All the rocks that outcrop within the Leeds district are sedimentary. This means that they were formed from the remains of previously existing rocks that were broken down by weathering, transported by water, ice or wind (or a combination of) either as fragments or solutions, before being deposited as layers of soft sediment. Conversion to hard rock is by a process called lithification. This involves compaction, whereby particles are forced closer together, and cementation, whereby mineral salts that are dissolved in water trapped between grains or passing through the sediment, crystallise in the spaces between grains, binding them together.
Individual layers of sedimentary rocks are known as beds and can vary in thickness from submillimetre to several metres. The boundaries between beds are called bedding planes and these often form observable patterns or distinctive shapes known as sedimentary structures andare an indication of the conditions the original sediment was deposited under. Cross-bedding and ripple marks are sedimentary structures often seen in sandstones in the district.
There are a number of ways that sedimentary rocks can be placed into groups; one method uses the origin of the material they are made of:
Clastic Rocks:composed of fragments (grains) of previously existing rocks and/or minerals.
Chemical Rocks:composed of minerals precipitated from solutions.
Organic Rocks: composed of the remains of once living organisms.
The size, shape and composition of fragments vary a great deal and these characters, along with structures formed at the time of deposition, can be used to determine the conditions under which the sediment was formed.
Size of fragments are an indication of the strength of the current that have transported and deposited them; the bigger the grains, the stronger the current.
Shape of fragments are an indication of the amount of wear the fragment has experienced; the more rounded a grain is, the more wear it has suffered.
Composition of fragments are an indication of the parent rock as well as the amount of wear the sediment experienced. The greater the proportion of resistant minerals in a rock is an indication of greater wear.
Clastic rocks are sub-divided on size of their fragments:
These are rocks composed of rock and or mineral fragments that are between 0.0625 – 2 mm in diameter andso can be very variable. Sandstones that occur in the Leeds district include:
Gritstone(Fig 16) These are coarse-grained sandstones with most of their grains being at the upper end of thesize range, along with ones bigger than 2mm. Grains are angular in shape and consist of different types ofmineral and rock fragments. There are many layers of gritstones in the Millstone Grit Group some of which canbe traced over a wide area. The most distinctive units have been given names that relate to the place wherethey are best seen e.g., Guiseley Grit, or their character e.g. The Rough Rock – a very pebbly grit, or acombination of both e.g, Huddersfield White Rock.
Fig 16 Gritstone
Sandstone (Fig 17) These are made of grains that lie within the size range for the group and can be described as being coarse, medium or fine grained depending on what size the majority of them are. The grains areusually sub-angular in shape and most are made of the mineral quartz.
Fig 17 Sandstone
Flagstones: (Fig 18) Theseare composed of fine to medium sized grains and the rock readilysplits into layers afew centimetres thick. This is because they were originally deposited as thin layers of sediment and it is alongthe bedding planes that the rock breaks.
Fig 18 Flagstone
Desert Sandstone: (Fig 19) These have grains that are usually quite fine but all the same size (well sorted), wellrounded, polished and composed of one mineral; quartz. They were formed in the Permian age deserts havingbeen blown around by the wind and where there was nothing to cushion the impact when they struck othergrains or rocks.
Fig 19 Desert sandstone
These arerocks made of particles between 0.002 – 0.05 mm in diameter which are composed of very finely ground down rock and mineral fragments. Being so small these particles are only deposited in places where current energy is low.
Fig 20 Siltstone
These are made of particles smaller than 0.002 mm in diameter, which are too small to be seen with thenaked eye. The particles are mostly of clay minerals, formed by chemical alteration of previously existingminerals. Being so small the particles are only deposited in water that is very slow moving. Mudrocks are oftendark in colour which is due to high levels of carbon they contain. This comes from organisms that were living in the water at the time of deposition and which didn’t fully decompose on death due to the waterlacking in oxygen. Mudrocks are generally soft and easily weathered & eroded.
The most common types are:
Mudstone(Fig 21) which has little internal structure and breaks into blocks.
Fig 21 Mudstone
Shale(Fig 22) which is very finely layered (laminated), so breaks into thin sheets.
Fig 22 Shale
Clay which is soft and pliable as it is unconsolidated.
Chemical and Organic Rocks
Limestones are composed of more than 50% of carbonate minerals (XCO3). Most are formed in clear, shallow,warm seas. The carbonate is either Calcite (CaCO3) or Dolomite (CaMg(CO3)2 ) or mixtures of both. Calcite canhave a chemical origin, forming as a precipitate when sea water evaporates and tiny crystals of calcite form a lime mud, or an organic origin, forming from the breakup of shells and skeletons of animals or as droppings ofanimals burrowing through lime muds.Dolomitic limestones are composed of a mixture of calcite and dolomiteand are formed by the chemical alteration of previously formed calcitic limestone by high salinity waters.
Fig 23 Dolomitic limestone
Coal is composed of partially decomposed organic matter. When plants growing in swamp environments shed leaves, branches, spores or fall over, they become buried in an environment with little or no oxygen. Here thenormal decay processes performed by animals, fungi and bacteria are halted, or greatly reduced, and the organicmatter isn’t ‘recycled’ but turns into peat. When this is buried by overlying sediments the increase in pressurebegins to drive out water and a process of complex bio-chemical reactions begin to release gases, enriching theamount of carbon in the peat. Over time, deeper burial and its associated increase in temperature and pressure,sees this process continue and the peat harden to become coal.
Fig 24 Coal
These are rocks rich in iron minerals and occur in mudrocks as hard, thin layers or isolated lumps called nodules. They form as a result of complex chemical reactions acting on the products of partial decomposition of organicmaterial, soon after the sediment in which they occur was deposited.