bermuda during the 19th Century

Compared to sand dune activity of the geological past that built Bermuda, dune activity since the settlement of the islands has been of limited extent. Nonetheless episodes of heightened dune activity have been recorded in historical times.

In 1835 Bishop Inglis a visitor from Canada documented (in a letter archived at the National Museum of Bermuda) burial of houses by the accumulation of sand up to 12 feet deep in Tucker’s Town. He recounted meeting a man who “… showed us the ruins of the house in which he was born … and which is now covered to the depth of 8 feet (of sand), but the lines of the walls, and the chimney, are still visible”. At Elbow Beach in 1837, Lieutenant Nelson reported a sand dune which “supported by constant supplies (of sand) from the sea, has steadily climbed up the hill to the very summit, a height of 180ft.” (Figure 3e). He observed that “…. the dazzling dry, and almost snow-white sand is checked, before the front of the trees, in a steep bank, varying from 10 to 25 feet in height.” and “As soon as the mass shall have over-topped the woods, I know nothing capable of opposing its progress.”.

Elbow Beach painting of historic dune activity
Figure 3e.  Historical dune field – South Shore, Warwick Parish facing east. This painting presents an 1830s view of an active dune field near the site of the present Elbow Beach Hotel, which today is located in the middle distance between the two forested hills. The painting is from the Johnson Savage MD Collection, National Museum of Bermuda.

Some sand dunes were still active when visited, as an object of scientific curiousity, by British scientists of the Challenger expedition in 1873 (Figure 3f). Later still in 1889, Angelo Heilprin (HE9) observed active dunes at Elbow Beach and Tuckers Town which he described as “great tongues of sand” and “sand glaciers…stealthily encroaching on hilltops of the interior and burying everything” including houses (Figure 3g) at both locations.

Dune activity apparently dwindled through the early 1900s to the point that in 1931 Sayles (SA1) declared an absence of active dune-building on Bermuda. In making this observation, he surmised that conditions quite unlike those of recent times were required to trigger a meaningful dune building event. He was the first to make the association between dune activity and sea level fluctuations of the Pleistocene Ice Ages.

Although of limited significance in terms of an impact on the geological record,  the cause of historical bouts of dune activity could be of relevance to our understanding of more prolific events of the distant past. It had been suggested, for example, that 19th century dune activity on Bermuda was triggered by “clearing of brush” during construction of coastal fortifications (VE1). Similarly, Robert Sayles (SA1) and Stanley Herwitz  (HE10) advocated a connection between heightened dune activity and climate-related degradation of vegetation-cover. However, they failed to account for fossil evidence of flourishing plant life, such as a proliferation of herbivorous land snails and  palmetto leaf impression, preserved within the dunes. Sayles (SA1) did to his credit recognise that, regardless of the magnitude of any botanical effect,  a key prerequisite for the episodic  generation of massive landward advancing dunes (Figure 3k, see later) was the delivery of an  abundant supply of  sand via expansive beaches.

Stratification of Bermuda’s limestone dunes

Sedimentary rocks are the dominant rock type on the surface of our planet and thicknesses typically exceed 1 kilometre. They are formed by the accumulation of layers of loose sediments which through the effects of time became consolidated or “lithified”. These ancient lithified sediments can be classified on the basis of their chemical composition, structure and particle size, which can range from that of clay or silt, through sand to pebbles and cobbles. When sedimentary rocks are exposed in a rock face, small differences in colour,  cementation or in the size of particles between one layer and the next are manifested as bands known as “strata”. Such strata can range from less than one millimetre to several metres (yards) in thickness.

The majority of sedimentary rocks were deposited  on deep sea-beds or lake-floors in horizontal layers (of mud, silt or sand) but subsequently may have been tilted and folded by tectonic forces. In some depositional environments, however, the sediment did not simply settle in layers, but was transported along by water currents or by the wind. Rivers, shallow seas, sandy shores and deserts are examples of such environments.

A sediment surface which is exposed to a strong  current is sculpted into moving bedforms, such as ripples or dunes. The accumulation of cascading sediment on the downstream side of these features represents lateral growth, or lateral accretion. This creates series of sediment layers which slope down in the direction of the current or wind flow. When, and if, they are preserved in rock, they are termed “cross-stratification” (Figure 3h and 3i) to distinguish them from originally flat stratified deposits which simply accumulated by settlement of sediment.

