When we talk about planet Earth, that big planet brimming with secrets, we usually envision the familiar image of the internal form of Earth consisting of four huge main layers: the crust, the mantle, the inner core, and the outer core. However, we rarely think about the main layer, which is the Earth’s crust, and what it contains of multiple layers formed throughout millions of years, enclosing millions of fossils of creatures that once roamed the Earth in Ancient Times.
All that aside, we have all the peculiar and magical forms of the deposited layers that have formed Earth’s crust; this is what specialists name “stratigraphy”. It is the science field concerned with studying the layers of sedimentary rocks and what they contain from deposits and fossils, indicating how these layers were created and formed. First, we need to know some of the important terminologies to understand the subject better.
“Stratification” is the classification of rock layers on top of each other, and it is a characteristic that distinguishes sedimentary rocks from other types of rocks; the unit of stratification is known as “stratum”. It is the smallest lithostratigraphic unit; it is known to be a homogeneous rocky thickness differentiated from what is above and what is below it.
The reasons why we can distinguish between strata in nature is known as “the basis of stratification”; if you examine two adjacent strata, you will definitely find out what makes them two strata and not one stratum. We can identify the reasons based on the types of rock that form the strata and their colors, the differences in the size and form of rock sediments, the variation in the degree of cohesion of sediments, as well as the differences of the cohesive material keeping the sediments together to form the strata.
Just as stratification has its basis, strata could be deposited in one of two manners: “flat-lying”, which is the original nature, or “inclined”, which could be original or structural. As for originally inclined strata, they are the result of rock sedimentation on a non-horizontal and rough surfaces—such as the sedimentation on the sides of sand dunes, beaches, or sea bottoms full of bumps.
The formed stratum is at the same angle as the surface’s slope on which it is deposited, and that is the feature it gained during its formation. As for structurally inclined strata, they are formed due to structural movements after the formation and stiffening of the strata; when the sediments settle down above the crooked surfaces in great thicknesses, the original inclination starts to gradually fade away, eventually making the strata horizontal.
As for the forms of stratification, there is “cross-stratification”, which is the common type in nature; in this form, the stratas are horizontal and there is nothing to characterize them—such as repetition, gradation, or intersection. Then there is “imbricate stratification”, which occurs when grainy sediments deposit at a certain angle on a sloping surface, as we find in estuaries; the inclination of the angle is about 20 degrees. There is also “graded stratification”, which occurs when the speed of water currents decreases, causing the sedimentation of rock fragments carried by these currents. The sediments will be filtered so that the coarse-textured sediments (gravel, for example) is deposited first, followed upward by pebbles, granules, sand, silt, and clay.
Uniformitarianism, also known as the “Doctrine of Uniformity” is one of the fundamentals of geology; it states that “the present is the key of the past”, meaning that the different geological processes that are at work in the present time to form the surface of the Earth are the same processes that formed the surface of the Earth in ancient geological times.
To learn the geological history of a certain region, we have to describe the highlighted rocks in that region; in order to describe the stratification of layers, we should cut it into pieces using a scientific method. A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant features (facies) that characterize it. Sequences of sedimentary and volcanic rocks are subdivided on the basis of their lithology; going from smaller to larger in scale, the main units recognized are Bed, Member, Formation, Group, and Supergroup.
Planet Earth is filled with all that is amazing, bewildering, and curious; scientists are continuously attempting to unravel its mysteries and riddles, and the greater picture is gradually becoming clearer. This was but a tiny journey into the world of stratification and folds; I can hardly wait to take you on another trip to uncover more secrets soon.
References
Charles W. Finkl (1984), Applied Geology.
R. K. Bopche (2017), Objective Applied Geology.