Welcome to the World of Deep Time!
Ever looked at a cliff face and wondered which layer of rock formed first? Or how geologists can tell that a rock in Wales is the same age as one in Canada? That’s exactly what we are going to explore. While radiometric dating gives us the "numbers" (how many millions of years old a rock is), relative dating and biostratigraphy give us the "story order."
Think of it like a stack of newspapers: you might not know the exact date on each one immediately, but you know the ones at the bottom were delivered first. Don't worry if some of these terms seem like a mouthful at first—we'll break them down step-by-step!
Section 1: The Golden Rules of Relative Dating
Geologists use a set of logical principles to figure out the sequence of events in Earth's history. These are the "common sense" rules of geology.
1. The Law of Superposition
In any undisturbed sequence of rocks, the oldest layer is at the bottom and the youngest is at the top.
Analogy: Imagine your laundry basket. The clothes you wore on Monday are at the bottom, and the ones you wore on Friday are on top (unless you've been digging through it!).
2. Original Horizontality
Sedimentary layers (strata) are originally deposited in flat, horizontal layers due to gravity. If you see rocks that are tilted or folded, it means a tectonic event happened after they were laid down.
Analogy: If you pour soup into a bowl, the surface is always flat. If you see the bowl tipped over, you know someone moved it after the soup was poured.
3. Cross-Cutting Relationships
If a geological feature (like an igneous intrusion or a fault) cuts through another rock, the feature doing the cutting must be younger than the rock it cuts through.
Analogy: You cannot cut a cake unless the cake already exists. The "cut" is always younger than the "cake."
4. Included Fragments (Inclusions)
If a rock contains pieces (clasts) of another rock inside it, those fragments must be older than the rock they are stuck in.
Analogy: The chocolate chips in a cookie had to be made before the cookie was baked!
5. Unconformities
An unconformity represents a gap in time. It is a surface where rock layers are missing because of erosion or because no sediment was deposited for a long time. An angular unconformity is particularly famous—this is where older tilted rocks are overlain by younger horizontal rocks.
Quick Review:
• Superposition: Bottom = Oldest.
• Cross-cutting: The "cutter" is younger than the "cuttee."
• Inclusions: The pieces inside are older.
• Unconformities: Missing "pages" in the history book.
Section 2: Way-Up Criteria
Sometimes, tectonic forces are so strong they flip rock layers completely upside down! To figure out which way was originally "up," geologists look for way-up criteria.
Common indicators include:
• Graded Bedding: Heavier, larger grains settle first at the bottom of a layer, with finer grains at the top. If the big grains are at the top, the rock is upside down!
• Ripple Marks: The "peaks" of ripples in sand usually point upwards.
• Desiccation Cracks (Mudcracks): These V-shaped cracks are wide at the top and narrow towards the bottom.
• Fossils: Some fossils, like certain corals or burrows, have a clear "top" and "bottom" based on how the animal lived.
Section 3: Biostratigraphy - Dating with Life
Biostratigraphy is the branch of geology that uses fossils to correlate (match) the ages of rock layers. This is often more reliable than just looking at the rock type.
What is a Zone Fossil?
Not all fossils are helpful for dating. A "good" zone fossil (or index fossil) is one that allows us to pin down a very specific moment in time. To be a superstar zone fossil, an organism needs to meet these criteria:
1. Evolved rapidly: So different species only existed for a short time.
2. Common: Easy to find in the field.
3. Widespread: Found all over the world.
4. Easily identifiable: You don't need a super-computer to tell what it is.
Key Groups Used as Zone Fossils:
• Trilobites: Essential for the Palaeozoic era.
• Graptolites: Small, colonial sea creatures, great for the Ordovician and Silurian.
• Ammonites: The "gold standard" for the Mesozoic (the age of dinosaurs).
• Microfossils: (Like foraminifera) These are vital for the oil industry because they can be recovered from small drill cores.
Did you know?
Ammonites evolved their shell patterns so quickly that geologists can sometimes use them to date a rock layer to within less than a million years! In geological terms, that’s like being able to tell what time it is to the nearest second.
Section 4: Correlation and its Challenges
Correlation is simply matching rock layers from different locations. There are two main ways to do this:
1. Lithostratigraphic Correlation
Matching rocks based on their physical characteristics (rock type, color, thickness).
The Catch: This can be tricky because of diachronous beds. A "diachronous" bed is a single layer of rock (like a sandstone) that was actually formed at different times in different places. Imagine a beach moving inland as sea levels rise—the sand is the same, but the sand in the west was deposited later than the sand in the east.
2. Biostratigraphic Correlation
Matching rocks based on their fossil assemblages. We look for:
• First Appearance Datum (FAD): The earliest time a species appears in the record.
• Last Appearance Datum (LAD): The time the species went extinct.
• Stratigraphic Range: The total time between the FAD and LAD.
Section 5: The Geological Column
By combining relative dating and biostratigraphy, geologists created the Geological Column. You need to know the major divisions of the Phanerozoic Eon (the time of "visible life").
The Three Eras:
1. Palaeozoic: "Ancient life" (Trilobites, early fish, first land plants).
2. Mesozoic: "Middle life" (Dinosaurs, Ammonites).
3. Cenozoic: "Recent life" (Mammals, Birds, Humans).
The Periods (from oldest to youngest):
Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, Quaternary.
Memory Aid (Mnemonic):
Can Ordinary Students Drink Cold Pepsi? They Just Can't Probably Not Quite!
(Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, Quaternary).
Summary: Key Takeaways
• Relative dating tells us if a rock is older or younger than another, but not its age in years.
• Superposition and Cross-cutting are your most important tools for solving geological puzzles.
• Biostratigraphy uses Zone Fossils (like Ammonites) to match rocks across the globe.
• Way-up criteria are essential for figuring out the original orientation of folded or faulted rocks.
• Always be careful with Lithostratigraphy—rock types can be "diachronous" (varying in age over distance).
Top Tip for Exams: When looking at a geological cross-section, always look for the unconformities first! They help you break the history down into "chapters" that are much easier to manage.