Cross-stratified deposits typically feature intersections of sets of strata at “bounding surfaces” across which there can be a  marked change in the orientation of the strata (Figures 3h and 3i). These bounding surfaces represent transitions between phases of accumulation and phases of erosion – reflecting changes in: wind/current direction,  sediment supply and, in the case of aeolian dunes, vegetation cover.

Khybe Pass St Georges strata
Figure 3h. Slip-face cross strata at Khyber Pass, St. George’s Parish. Cross-strata displayed in this rock face record the accumulation of layers of wind-blown sand on the leeward slope or “slip-face” of a sand dune.    Advance of the dune from the north (right to left) is indicated. The variation from distinct to indistinct stratification across the exposure likely records changes in wind strength,  with the less stratified, more homogeneous, layers representing faster accumulation. (Scale is provided by 1 m rule).
Verdmont Road annotated.jpg
Figure 3i. Bounding surface on Verdmont Road, Smith’s Parish. Sand dune deposits in this 2.5 m (8 ft) road-cut display good examples of cross strata. They are divided into two main sets (A and C) separated by a bounding surface (B). The lower set (A) comprises steep, leeward slip-face strata, or “foresets”. The upper set (B) comprises low angle undulating windward strata, or “backsets”. The lower dune  (A) underwent erosion of its top surface (B) as it advanced from the south (left). A subsequent increase in sand supply and upward growth is recorded by the accumulation of low angle strata (C). (Scale is provided by 1m rule)
Dune at Gunpowder Taven anotated
Figure 3j. Ancient dune at Old Military Road, St. George’s at site of former “Gunpowder Tavern”.  Similar to Figure 3i, this 10m (30 ft) high rock face displays distinctive sets of cross-strata separated by a bounding surface (broken white line). Dune (A) advanced southward (left to right) across  the top of a 37 m (120 ft) hill before stopping in its present position. It was subsequently entombed by accumulations of low angle strata which likely fed into a larger dune to the south. It can be concluded that at the time these dunes were active, approximately 120,000 years ago, vast quantities of platform sand were being swept landward from beaches on the north shore positioned just beyond (to the north and  east of) the present coastline.

A particular dune geometry, consisting of low angle strata which feeds into or over-rides and truncates steeply sloping slip-face cross-strata (Figure 3i), is displayed in rock faces and cliffs throughout Bermuda. Variations in the proportions of these two types of strata – low angle and high angle – are attributable to a dune’s behaviour, which can  range from a static structure which stores sand to a mobile structure which transports sand.  (Figure 3k). In noting “the establishment of a slip face has a profound effect on the subsequent life history of the dune” Ralph Bagnold (BA1), a pioneer in sand-dune science, enunciated  a fundamental relationship between dune form and dune behaviour – that the existence of a slip-face is diagnostic of dune mobility.

In their simplest form, mobile sand dunes are wedge shaped structures. There is a gently sloping windward face, up which sand is blown from the general direction of the sand source, such as a beach. At the top end of this slope there is a brink over which sand cascades onto the steep  leeward face of the dune, or “slip-face”. The slope, or dip, of this face – approximately 30 degrees to horizontal – is determined by the “angle of repose” of dry sand.

Convict Bay Dune.jpg
Figure 3k. Evolution of an ancient dune at Convict Bay, St George’s. The  growth and behaviour of this dune  is recorded by the orientation and slope of the layers , or “strata”. This dune advanced laterally for a while (left to right) before growing in height. Sand supply and the presence of vegetation are factors which dictate whether dunes are mobile and advance laterally (as in 1) or are fixed in position and grow upwards, or accrete (as in 3).

Characteristics of Bermuda’s limestone dunes

As noted above, the geometry of sedimentary strata is indicative of environmental conditions, such as current/wind strength and sediment supply, at the time of deposition. The high proportion of slip-face cross-strata, or foresets, present in Bermuda’s eolianites (Figures 3l and 3m) demonstrates significant transfer of sand from the windward face and dune-top to the leeward face; and is diagnostic of dunes – in this case coastal dunes – which have become unstable and mobile. Such dunes, which transport sand laterally, are known as “advancing dunes”  (MA3, RO2). Their existence reflects the inability of vegetation to stabilise wind-blown sand, which likely coincided with episodes of high sand supply.

