Purbeck Stone

["Dr. Tim Palmer" <tjp@xxxxxxxxxx>]

Many people who work with stone have been kind enough to express an
interest in the articles that I write on the geological origins of famous
British stones.  They appear intermittently in 'Natural Stone Specialist'
Magazine, and there have been about a dozen so far (including Portland, Ham
Hill, Blue Lias, Doulting, Flint and Chert, Grinshill, Stainton, Bath).
The idea, not always successful, is to describe how these stones came into
existence, without recourse to the worst excesses of unexplained
geologists' jargon

The latest, on Purbeck Stone, is just out, so I thought I'd try it out on
people here.  The original is illustrated, whereas the text pasted below is
not.  All further suggestions are welcome, and I will try to assemble the
whole lot onto a website if anyone would be interested.

'The Isle of Purbeck in eastern Dorset produces a range of limestones that
have great historic importance as walling and paving materials across
southern England.  Some take a good polish, none more so than the famous
Purbeck Marble, which was of such fame and importance for ornamental work
in medieval ecclesiastical building.  There are several distinct types of
Purbeck stone, attractive, mostly shelly and still available.  All date
 from the period of geological time about 135 million years ago when the
Jurassic period was giving way to the Cretaceous.  Bed heights tend to be
small, usually less than 0.4 m; hence their suitability for walling, paving
and ornamental work rather than ashlar.  The environments in which the
sediments were originally deposited are rather unfamiliar in today's world,
at least to those of us who live in the UK, but they were crucial in
determining the shell-rich nature and the slabby character of the stone.

As a preamble, we need to ask the question: why are some beds of stone
thin, and some thick?  Or to take the question back in time, closer to the
geological answer, why were some beds of sediment deposited as thin layers
- maybe only a matter of millimetres, whereas others achieved thicknesses
of tens of centimetres to metres?  The answer to this question lies in the
fact that, in different environments, different processes influence the way
that layers of sediment accumulate.  On a river bed or in a delta, for
example, the depth and flow-speed of the river, and the amount of erosion
upstream (the ultimate source of the sediment) play a part.  When the river
is flowing rapidly, during flood conditions, it moves a large load of
sediment.  Some of this load is actually suspended in the water, and some
is merely rolled along the river bed.  When the water movement slows,
either because the flood abates or because the rapid river enters the sea,
the sediment all falls to the floor and no longer moves.  Most of the
sandstones of northern England originated in this way, in the huge rivers
that covered the region 300 million or so years ago.  The thickness of the
resulting bed of sediment is never greater (nearly always much less in
fact) than the depth of the water that originally transported and deposited
it.

On a shallow sea floor like the modern North Sea, it is tidal currents,
both during episodes of fair weather and during storms, that play a major
role. When large masses of water move under these influences, they can
shift large dune-like accumulations of sand across the sea-floor. Again,
when the water stops moving, so does the sediment.  Because the water in
the sea is usually deeper than the water in rivers, thicker accumulations
of sand, often as a series of stacked beds, are typical of ancient
sandstones that have a marine origin.

In shallow tropical seas where lime sediments (the forerunners of
limestones) accumulate, biological processes are often an important
additional component.  Reefs and shell banks flourish, and in only a few
years or tens of years (a mere lightening flash in geological time) may
form layers many centimetres thick.  In these environments, which are
almost invariably tropical, storms and hurricanes were and are a major
modifying influence.  Every few years they sweep past, the currents and
huge storm-waves scouring and eroding wide tracts of sediment from one
place and dumping it as a pile on the sea floor of an adjacent area as the
storm abates.  Each such pile of limey sediment or shell debris is a single
bed and ultimately, millions of years down the line, forms a single bed of
extractable stone in a quarry.

This contrast between times of water and sediment movement, and
interspersed episodes of quiet water when the sediment load is dumped on
the river floor or the sea bed, is critical in an understanding of  how
beds of sedimentary rock first came about.  Geologists have compared the
process of sedimentation to the life of a soldier: long periods of boredom
punctuated by brief moments of terror.  The thicker beds almost always
represent the deposition that occurs at the end of an episode of violent
water movement (geologists refer to these as high-energy events.)  The
intervening quiet (low energy) episodes often occupy a much greater
proportion of the overall time - maybe 99% or more - but they are seldom
represented by any greater sediment deposition than just a thin smear of
clay - perhaps just enough to form a horizontal separating layer at the
base of the next high energy deposit.

