Sedimentary rock formations are those which are formed by layer upon layer of sediment being laid down by natural forces. It is the stone in which history is hidden.

On the subject of human endeavors to classify their surroundings into neat categories, James Murray, the original editor for the grand Oxford English Dictionary, who made a name for himself attempting to do this very thing with words, wrote the following:

How often does man hairsplit, and sever, and part asunder what Heaven has made whole!... Man is fond to classify, to separate, to discriminate, to set apart in the little cells of memory the mass of facts he gathers from the field of nature. But nature has no such isolating method – her facts and laws are a continuous, all-connecting network.

This seemingly irrepressible need for classification is especially true in geology. And in the subject of sedimentary rocks, in particular, one certainly finds more than a little difficulty in performing this seemingly simple task.

What Are Sedimentary Rocks

As its name would suggest, sedimentary rock is formed from layers of sediment. That is, it is built up as sediment (dust, dirt, other, smaller rocks) is spread across an area by water, wind, ice, etc… As time passes (and a lot of it, usually) these sediments begin to feel pressure from the further layers continually being added above, and after enough time undergoes a process known as lithification (lithos being the Greek word for “stone”) occurs.

The most interesting and unique feature about sedimentary rocks are the fact that they tend to be laid down in layers, known as strata. These various layers are quite evident in places where the rock has been split open or eroded, such as in canyons and cliff faces, and a great deal about the Earth’s vast history can be learned by studying the patterns thus created.

The Study of the Sedimentary

By observing the different minerals which form the different strata of a sedimentary formation, scientists can begin to understand a little bit more about what the Earth may have been like during certain periods of time (the dates of which can then be determined somewhat accurately by way of radiometric dating).

In addition, they can learn about the various stages of animal life which has existed upon the Earth by determining exactly which layers hold certain fossils and then discovering the patterns that this might lead to.

Indeed, sedimentary formations are some of the most important geological features on the planet, as far as history and paleontology are concerned.

The Breakdown of Categorization

Unfortunately for the neat little igneous/metamorphic/sedimentary categorization system, however, these rocks by their very nature are made up quite the variety of substances, including other types of rocks.

This means that there must be layers of sedimentary rock which are made up of either igneous or metamorphic rocks, thereby adding a crucial gray area into the classification scheme. Yet, one may overlook this as soon as lithification occurs, making a new rock out of the old.

Variations of Sedimentary

Sedimentary formations can be classified into three main subcategories:

1) Clastic: The most traditional type of sedimentary formation, clastic (which is defined as “having separable parts or removable sections”) rocks are formed from small grains of previously existing rock (of any type) which after vast amounts of time and pressure lithificate into a layered rock. These rocks are then classified even further by the coarseness of the grains from which they are produced using what is known as the Krumbein phi (j) scale, from the finest grain (shale, made up of just tiny particles of dust and debris) through moderate grains (sandstone), and even up through sedimentary layers made up of entire large rocks or boulders, known as conglomerates and breccias.

2) Chemical Precipitate: These sedimentary rocks are formed when liquid chemicals undergo certain reactions which cause them eventually to harden into stone. Most commonly this process occurs with mineral-rich water, which then dissolves, leaving only the minerals. In this manner are formed such classic structures as stalactites and stalagmites in caves, as well as the salt formations around the Great Salt Lake and the Dead sea.

3) Biochemical: The final category of sedimentary rocks is the one which, to some, probably shouldn’t be considered a rock at all. Where clastic rocks may stretch the bounds of the classification scheme by admitting that sedimentary rocks may actually be made up of any of the other types of rocks, thus creating a vast gray area, biochemical sedimentary rocks go even a step further by allowing rocks to be made up of the remains of plants and animals. When sufficient pressure is thus applied to such deposits, lithification can occur and hard minerals can be created. Such important minerals as coal, oil and gas deposits (which are not stone yet, but are at an intermediate stage of development) fall into a gray area in this category. Limestone is another example of a biochemical rock, formed from layers of calcite often scattered along the ocean floor by dead coral, plants and the shells of mollusks.

Sedimentary rocks are truly important in many ways, both practical and from a purely scientific and inquisitorial perspective. The continued study of these rocks will help humans to know more about their history, and more about the naturally processes which have shaped the earth over time.

See Also:

Igneous Rocks

Metamorphic Rocks

References:

“How Sedimentary Rocks are Formed.” Rock Hounds.

“Sedimentary Rocks.” United States Geological Survey.