Astwood Cove.jpg
Figure 3l. A large landward advancing ancient dune at Astwood Cove, Warwick Parish.  Steeply sloping slip-face strata are dominant in this dune, which advanced landward from the south (left to right) onto a flat vegetated land surface recorded by a thin brown fossil soil layer, or “protosol” .  Note the truncation of the slip-face strata near the top of the cliff and the subsequent accumulation of horizontal strata. This sequence represents  a transition from a mobile advancing dune to a stabilized dune.  (Persons on beach provide scale)
NonSuch Dune.jpg
Figure 3m. Ancient mobile advancing dune on Nonsuch Island, Castle Harbour. This flat-topped dune consists almost entirely of northward sloping slip-face strata  from which a northward direction (right to left) of advance, or migration, is inferred. This dune of the Rocky Bay Formation – approximately 120.000 years – must have been active when sea level was lower than today.  (Scale: 1.5 m (5 ft) child standing on the rocks near the water’s edge provides scale)

It can be concluded, that the dunes which formed the hilly topography of  the Bermuda islands were   not static mounds of sand, fixed in position by vegetation, or by rapid cementation as had previously been thought (BR1, LA2). It is evident  that they were mobile structures  which advanced, or migrated, landward from the beaches and up onto the bordering hillsides burying plants, including tall trees, in their path (RO2) (See Chapter 9).

An illustration of typical Bermuda dune types, their stratification and their evolution can found here.

The arrangement of Bermuda’s sand dunes

Bermuda’s earliest limestone deposits would have co-existed with remnants of the volcanic island. Evidence for this exists in the form of isolated deposits of volcanic sands (at Whalebone Bay) and volcanic pebbles (at Government Quarry and Stokes Point) inter-layered or mixed with the dominant carbonate sediments. The oldest islands of Bermuda, including remnants of the volcanic island served as nuclei for the accumulation of later limestone deposits. Beaches which formed around the edges of pre-existing islands intermittently provided sources of sand for large wind-blown dunes. Some of the oldest dunes of the Walsingham Formation (e.g. at Government Quarry, Hamilton Parish), although overwhelmingly constituted of calcium carbonate sand, include black or rust-coloured flecks representing the occasional grain of volcanic origin.

Bermuda’s dunes began life as sand mounds, or foredunes, which formed amongst vegetation at the back of beaches (Figure 3a). During episodes of plentiful sediment supply they grew into, or were entombed by, much larger and more mobile desert-like dunes. These dunes coalesced into linear ridges at the back of extensive beaches, the likes of which do not exist today. Their structural resemblance to transverse dunes of the Oregon coast (Figure 3c) was noted by Fred Mackenzie (MA1). One ancient dune ridge, of the Rocky Bay Formation, now occupies the entire length of the North Shore in the central parishes (See Geological Map, Figure 5a, Chapter 5) . At the time of formation this would have been associated with a beach approximately 5 km (3 miles) in length situated to seaward of the present north shore. It is thought that this beach and associated dune ridge formed during the last Interglacial period approximately 120,000 years ago.

Present climatic conditions at Bermuda produce south-westerly prevailing winds and westerly gale force winds. Had these same conditions existed in the past, as is likely, then one might have expected ancient wind-blown dunes to have advanced in a generally east-north-easterly direction. However, the orientation of their leeward slip-faces indicate that Bermuda’s dunes advanced predominantly in an inland direction (Figure 3n). Those on the north shore advanced south, those on the west shore advanced east and those on the south shore advanced north. (East-facing shores are very limited in length).

An analysis of over 3750 slip-face orientation measurements taken from Bermuda’s Pleistocene dunes was undertaken to determine the relationship between the direction of dune migration and the wind regime (RO2). A summary of the results in map-form can be found here.  It is apparent from these data that, regardless of their frequency, onshore winds were more effective at dune building than prevailing or gale force winds. This may be attributable to the relatively high capacity of non-turbulent wind,  which blows directly off the sea, to transport sand landward having crossed a beach-source (RO2),  as compared to turbulent wind which has traversed Bermuda’s hilly topography. In any case,  the record shows a tendency for dunes on all shorelines to advance inland away from the coast, climbing onto the flanks, and often over the top, of their predecessors (Figure 3o). The pattern of accumulation is reflected in the arrangement of the geological formations and in the topography of the islands, which rise up to ~80 m or 260 ft above mean sea level.

