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Compacted mud is an excellent building material
in the right place, like a hot, dry desert, but it is quite
unsuitable for the wet, northern European climate unless covered
and kept dry. Rocks that are made of mud and silt are the
Cinderellas of the useful sedimentary stones, overshadowed
by their more glamorous relations, limestone and sandstone.
Usually they are soft and weak, weathering easily and soon
breaking down under the influence of ice and rain to their
constituent particles of clay.
But mudrocks may be transformed from something
soft, grey and dull into a versatile and attractive material
that has the property of being much harder and easy to split
into thin sheets. It can be used in flooring, walling, cladding
and (literally above all) roofing. This splendid material
is slate. The story of slate starts in the ancient seas that
once covered much of the land around us. Rivers that drained
the land carried with them a sediment load of silt that dropped
out onto the sea floor, building deltas and barrier islands
where the rivers met the sea. Close to the shore the coarser
sediments (composed of the larger grains) formed sandstones,
but the smaller silt and clay particles were carried further
away from the shoreline and settled out more slowly into the
Eventually these fine-grained sediments
built up into thick piles of mud on ancient sea-beds, slowly
compacting in thickness as the water was squeezed out of them
by the increasing weight of the accumulating sediment pile.
The grains in muds are of two main sorts. Some are minute
particles of quartz, much smaller than the quartz sand-grains
in a sandstone. Thin layers (laminations) enriched in quartz
particles sometimes show up as slightly lighter-coloured layers
in a slate and give away the original direction of the natural
sedimentary bedding of a rock.
Most of the particles, however, are made
up of clay minerals - a geological mixed-bag of chemically
similar but distinct silicate minerals that are the end products
of the weathering reactions of the minerals that composed
the rocks of the land surface. Besides their small size (only
a few thousandths of a millimetre across), clay mineral particles
have one other important property. The minute individual crystals
of which they are made are cut by planes of weakness in one
direction, in the direction in the crystal structure along
which they break easily, thus producing smaller grains that
Deposition of muddy sediments on the seafloor
sets the stage for the formation of slate, but they then have
to be altered into something stronger and harder by the effects
of heat and pressure - the process known to geologists as
metamorphism. Although slate represents the results of a fairly
low level and gentle metamorphic alteration, the temperature
and pressure increases involved still require something more
than just burial of an increasingly thick sediment pile on
The lateral forces provided by the sideways
movement of continents and the tectonic plates that support
them across the surface of the globe can provide the necessary
changes. Squeezing and folding by compression of formerly
quiet, muddy sea floors, has often happened in geological
history. Not only is direct squeezing pressure applied, but
the buckled sediments may become deeply buried to where pressures
and temperatures are higher, until some such time as erosion
brings them to the surface again. Often, such zones of squeezing
mark the lines where ancient continents, formerly separated
by an ocean, approached one another and collided.
The slate-belt of north Wales and southern
Scotland, for example, marks the site of a former ocean, the
Lapetus Ocean, that ceased to exist about 450 million years
ago. In north Cornwall, as well as in Brittany and Spain,
the same scenario happened, about 150 million years later.
The increase in temperature and pressure that the muddy sediments
are subjected to causes several changes, all of which combine
to produce a harder rock with a well-developed rock cleavage,
that is, a preferred direction of splitting.
The compression rotates some of the clay
particles so that their flat, flaky faces are perpendicular
to the direction of stress. Other clay particles start to
re-crystallise and grow with a different orientation, again
with their flat faces aligned in the same way. Some new clay
minerals, such as chiorite, and other flaky minerals such
as mica, may also form and grow with similar orientations.
All these flaky minerals growing in an aligned direction also
have their internal planes of crystal weakness minerals growing
in an aligned direction and the effect is that the whole rock
splits easily in a direction of 90 degrees to the original
Of course, this direction need not have
any relation to the direction in which the original bedding
in the mud ran, so faint bedding laminations sometimes cross
the planar surfaces where the rock has split along the cleavage.
Slates may be quite varied in colour because of the different
minerals that grew during the metamorphic alteration. Chlorite
is green, so gives a greeny tint to the grey of the slate.
Some slates contain quite a lot of iron, and are pink or purple.
Locally, gobstopper-sized blobs within purple
slates have undergone alteration of the iron to a green form
that is chemically reduced, giving the famous spotted slates
of north Wales. The green blobs were there before the squeezing
started, because they have also been stretched out and deformed.
Rarely, fossils are found in slates that have been stretched
on the geological rack in the same way - the Delabole Butterfly
(actually a brachiopod that lived on the muddy sea floor)
is an example.
A few slates started as segments other than
simple muds. Some of the Lake District slates formed from
the ash that was spewed out of the volcanoes that lined the
shores of the Lapetus Ocean all those millions of years ago.
The re crystallisation of minerals during the metamorphic
alteration of slate can sometimes be a potential source of
trouble. The original muds varied in the proportion of fine
particles of organic matter that they contained. Organic-rich
muds tended to develop iron sulphides (pyrites) as a waste
product of the activities of bacteria that thrived in them
on the seabed.
Metamorphism can cause the pyrites to grow
as quite large crystals which are hard and shiny when the
slate is freshly split. But beware! Pyrites are notoriously
unstable when exposed to air and rain and will soon rot away
to a rusty stain, leaving a hole. Some pyritic slates will
rot away completely. Some of the slates imported into Britain
in recent years have turned out to be expensive mistakes for
just this reason.