Stacks of slabby thin beds (usually of very small individual particle
size), frequently characterize environments of shallow standing water that
aren't (or weren't) normally influenced by river flow or tidal action.
Inland waters and  lagoons isolated from the open sea by barrier-islands
are typically floored by thin layers of fine sand and clay.  A variation on
this theme is found in warm conditions, particularly those that show
seasonal variation in temperature and rainfall.  Intermittent high tides or
storms may wash sea water into the lagoons, giving shallow warm sea-water
ponds that teem with invertebrate life.  But the water slowly evaporates,
and the water become more concentrated and briny.  Sea water contains about
3.5% dissolved salts by weight.  By the time evaporation has concentrated
this to about 7%, all but the most primitive life forms have been killed
off or survive only as resistant eggs in the muddy lagoon floor.  By the
time the concentration reaches about 11%, crystalline gypsum and then salt
crystals start to grow.  These conditions are referred to as hypersaline.
Eventually, another inundation of the sea or a period of heavy rain will
restore the salinity levels to something tolerable, and the life forms will
hatch out or return.  In isolated corners of these lagoons, or across
larger areas where barrier islands emerge to cut off any connection with
the sea, rainfall or freshwater run-off from land may predominate and the
water conditions become brackish or fresh.  Today, just such a mosaic of
environments with varying salinities can be studied in western and southern
Australia, along the coastal zone of North Africa and around the Persian
Gulf.  But in late Jurassic time, as the seas that deposited the Portland
limestone retreated and dried up or became restricted to land-bound gulfs
and embayments, these were the conditions that prevailed across southern
England and parts of Northern France.  Geologists refer to such a range of
freshwater / brackish / hypersaline environments as 'marginal marine', and
the Purbeck sediments of southern England are a classic example, differing
only from the modern examples in that they have been buried and hardened
into stone over their 130 million year history.

Although animal life teems in these lagoonal environments, it tends to be
dominated by just a few species that can handle the harsh conditions
represented by the seasonal fluctuations in salinity.  Many animal groups
that are familiar in ordinary shallow seas are excluded, so the few species
that can survive have little competition and occur in vast numbers.  For
this reason, the beds of the Purbeck contain fossils of only a small number
of different species of invertebrate animals - not much more than half a
dozen - dominated by water-fleas, brine shrimps, a couple of different
types of clams, an oyster, and one or two types of snail.  But these occur
in huge numbers, and some of the different beds of stone represent thick
piles of one or two species of shell, piled on the lagoon floors by passing
storms.  Some beds, such as the Grub and the Vye Bit are entirely made up
of oysters and a species of small clam. Under the microscope, the abundance
of shell is clear; the only other component in the stone is the natural
calcite cement that grew between the shells when the sediment became
buried, and which cemented them into the hard stone seen today.  Other
beds, such as the Cap Bed, show signs of the shrinkage cracks that
developed in the surface of the lagoon floors when they dried out for a
short interval.  The famous Purbeck Marble is dominated by a snail called
Viviparus, which thrived in an area of lagoon that became isolated from the
influences of the sea for a time, so that seasonal rainfall kept the water
brackish or fresh for a period of years or decades.  At the top of the
Purbeck Marble bed is a layer with the freshwater mussel Unio, which
confirms this interpretation.  In between the shelly beds are more
conventional limestones, some displaying the burrows of shrimps that dug
their way into the muddy sea floor.  If you visit the quarries, you might
even be lucky enough to see the large 3-toed footprints of the dinosaurs
that waded across the brackish ponds.'
Dr Tim Palmer C.Geol., F.G.S.
Executive Officer, The Palaeontological Association
I.G.E.S., University of Wales
Aberystwyth SY23 3DB
Wales, U.K.

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