The Work Of the Weather & Stratification and Jointing

     At the sandstone ledges, we saw that the rock was divided into parallel layers. The thicker layers are known as strata, and the thin leaves into which each stratum may sometimes be split are termed laminae. To a greater or less degree, these layers differ from each other in fineness of grain, showing that the material has been sorted. The planes which divide them are called bedding planes.

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Besides the bedding planes, there are other division planes, which cut across the strata from top to bottom. These are found in all rocks and are known as joints. Two sets of joints, running at about right angles to each other, together with the bedding planes, divide the sandstone into quadrangular blocks.

huddersfieldgeology.org.uk

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Sandstone: Examining a piece of sandstone we find it composed of grains quite like those of river sand or of sea beaches. Most of the grains are of a clear glassy mineral called quartz. These quartz grains are very hard and will scratch the steel of a knife blade. They are not affected by acid, and their broken surfaces are irregular like those of broken glass.

    The grain of sandstone is held together by some cement. This may be calcareous, consisting of soluble carbonate of lime. In brown sandstones the cement is commonly ferruginous, hydrated iron oxide, or iron rust, forming the bond, somewhat as in the case of iron nails which have rusted together. The strongest and most lasting cement is siliceous, and sand rocks whose grains are closely cemented by silica, the chemical substance of which quartz is made, are known as quartzites.

We are now prepared to understand how sandstone is affected by the action of the weather. On ledges where the rock is exposed to view, its surface is more or less discoloured and the grains are loose and may be rubbed off with the finger. On gentle slopes, the rock is covered with a soil composed of sand, which evidently is crumbled sandstones, and dark carbonaceous matter derives from the decay of vegetation. Clearly, it is by the dissolving of the cement that the rock thus breaks down to loose sand. A piece of sandstone with calcareous cement, or a bit of old mortar, which is really an artificial stone also made of sand cemented by lime, may be treated in a test tube with hydrıchloric acid to illustrate the process.

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The element of geology - Section of Limestone Quarry

mn: the pitted surface of rotted limestone

bb: limestone divided into thin layers

c: thick layers of laminated limestone, the laminae being firmly cemented together

jjj: Joint

A limestone quarry. Here also we find the rock stratified and jointed. On the quarry face, the rock is distinctly seen to be altered for some distance from its upper surface. Below the altered zone the rock is sound and is quarried for building, but the altered upper layers are too soft and broken to be used for this purpose. If the limestone is laminated, the laminae here have split apart, although below they hold fast together. Near the surface, the stone has become rotten and crumbles at the touch, while on the top it has completely broken down to a thin layer of limestone meal, on which rests a fine reddish clay.

    Limestone is made of minute grains of carbonate of lime all firmly held together by a calcareous cement. A piece of the stone placed in a test tube with hydrochloric acid dissolves with brisk effervescence, leaving the insoluble impırities, which were disseminated through it, at the bottom of the tube as a little clay.

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We can now understand the changes in the ıpper layers of the quarry. At the surface of the rock, the limestone has completely dissolved, leaving the insoluble residıe as a layer of reddish clay. Immediately below the clay, the rock has disintegrated into a meal where the cement between the limestone grains has been removed, while beneath this the laminae are split apart where the cement has been dissolved only along the planes of lamination where the stone is more porous. As these changes in the rock are greatest at the surface and diminish downward, we infer that they have been caused by agents working downward from the surface.

     At a certain point, these agencies have been more effective than elsewhere. The upper rock surface is pitted. Joints are widened as they approach the surface, and along these seams, we may find that the rock is altered even down to the quarry floor.

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Sources: The Element of Geology-Wıllıam Harman Norton

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