6. Fossil Soils

Types of Fossil Soils

Fossil soils, or palaeosols, are thin undulating layers of sandy clay that  run through Bermuda’s limestones and, in some cases,  separate the formations (Figure 5a). They originally formed as surface soils on ancient landscapes where plants and trees grew. They are preserved today because they were buried by advancing sand dunes which eventually became cemented into limestone rock, known as eolianite (Chapter  3).

Figure 5a. Palaeosols in a rock cut  near the Airport Fire Station, St David’s.  These two moderately well developed fossil soils, or palaeosols, separate three eolianites mapped as the Town Hill Formation. The soils, including the modern soil at the top, represent lengthy episodes of plant growth.

The best developed palaeosols accumulated in valleys or on sheltered hillsides over periods spanning tens, or hundreds, of thousands of years. They tend to have a  reddish brown colour (Figures 5b, 5c and 5d) attributable to high content of iron rich clay. These are classified as terra rossa palaeosols and typically range in thickness from 0.25 to 0.5 m (1 to 1.5 ft).

Figure 5b. Palaeosol near the entrance to Westgate, Ireland Island North. This well developed 0.4 m thick terra rossa palaeosol separates the Walsingham Formation and the Town Hill Formation.
Figure 5c .  Palaeosol at the Argus Building site on  Wesley Street in Hamilton. This well developed palaeosol separates the Lower and Upper Town Hill formations.  Note the variation in the character of the palaeosol  along its length. Where it slopes downwards into a shallow valley,  it changes from a brownish layer of even thickness (0.3 m) to one with deep solution pockets filled with dark reddish-brown clay typical of a terra rossa palaeosol.
Figure 5d. Sloping palaeosol on Cavendish Road, Pembroke Parish. This 0.4 m thick palaeosol separates the Lower and Upper Town Hill formations.  Its strong reddish-brown colour, which qualifies it as a terra rossa palaeosol, is an indication, that it probably formed over a long period. Its relatively even thickness is attributable to its development on a gentle constant slope.

Terra rossa palaeosols, which represent significant time boundaries, developed across the extent of the, now submerged, Bermuda platform during glacial periods when sea levels were much lower than at present (Figure 1f, Chapter 1). Exploratory drilling through the seabed of the Bermuda platform has confirmed the presence of these palaeosols, submerged and entombed within limestone sediments (V03). Forestation of the platform in quite recent  times is corroborated by the discovery of submerged stumps of cedar trees found in Mills Creek, at Gurnet Rock and in the St George’s Town Cut. The genesis of terra rossa palaeosols will be discussed in more detail later in the chapter (below).

A second category of palaeosol found on Bermuda are the “protosols”. These soils are light coloured, varying from white (Figure 5e) to light brown (Figures 5f and 5g) to pinkish (Figure 5h), and would have accumulated over relatively short periods of perhaps a few thousand years. Their thickness is typically about 0.5 m, but can range up to  2 m (6 ft). They are constituted of a high proportion of wind blown sand and often include well preserved fossil land snails (Poecilozonites). These features of protosols reflect the trapping of aeolian sand in a vegetated terrain on temporarily stabilised dunes. Unlike terra rossa palaeosols, they do not represent significant time boundaries in the geological record.

The genesis of terra rossa soils

The best developed soils in a  limestone terrain are terra rossa soils. They occur as surface soils (Figure 5i) and also as buried fossil soils, in which case they are known as terra rossa palaeosols (Figures 5b, 5c, 5d, 5j) .  Their reddish brown hue is attributable to non-carbonate particles (including iron oxides) which are otherwise  a rarity  on limestone islands such as Bermuda.

Figure 5i. Surface terra rossa soil near Valley Road, Paget Parish. This terra rossa soil is actively forming on the land surface.  The underlying rock is mapped as the Upper Town Hill formation, which means the soil has developed uninterrupted for several hundreds of thousands of years without being buried by dunes. The deep irregular penetration of the soil into the limestone on which it rests is confirmation of the soil’s age.
Figure 5j. Terra rossa palaeosol at “Seon Place” building-site, at east end of Front Street near Spurling Hill, Hamilton.  This buried fossil terra rossa soil, or palaeosol, is mapped as the boundary,  between the Lower Town Hill formation (below) and the Upper Hill Formation.

As a terra rossa soil forms, the underlying limestone surface is simultaneously being dissolved and lowered by the chemical action of acidic water within the soil. The dark clay material within the terra rossa soils was once considered to be exclusively the residue of non-soluble impurities within the limestone, which were concentrated in the soil as the limestone was dissolved (SA1). However, because of the high purity of Bermuda’s limestones it has been calculated (LA2) that the thickness of limestone that would have to be dissolved to produce sufficient insoluble residue to form a typical Bermuda terra rossa palaeosol is unrealistically large. Other sources of the non-carbonate minerals within these palaeosols, and soils, therefore has to be found.

There is evidence that  “foreign” minerals found  in Bermuda’s soils arrived as airborne dust which was carried from the Sahara desert by winds which circulate high in the atmosphere. These air currents flow westward near the equator and then northward, across the Caribbean and North America, towards Bermuda. The chemistry of the particles seems to support this hypothesis (HE12).

An alternative explanation for the composition of these “red” soils  is that the non-carbonate minerals, including silica, are actually not so foreign, and largely comprise material that was derived locally from the Bermuda volcanic seamount (PR1). When sea levels were low, during glacial periods of the Pleistocene Epoch,  the edge of the Bermuda platform would have been exposed to wave erosion. Resultant beaches of volcanic sand would, it is argued, have provided a source of fine material to be carried landward by the wind and accumulate on the land surface as a component of the soil.