Dune arrangement
Figure 3n. A cross-section through Bermuda showing a typical arrangement of ancient dunes. Bermuda’s dunes advanced inland in response to onshore winds. They climbed onto the flanks and sometimes over the top of their predecessors.  Numbers indicate the order in which the dunes accumulated and are equivalent to geological formations. In some cases dunes advanced from  opposite coasts during the same episode of dune building e.g. 1, 2 and 4. At other times dunes  advanced from only one direction e.g. 3.  The arrangement varies from one part of Bermuda to another but the general pattern remains the same.
OLYMPUS DIGITAL CAMERA
Figure 3o. Cross-island  dune migration near St David’s Head. At this location a younger mobile dune (light coloured), advancing from the north-west (left), over-topped a forested hill of older dunes. The seaward-dipping brownish surface at the base of the cliff records the position of an ancient soil in which trees would have grown.  It marks the boundary between the two dunes. It is quite common for Bermuda’s ancient mobile dunes to have advanced onto and over the top of their predecessors in this manner. These limestones at St David’s Head are mapped as the Town Hill Formation. (Scale: light coloured limestone cliff is approximately 20 m (65 ft) high.

The debate over the timing of dune-building

Robert Sayles (1931) attributed the initiation of dune building on Bermuda to harsh climatic conditions and falling sea levels at the onset of glacial periods during the Pleistocene Epoch. He correlated maximum dune activity with a sea level that had been lowered by 20 metres or more from its current position,  exposing the seabed on the Bermuda platform. He depicts “great flats covered by marine shells exposed to the air” which were caused “to pile up as dunes” (SA1).

J. Harlen Bretz (1960), rejected  Sayles’ interpretation. Based largely on his observation of supposed beach-dune transitions above present sea level, notably on the North Shore  at Blackwatch Pass and Whalebone Bay, he asserted that major dune building activity coincided with sea level rising to a peak,  following a small oscillation (BR1).

Leonard Vacher (VA1) generally agreed with the conclusion of Bretz with respect to North Shore dune deposits, but argued that South Shore dune activity was not similarly “forced” by a rising sea level nor, for that matter, a falling sea level (VA3). It was instead the outcome of stable warm conditions at the peak of an interglacial period when sediments being generated in the “carbonate factory” on the south shore ledges would eventually fill “accommodation space”. He postulated (VA1) that, at this point, the carbonate sand would have spilled shoreward onto widening wind-swept beaches and, thence, into large dunes.

The historic debate, briefly summarised above, as to whether dune-building on Bermuda occurred at high sea levels (interglacial periods) or low sea levels (glacial periods) has moved on. No-one now doubts that dune building shut down during the long periods when sea level was below the edge of the Bermuda platform. The presence of vegetation and the absence of active beaches, on the platform, would have assured this. Some may still argue that dune building episodes correlate with peak sea levels, but if this were so, we would be experiencing dune building on Bermuda today.  The dune building models which, now,  are considered most credible, incorporate a sequence of destabilising sea level changes which span or succeed peak sea level. Stability is an anathema to  coastal dune construction. It is conducive to sediment depletion and stabilisation.

Early dune-building hypotheses, outlined above, relied on incorrect or incomplete interpretation of key sequences of coastal deposits on Bermuda’s shores. For example,  strata on the North Shore  once attributed to marine deposition on a beach, are now considered to represent accumulations of aeolian sand sheets on the windward side of a dune (KI1, RO3). The elevated beach at up to 10 m (30 ft) above present sea level identified by Bretz, as feeding into large dunes, does not exist (see Blackwatchwatch Pass field guide, ……..).  On the south shore, recurring critical geological sequences, which comprise a weak fossil soil, or protosol,  sandwiched between a subjacent marine deposit and an overlying dune (Figure 3p), had never been satisfactorily accounted for. It is inferred from a new detailed interpretation of such sequences  that a rise and then, critically, a fall in relative sea level were prerequisites for major dune building events (RO3). Indeed two such “high order” oscillations may have been needed to manoeuvre and rework sufficient volumes of  sediments close to the shore to account for some of the very largest Pleistocene dunes on Bermuda.

 

Spittal Pond west
Figure 3p. Marine-dune sequence in the Belmont Formation at Spittal Pond West, Smith’s Parish. This Belmont Formation sequence is found at numerous localities along Bermuda’s south shore. A shoreface bar (bs) and beach-face deposits (Bb) are truncated at a horizontal surface which is superposed successively  by a fossil soil, or protosol, (Bp) and a dune (Bd). The sequence is believed to be the product of: 1).  a rising sea level which formed a laterally expanding, or prograding, beach; 2). truncation caused by backshore flooding and deflation; 3). an episode of low sediment supply as sea level peaked, represented by the protosol, and 4). a fall in sea level, which produced widening beaches and advancing dunes. By this interpretation, significant dune development is correlated with the commencement of  sea level retreat, or a marine regression. (Scale: The top of the truncated Belmont marine deposits (Bs and Bb) are at 2.5 m (8 ft) above present mean sea level)