The planet we live on: The beginnings of the Earth Sciences

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1.3 Sedimentary rocks - formed by a range of surfaceprocesses in a variety of environmentsYou can ‘read’ sedimentary rock exposures by understandingthat these rocks <strong>we</strong>re once loose sediments thathave been cemented or compressed into sedimentary rocks.<strong>The</strong>y therefore contain the same clues that sediments do,to the ways in which they <strong>we</strong>re originally moved and laiddown. <strong>The</strong>se clues include the size, shape and chemicalmakeup of the grains, the layers these grains form, likethe cross bedding in Figure 1.16, and the fossils and overallsequences that the rocks contain. Since most sedimentaryrocks have layers, called beds (see Figure 1.17), youcan usually use this to distinguish them from igneous andmetamorphic rocks. <strong>The</strong> good ‘rock detective’ can readsedimentary rock clues of composition (chemical makeup),texture (grain size, shape and arrangement) and structure(including structures like bedding and cross bedding)to discover the environment in which the sediments<strong>we</strong>re first deposited.Sediments are generally eroded from higher land and depositedin lo<strong>we</strong>r areas. Such lo<strong>we</strong>r areas include puddles,ponds, lakes, gutters, streams, rivers, valleys, general lowlandareas, coastlines and shallow and deep seas. <strong>The</strong>selo<strong>we</strong>r areas span a range of latitudes, from polar to temperateto arid and tropical. Deposits of boulders, gravel,Figure 1.16: A sedimentaryrock face showing some of theclues into how the sediments<strong>we</strong>re originally laid down.sand or mud can be laid down at all latitudes but calcium carbonate deposits (limestones)and evaporites are formed mainly in the tropics.Most sediments are formed from the <strong>we</strong>athering and erosion of rocks, which could be ofsedimentary, igneous or metamorphic type. Deposits near these source rocks have not beenmoved very far and so the make up of the sediment is similar to the mineral compositionof the source rocks. If the source rocks contained quartz, feldspar and mica, so will thesediment. Similarly, there has been little chance for the sediment to become sorted intodifferent grain sizes or for the grains to be worn down. Thus, sediments near source rocksusually have mixed compositions and mixed sediment sizes, whilst the individual sedimentfragments have sharp corners and are angular, as in Figure 1.18, which shows a breccia(a sedimentary rock of mixed sizes with angular grains).Gravity moves these sediments downhill and they become picked up by water and movedalong streams and into rivers. Minerals like mica, and later, feldspar, become brokendown. <strong>The</strong> corners of individual grains are worn away and they become rounded andreduced in size. <strong>The</strong> abrasion of the grains to smaller sizes and rounded shapes is calledattrition. As rivers and later, coastal currents, carry the grains along, they are sortedinto different sizes, since fast flows can carry large grains whilst small grains only settle11
Figure 1.17: Sedimentary rocks showingtheir layers or beds. <strong>The</strong> surface of eachbed is called a bedding plane.Figure 1.18: A sedimentary rock made ofangular fragments, a breccia.in quiet conditions. So, <strong>we</strong> find boulders, pebble deposits, sand deposits and deposits ofmud in different parts of rivers and coastal areas, depending upon the speeds of currentflow. <strong>The</strong> proportion of quartz in sediments increases as they are carried along, sincemica and feldspar are broken down physically and chemically. <strong>The</strong> chemical breakdownof mica and feldspar produces clay minerals so there is a high proportion of these veryfine-grained minerals in quiet areas of deposition.So, <strong>we</strong> can use the sizes, shapes and compositions of the grains to tell us about how thegrains <strong>we</strong>re transported and deposited. Poorly-sorted angular sediments of mixed compositionsare found near source areas, whilst <strong>we</strong>ll-rounded gravel deposits, <strong>we</strong>ll-sorted,quartz-rich sands, and muds rich in clay minerals, are found far from their sources. <strong>The</strong>gravels can become cemented into conglomerates (Figure 1.20), the sands into sandstones(Figure 1.21) and the muds can later be compressed into mudstones (Figure1.19) and shales (shales are <strong>we</strong>ak mudstones that tend to fall apart in your hand).Deserts are mostly very dry but sometimes have torrential storms, so <strong>we</strong> can find a widerange of deposits. <strong>The</strong>re are accumulations of angular fragments that could becomebreccias, whilst dried rivers can have pebble and sand deposits that could become conglomeratesand sandstones (Figure 1.22). Dried up lake beds can have muds and saltdeposits (as in Figure 1.23) that could be preserved as mudstones with evaporite layers.<strong>The</strong>se deposits often contain more clues as <strong>we</strong>ll.Most of the sands have layers that will become sandstone bedding. When sands aredeposited in fast flowing rivers, they are usually laid down in small underwater dunesas sloping layers that are seen as the cross beds in many sandstones. Since the crossbeds always slope downstream, they tell you the direction of the current flow that laiddown the sand (as in Figure 1.24). As water currents slow down, the surface of the sandoften forms into current ripple marks that have a shallow up-stream slope and slope moresteeply downstream. <strong>The</strong>se asymmetrical ripple marks or current ripple markstell you the direction of the current that deposited them and these can be preserved in12
Page 1 and 2: Basic Books in ScienceBook 6<strong
Page 3 and 4: BASIC BOOKS IN SCIENCE- a Series of
Page 5 and 6: BASIC BOOKS IN SCIENCE- a Series of
Page 7 and 8: Looking ahead - If you came across
Page 9 and 10: 3.2 Plate tectonics (20th Century)
Page 11 and 12: 1.30 Coal seams in an opencast coal
Page 13 and 14: 3.4 Alfred Wegener, the polar explo
Page 15 and 16: 5.14 A GPS remote volcano monitorin
Page 17 and 18: Figure 1.2:minerals.A sandstone roc
Page 19 and 20: Even diamond can have different col
Page 21 and 22: Figure 1.8: A red garnet crystal in