Soil pipes or “palmetto stumps” – which?

Mature terra rossa soils, or palaeosols, often have an irregular contact with the underlying rock (Figure 5i), representing  uneven dissolution of limestone by acidic soil-water. Where this surface, or contact, is deeply pitted it is known as “karst” of which “dissolution pipes” or “soil pipes” are a subset. They are tubular or conical soil-filled structures which can penetrate multiple metres into the  limestone bedrock.

Soil-lined pipes or conical hollows are found in abundance along Bermuda’s rocky shorelines (Figures 5k and 5l). They are described in 19th and early 20th century  accounts of Bermuda’s geology, at which time there was no consensus on  their origin – and so commenced a long-running debate.

Figure 5k. Soil-lined  hollows at Grape Bay east, Paget Parish. These 0.25 m (10 in) wide circular structures referred to as “palmetto stumps” by some and “soil pipes” by others, occur here in ancient, Belmont Formation, beach deposits. At the top of the photograph the remnants of a fossil soil, or palaeosol, can be seen. This soil once covered and filled these pipe-like features and has now been stripped away by wave erosion.
Figure 5l. Soil-lined hollows at Grape Bay east, Paget Parish. These circular pipe-like structures are similarly situated to those in Figure 5k. The terra rossa soil which once covered and filled the structures is better preserved at this locality.

Nelson (1837) (NE1) proposed that these hollows in the limestone  were the product of root growth or the action of water. Verrill (1907) (VE1), on the other hand, named the soil filled circular structures “palmetto stumps” – a term that persists in Bermuda’s geological vernacular today.  He envisaged the partial burial of palmetto trees by accumulations of sand. Following their death and decay, he argued that each tree left a cylindrical void in the hardening limestone which subsequently became filled with soil.

Sayles (1931) did not agree with Verrill’s assertion that trees were directly involved, stating that “….these “palmetto stumps” are nothing but solution holes caused in the development of soils”. Some people have continued to favour Verrill’s explanation, but the consensus among  geologists who have studied these features in recent decades is that they are indeed solution holes, commonly known as “soil pipes”. This conclusion is supported by the close relationship, which invariably exists, between the soil pipes and an overlying terra rossa soil, or the remnants of one (Figures 5i, 5l, 5n). Only where this soil has been  stripped away (Figure 5k), can the false impression be given that these features might have extended upwards through the limestone and represent the position of  ancient buried tree trunks.

Figure 5m.  Deep soil-filled hollow at Aubrey Road, Hamilton Parish. This 2 m (6 ft) long pipe-like soil-filled structure has formed below the modern land surface within limestone of the Town Hill formation.  The top of the structure coincides with the surface soil (replaced with concrete) and the bottom of the structure ends in the limestone. It can be concluded that the structure penetrated the limestone from the top down as a “soil pipe”.  Root growth and infiltration of acidic soil water are factors which would have contributed to the pipe’s development.
Figure 5n. Deeply pitted sub-soil limestone at Wilkinson Quarry, Hamilton Parish. The mature terra rossa soil featured at this locality coincides with a step in the quarry face so appears to be a surface soil. It is actually a buried soil, or palaeosol, separating the Walsingham formation (below) from the Lower Town Hill formation. The limestone surface directly underneath the palaeosol is deeply pitted and is penetrated by up to 3 m (10 ft) long “soil pipes”. Those “pipes” which are not vertical, intersect the quarry face at an angle and therefore appear as detached pipe segments. Chemical dissolution of the limestone at the base of the palaeosol must have continued over a very long period to produce this degree of karstification.
Soil pipe
Figure 5o.  The formation of  soil-filled pipes – two hypotheses. These two illustrations represent the alternative explanations of how vertical soil-filled pipes were formed within limestone. In the first, a cylindrical void, or mould, formed  in the limestone by decomposition of a buried tree, was infilled with soil. This created  a “palmetto stump”. In the second illustration, concentrated dissolution of limestone at the base of a terra rossa soil created a conical pit or “soil pipe” which penetrated downward from a soil.

Processes that might initiate the development of solution holes or “pipes” at the base of soils include the concentration of rain water flow (“stemflow”) into the ground around tree trunks (HE11, HE13) and the growth of tap roots (BR1). Either of these, or a combination, amount to the same explanation proposed by Nelson in 1837 (NE1)

There is no question that palmettos and cedar trees were buried by advancing dunes. Evidence is seen in the form of structureless pillars, or columns, of sand found within ancient cemented dunes, which are the casts of former tree trunks (Figure 5p and Figure 9j, Chapter 9). They are preserved within the limestone strata, often extending upwards from fossil soils in which the trees once grew (Figure 5p) . The sand columns are white,  however, and cannot be confused with reddish-brown soil-filled putative “palmetto stumps”.

There are many instances, observed in the field, of soil-filled pipes, cones or pits which extend downwards from a terra rossa soil, or palaeosol, as illustrated in Figure 5o. These occurrences corroborate the “dissolution pipe” hypothesis. In contrast, there is no documented example of a red soil-filled pipe which extends upwards from a  palaeosol, as would be expected had any originated as “palmetto stumps”.

Figure 5p. Large fossil tree impression at Saucos Hill, Smith’s Parish.  This cast of a tree-trunk is constituted of structureless dune sand. It represents a large tree (probably Bermuda cedar – Juniperus bermudiana) which was entombed by an advancing dune. After the dune had become consolidated and the tree had rotted away, the resultant void  was infilled with sand from a later dune. Note that the soil in which the tree was rooted is preserved as a protosol.  A demonstrable tree cast such as this is never found filled with red soil. (1m (3ft) white rule standing vertically to the right of the tree cast, provides scale)