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Local Geology - Weathering |
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The two rock types most commonly found in this district are
sandstones and
shales.
These rock types weather down quite differently, both by chemical and physical
processes, during exposure to water and extremes of temperature.
Quartz grains, in sandstone, are made of silica which does not break down chemically. Sandstones are resistant to chemical weathering when held together with a quartz cement, but wherever the cement is either clay or iron rich or missing, the sand grains become loose and will be washed out.
Flaggy sandstones with numerous bedding planes will decompose much more readily than a massive quartz sandstone which is resistant enough to produce exposed cliffs and benches. Shales contain clay particles in finely laminated beds and break down easily when exposed. Organic acids produced from plants and rain water react chemically with shale to speed up the weathering process. Most shale outcrops are hidden by layers of debris and vegetation. Freeze-thaw weathering occurs when temperatures fluctuate around zero degrees Centigrade. Water present within pore spaces, along with bedding planes and in joints, expands on freezing, causing blocks to fall from cliff faces.
Sandstones can have a variety of colours. The brown colouration of weathered rock is caused by the oxidation or rusting of iron minerals cementing the particles together. Black surface colouration on the rock results from soot being deposited - a legacy of the burning of coal in domestic hearths, mills and factories before the Clean Air Acts of the 1950s. |
| Because the atmosphere now has less soot and dust the buildings are slowly cleaned by acid rain-water, although the rain-wash is more effective on smooth ashlar sandstone than it is on rougher wall stone. Fine ashlar buildings in Huddersfield, such as the Town Hall and the Railway Station have responded well to being commercially cleaned, with the sharpness of carving and the pale colour of Crosland Moor sandstone ( Rough Rock ), well displayed enhancing the townscape. |
Graveyards are fascinating places for geologists to study. Many headstones
are made of the best local sandstones. These are easily worked, with
inscriptions often legible after 150 years of weathering. During the 19th
century the use of limestone or marble headstones was unfortunate because
acid rainwater reacted unfavourably with
calcite, etching the polished
surface of the stone. The use of lead letters inset into marble headstones
is an improvement but as the stone weathers, the letters are loosened and
may fall out. There is a long history of the use of exotic stones from other
parts of Britain and overseas. Most of these are
igneous rocks, such as
granites and
gabbros,
which take a high polish. They are hard and therefore more difficult to letter,
and are less prone to most forms of weathering as their crystalline
texture has few weaknesses. Their highly polished surfaces do not allow
the accumulation of soot or lichens as readily as other rock types. Nowadays
many churches place restrictions on the use of these highly polished headstones,
preferring local materials.For more information about the Rocks and Landscapes of Huddersfield see our other pages on Local Geology and our guide to the Geology of Huddersfield. Publications |
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Variable types of cement result in differential
Angular blocks form scree slopes which are found below many gritstone edges.
Freeze-thaw conditions continue to break down sandstone blocks into quartz
grains or fragments. This is why some moorland footpaths appear to be covered
with a white sharp sand bleached by acids from the moorland peat.