Page 23 and 24: Figure 1.12: Crystals of minerals i
Page 25: Hematite, Fe 2 O 3 - earthy red, me
Page 29 and 30: Figure 1.20: A close up view of a p
Page 31 and 32: Figure 1.26: Ancient wave ripple ma
Page 33 and 34: Figure 1.30: Coal seams in an openc
Page 35 and 36: Figure 1.32: Close up view of a pie
Page 37 and 38: Figure 1.35: A fossil colonial cora
Page 39 and 40: Figure 1.38: Fossil ammonites, indi
Page 41 and 42: Figure 1.39: ‘Massive’ igneous
Page 43 and 44: Figure 1.41: A close up view of a p
Page 45 and 46: Figure 1.44: A basalt flow that coo
Page 47 and 48: Figure 1.47: Deposit of volcanic as
Page 49 and 50: Figure 1.49: An old slate quarry, s
Page 51 and 52: Figure 1.51: Slate, a low-grade met
Page 53 and 54: Figure 1.53: Schist, a medium-grade
Page 55 and 56: Figure 1.56: Metaquartzite, produce
Page 57 and 58: Figure 1.58: A region of folded roc
Page 59 and 60: Figure 1.61: A normal fault. This s
Page 61 and 62: Figure 1.64: An unconformity. Older
Page 63 and 64: proper geological maps. Fossils hav
Page 65 and 66: Cephalopods are not extinct, but th
Page 67 and 68: Figure 1.68: A shelled cephalopod f
Page 69 and 70: You can try the rock sequencing pri
Page 71 and 72: Chapter 2Reading landscapes: how la
Page 73 and 74: Figure 2.3: Rock fragments loosened
Page 75 and 76: Figure 2.6: The jo
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Figure 2.10: The m
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are forming them. Similarly the sha
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Figure 2.19: This photo was taken f
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Figure 2.25: Dinosaur tracks conser
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• On coastal sections, whenever p
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found evidence that the Earth was a
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Figure 3.3: James Hutton, the ‘Fo
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Figure 3.5: A page of Wegener’s 1
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Figure 3.7: The st
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Figure 3.10: The r
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Figure 3.13: The m
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Figure 3.15: The S
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Figure 3.19: An ocean versus contin
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Figure 3.23: Map of the major tecto
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Figure 3.26: Global temperature cha
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Figure 3.29: A computer generated p
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Figure 3.31: The
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planet extended th
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Figure 4.1: William Smith’s geolo
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Millionsof yearsago (Ma)01000Some M
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ocks whilst the first definite plan
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Figure 4.9: The ch
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Figure 4.12: The 4
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Millionsof years Some Major Earth E
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Figure 5.2: Strike-slip movement (r
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Figure 5.4: The So
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Figure 5.6: An ash eruption rising
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Figure 5.10: A hazard zone map of t
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isk to humans. The
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Figure 5.14: A GPS (global satellit
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None of these methods has yet prove
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Figure 5.17: A windfarm in Ireland.
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Figure 5.19: A beautifully preserve
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Another ‘missing link’ find has
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Figure 5.22: A dinosaur reconstruct
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Figure 5.25: A working aggregate-pr
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Figure 5.26: The E
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Chapter 6Understanding what geologi
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Figure 6.2: A drilling rig used for
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When an oil/gas field has been foun
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Figure 6.6: Groundwater flowing out
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Dam disaster in Italy, when the wav
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Figure 6.10: A slab foundation, bui
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An example of this is investigation
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GlossaryAbsolute age The</s
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Carbon capture The
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Crustal shortening This results of
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Evaporite deposits (or evaporites)
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Geophysical survey Using the method
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“Integrated waste management” <
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Metamorphism The r
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Pore spaces (or pores) Gaps bet<str
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Saltation Sediment movement by flui
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Suspension Sediment movement by flu
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AcknowledgementsPermission to repri
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Figure 2.3 A scree slope. Photo ID:
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Figure 1.15a Hematite.Figure 1.15b
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Figure 1.28 Dune cross bedding in s
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Figure 3.18 An island arc volcano,
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Figure 5.21 Excavations at the dino
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