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Strata and Land Eras Help

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Introduction to Strata and Land Eras

Land eras are the broad ranges of time that geologists use to group different information. For example, if a geologist wants to talk about the time of the dinosaurs, the Jurassic period might be mentioned. Remember, Jurassic Park , the “science-gone-terribly-wrong” movie where scientists use genetic material preserved in petrified tree sap to produce prehistoric dinosaurs? It couldn’t be named Cambrian Park because the much earlier Cambrian period was home to mostly microorganisms, ancient horseshoe crab-like invertebrates ( trilobites ), and other shelled inhabitants. They aren’t nearly as exciting to watch on the big movie screen.

We will learn more about the specific types of sediments laid down by rivers, glaciers, ocean movement, decay of microorganisms, but first, let’s step back and look at the bigger picture. Why are geologists interested in studying sedimentary layers in the first place? What kind of geological history can be discovered through studying different rock forms?

Well, geologists look at strata (rock layers) like pieces in a history puzzle. The make up, depth, type, angle, and compression of sedimentary rock give geologists an idea of the “how and when,” of rock deposition.

Strata is the layering of the Earth’s sedimentary rock layers into beds, either singly or layers upon layers over geological time.

When visiting deep cuts into the Earth’s sedimentary rock layer, like the Grand Canyon, millions of years of sedimentary layering can be seen. Geologists try to reconstruct the Earth’s developmental history and formation by studying these rock layers. The theory of plate tectonics came into full acceptance after decades of careful measurements and study of the formation and movement of the Earth’s strata by geologists. The study of the Earth’s strata, known as stratigraphy , also allows the sequencing of formation events.

Stratigraphy includes the formation, composition, sequence, and relationship of sedimentary rocks in strata.

Law Of Original Horizontality

Most sediments were deposited beneath the seas and oceans of the world. Because of gravity and the more or less horizontal ocean floor, sediments are originally laid down in flat, horizontal layers. This nearly universal process has become known as the Law of Original Horizontality . These layers may get pushed, folded, erupted on, and other things, but they started out flat.

Principle Of Stratigraphic Superposition

Have you ever been in an ancient city and seen evidence of many civilizations who built their idea of the latest architecture on top of older structures?

When the basements of towering, new office buildings are dug, often times an area’s history is revealed as well. Even cross-sections of older city streets sometimes expose older and earlier layers of brick, cobblestones, and dirt beneath recent concrete paving.

When this happens with layers of sediment and sedimentary rock, it is called the Principle of Stratigraphic Superposition . This principle says that in any strata, which has not been folded or overturned, the oldest sediments will be found at the bottom of the sample with the most recent sediments layered on top.

The Principle of Stratigraphic Superposition says that the deeper you go into the Earth, the older the sedimentary rock.

This law applies to sedimentary, igneous, and metamorphic rock layers, as well, that haven’t been mangled by any other outside forces after their first layering.

The principle was first described by William Smith, a civil engineer who did a lot of surveying work for canal construction in western England. During the construction of the canals, he noticed that there were different layers of sedimentary rock in a predictable order of layering. In 1816, Smith published Strata Identified by Organized Fossils in which he listed 17 strata with specific plants and organisms unique to each for periods between Jurassic and Tertiary. The next year, he added 10 more strata downward until he reached bedrock granite.

Smith got so used to seeing the different rock layers that after a while, he could name the layer, the region it came from, and its position in the rock sequence. He was a regular rock detective! During Smith’s study of sedimentary rock layers, he also found there were certain fossils that seemed to be connected to specific layers. This fact helped him identify the layers and their most common order of deposition. Later, this fossil and sediment relationship became known as the Law of Faunal Succession .

The Law of Faunal Succession explains how fossil faunas and floras follow one another in a definite, identifiable order.

Geologists use the Law of Original Horizontality, the Principle of Stratigraphic Superposition, and Law of Faunal Succession to figure out the age, scattering, and order of different layers of sedimentary strata.

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