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What is Stratum?

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The study of the formation of rocks is classified as geology. It is basically a science that deals with the study of solid matter such as rock or rock strata. And the rock strata conveys the history of the earth and its life, especially what is recorded in it. This can be categorized into stratum geology. The rock strata meaning can be better understood by studying the stratum that is formed from deposits or piles of layers for many years. Stratum is used when there is a single rock consisting of many (several parallel layers) layers. And the term strata is used as a plural noun for stratum to describe a giant pile of the deposited sediments. 

Rock Strata and Stratification

Rock Strata Meaning- The term ‘rock strata’ is often used by geologists when referring to many rock layers in a generic sense that appears over large areas. The singular form stratum, which is derived from a Latin word that means spread out, can be used for a single layer, but individual rock layers or even rock beds are more commonly referred to using this specific name as a stratum. Now that you have understood the rock strata meaning, let us understand the formation and features. Rock strata are formed via stratification. 

Stratification - A bed or layer of sedimentary rock which is formed by the accumulation or deposition of mineral or organic particles at the Earth's surface, and is then followed by cementation of the deposits naturally over time that is visually distinguishable from adjacent beds or layers and this layering of such rocks or sediment is called stratification. Stratigraphy can be considered a sub-discipline of geology that involves the study of rock strata. A sequence of sedimentary layers stacked one atop the other is known as a stratigraphic section and though this is the basic layer of foundation its arrangement and sequence can completely vary according to Steno’s law of stratigraphy. Something that is formed in layers is referred to as a stratiform deposit by geologists. And the term stratification planes are referred to the planes of parting, or separation between individual rock layers. 

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Features of Stratum from Stratification

  • Formed from the igneous rocks on the earth’s surface, sedimentary rocks, from the volcanic lava flows and its fragments deposits.

  • The layers vary greatly in shape and thickness ranging from several millimetres to metres.

  • The strata can be a lenslike thick body that only extends a few metres.

  • The layers can also be very thin sheets that spread up to several kilometres horizontally.

  • The layers are horizontally aligned and a few inclinations are seen on the deposition sites.

  • The texture of the stratum changes and with time, some coarser particles become finer, colour changes are seen due to change in mineral composition as time passes. 

  • The thickness of the rock strata is independent of the time of deposition; an inch or 2.5 centimetres of stratum layer may take longer to form than strata with a 3-metre thickness.

  • The prominence and the details of the strata can vary vastly even within the same strata.

  • Rock strata are only a feature of strata formed by sedimentary rocks while the volcanic rocks formation can differ in a few ways as it is influenced by gravity, sea, liquid lava flow and wind. 

Variants in Formation of Rock Strata

There are many factors that can influence, interrupt and change the course of rock strata formation and all these variants help are of primary importance when studying to interpret the geological events and transformation that occurred on the Earth. They are:

  • Transporting ability of the depositing agent

  • Water

  • Wind flow direction

  • Size and weight of the mineral agents

  • The shape of the deposits

  • Homogeneity of the sediments that are deposited

Stratigraphy Laws

  1. Steno’s laws describe the patterns of rock layers formation of strata. The first law is the superposition law which states that the younger layers or the new deposits sit atop the older layers and this pauses the change of their growth and texture.

  2. The second law is the law of original horizontality that states the original deposition of sedimentary rock layers are flat but orientation may change and even can be found to be tilted when they are heavily influenced by variants.

  3. The Law of cross-cutting is the third law of stratigraphy which states that there is a disruption in the rock layers formed wherein there is no particular pattern and younger ones overlap with the older layers of deposition.

  4. The law of lateral continuity is the fourth law which states that the deposition to form rock layers continues laterally without any opposition till they encounter other solid-body matter and no deposition is possible. 

Uses of Rock Strata 

  • To study the stratification of volcanic rocks, especially the layered ones.

  • Used to study the preserved movements of the earth’s surface through the deformed surface.

  • Through the interpretation of geologic events, one can gain such practical results that can be helpful in tracing the petroleum fields, the location of mineral deposits, and groundwater reservoirs. 

  • The branch of geology that deals with stratification are also called biostratigraphy which uses fossils to study the earth ages.

  • Fossils are a great way to determine the relative ages of the rocks.

  • Fossils interpretation is helpful in correlating the successions of sedimentary rocks within and between depositional basins.


Stratum geology is a great way of understanding the eras gone by and the endurance of the planet earth through various seasonal changes. It is also a great way to predict what is possible ahead if there are repeating patterns of depositing nature. The Grand Canyon is a pandora’s box for studying the rock strata. It is remarkable that the stratification process that preserves so much information about the past earth’s movement still endures and sustains as new movements are also being recorded. And these recordings are extremely helpful to study earth patterns throughout their history since their formation and that can provide interesting details and help find missing pieces in historical studies. 

FAQ (Frequently Asked Questions)

Q.1) Does Every Deposition Lead to Stratification Forming Rock Strata?

Ans.) Every depositing agent does not result in rock strata formation through stratification. And not all sedimentary deposits are stratified because their course is dependent on so many other factors than just the depositing agents. There are many stratification processes that might even be destroyed over time due to recrystallization of limestones and other disturbances and interruptions post the deposition which can be natural or man-made. The deposits transported by landslides and even residual soils exhibit no sign of stratification. There are deposits that are transported by ice that similarly do not stratify to form any kind of rock strata.   

Q.2) What is Stratigraphy Related to Rock Strata?

Ans.) The term rock strata refer to the stacked-up layers that are often placed horizontally of the sedimentary rock. There are many varieties and different kinds of rocks that can also have layering in them, for instance, the volcanic rocks but the word strata are reserved for sedimentary rocks. The sedimentary rocks are composed of individual mineral fragments and even other rocks. Stratigraphy is the science of studying such rock strata and it has two related subfields which are lithostratigraphy or also called lithologic stratigraphy that study the layers and biostratigraphy or the biologic stratigraphy which is used to study fossils and ages.

Q.3) What is the Importance of Stratification in the Earth’s History?

Ans.) The study of rock layers and fossils are a great way of interpreting how the earth has endured, changed and adapted during the transformation and so many weather changes. The study is integral as it indicates the relative age of the rock layers and fossils which can be helpful in also determining the age of the earth and its movements during a particular era in its history. Stratification and its result of rock strata formation is a piece of physical evidence that helps you decipher the history of our planet Earth. The law of original horizontality according to Steno’s Laws states that most sediments were laid down horizontally originally. However, many-layered rocks are no longer horizontal according to recent evidence which can be undeniably interpreted as that Earth has adapted to new changes and a new geologic age has begun that can be studied.

Q.4) What Kinds of Rocks Exhibit Stratification?

Ans.) There are three types of rocks depending on the formation: igneous rocks, metamorphic rocks and sedimentary rocks. The rock strata formation is only supported when sedimentary rocks are formed through the deposition and accumulation of many minerals and agents or sediments over time. There are three types of sedimentary rocks, the one formed via accumulation of mechanical weathering debris and its lithification is called clastic sedimentary rocks, like sandstone, siltstone, breccia, conglomerate, shale. The precipitation of dissolved minerals from solutions forms the chemical sedimentary rocks like the dolomites, flint, limestone, rock salt, iron ore and chert.  When there is an accumulation of plants and animal debris it leads to the formation of organic sedimentary rocks like chalk, coal limestone diatomite.