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The form of weathering observed on gravestones is a product of four interconnected factors: Material (M), Environment (E), Process (P) and Form (F). All these can vary both spatially (s) and temporally (t).
As a simple aid to memory the verbal equation is:
W= ( f (s,t (M, E, P, F))
Material refers to the material, natural or artificial, from which the gravestone has been made. Different types of stone can react in different ways to the same sets of weathering agents and can produce different weathering forms even in the same environment. Variations in material properties upon the same gravestone can also produce different forms and rates of weathering on the same gravestone. Variations in grain size and resistance on granitic gravestones, for example, can result in the preferential removal of particular mineral grains.
Environment refers to different things at different scales. At the scale of the whole graveyard, the environment could be classed as urban or rural, coastal or inland for example. The definition of environment at this scale can change over time however. The Victorian leafy suburb or even rural fringe can become incorporated into the polluted urban sprawl altering the weathering environment experienced by the gravestone. Within the graveyard itself there may be variations in exposure of different gravestones depending on the proximity of trees or the position of the gravestone on a slope. Environment can be viewed as shorthand for the presence or absence of particular agents of weathering or the operation of particular processes.
At different positions on a slope, for example there may be differences in water flow and hence water supply to the gravestone base that result in different amounts of capillary rise. This could influence the amount of surface disruption on particular gravestones. Environment can also be viewed as shorthand for the complicated set of variables that characterized a given location at a given scale. It may not be possible to distinguish which, if any factor, alone is the most significant in producing decay, but it is clear that distinct patterns of decay occur when particular combinations of variables occur together. Environment can also include the micro-environments found around an individual gravestone. For example, gravestones with different carvings will produce different waterflow and wind patterns on the gravestone surface. This could influence the amount and location of surface loss.
It is customary to associated particular types of gravestones or environments with particular weathering processes. Marble gravestones, for example, are expected or rather assumed, to decay mainly by dissolution by acidic rainfall (remembering that natural rainfall is slightly acidic anyway being a weak carbonic acid with a pH of 5.6) Dissolution is assumed to remove matrix and grains from the surface of the gravestone producing a 'sugary' surface ( Image 1 ). If you pass your hand over the gravestone surface it will have a slightly roughened feel. In coastal environments, it is often assumed that salt weathering is the main decay mechanism producing spalling and flaking. Often a specific weathering process is assumed to produce a specific weathering form. There is an assumption of a one-to-one relationship between process and form. This is not necessarily the case. Process never works in isolation from a particular environment and upon a particular material.
The forms produced are dependent upon this combination of factors. Some forms such as flakes on sandstone or surface roughening in marble may be indicative of the operation of particular weathering agents such as salts or acid rain. These forms are still only visible because they occur on particular materials that decay in a certain manner in their presence. Often the presence of a weathering agent is taken to imply the operation of weathering mechanisms. The details of these mechanisms or sequences of changes associated with a mechanism are often hazy or unknown. Laboratory studies may help identify possible ways these agents operate , but when in the 'real' environment combinations of agents are more common than the operation of a single, isolated weathering agent. Despite this, weathering rates and decay forms are often presented in the literature as resulting from a particular mechanism. In a number of papers, for example, sulphur dioxide pollution is cited as a cause of high historic losses.
By implication acidic rainfall is the prime agent, despite the possible presence of salts and other weathering agents in these polluted environments. In defence of such assumptions it should be noted that often similar forms can be found in environment with high pollution levels or high salt levels. These similarities suggest that when dominant particular agents may be able to produce particular weathering forms.
Sandstone can weather in a different manner from marble. Flaking of the surface ( Image 2 ) can produce large and small coherent flakes of sandstone which become detached from the surface. Often the flake can remain attached to the surface for so long that organisms can begin to live in the gap between the flake and the gravestone. This could produce biotic weathering of the surface. Eventually the flake becomes detached and an uneven surface ( Image 3 ). Different rates of detachment produce a rough and discoloured surface which can be very friable ( Image 4 ). As on some marble gravestones, black crusts can be deposited on sandstone gravestones ( Image 5 ). These result from the deposition of particulate matter such as soot, but can also form by the reaction of sulphur dioxide with and calcium carbonate that may be present, for example, in the cementing matrix. In addition, carving a surface can produce micro-environments that can enhance weathering as noted for marble gravestones. In Image 5 the section of the cross which has lost its black crust is highly friable and exhibits flaking and granular loss. Underneath the arm of the cross, atmospheric moisture concentrates and has enhanced loss. Such 'underarm' weathering is relatively common in sandstone crosses. Similarly, in Image 6 the lower part of the small cross seems to have be preferentially weathered, possibly because of capillary rise concentrating moisture in the lower section of the gravestone.
Although it is difficult to generalize about weathering forms the following form an initial classification scheme that might be of use. It should be noted, however, that often weathering forms appear in combination and trying to classify a surface as being of one particular form neglects the interactions and relationships between forms. For some idea of how questions can be asked about weathering on gravestones and for more details on classification schemes it is useful to have a look at Inkpen (1992) in Geography Review .
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