Relative Dating?
First and foremost–
Don’t date your relatives!  In many places illegal, and generally just a bad idea!

OK, seriously,
Geologist refer to “relative dating” when we’re deciding which rocks (or layers) are older, and which rocks (or layers) are younger.
We don’t know how old or young, and we don’t really know how much time has elapsed between rocks or layer deposition.
As we’ll learn shortly, there are a number of basic rules for evaluating relative dating.
But before we get to these rules,
it’s worth defining “absolute dating” as the business of actually quantifying how old (or young) a rock actually is, in terms of years.
For example, when we say that a rock is 200 million years old, we’ve used an absolute date.
Finally, absolute dating is NOT exact dating.  Every absolute date (like every scientific measurement) has an associated error.  It’s not uncommon to say that a rock is 200my +/- 5my, meaning that the rock is very likely somewhere between 195 and 205 my old (my= million years).(In most cases, but not all– absolute dating relies on what we call isotopic or radiometric dating techniques.  This is the use of radioactive decay as a clock.)

RELATIVE DATING RULES

These rules were derived long ago.  Generally ascribed to Nicolaus Steno, who lived in the mid-1600’s and was one of the first to recognize fossils as relics of past life (as opposed to something that had “grown” in the rock itself!).

 Steno wrote a short book (a prodromus) with a rather odd name–  
      “De solido intra solidum naturaliter contento dissertationis prodromus” 
Even if you translate the Latin, it’s still a bit strange!
“Dissertation Concerning a Solid Body Enclosed by Process of Nature Within a Solid”
What he wrote about though makes sense in the context of the mysteries of nature, in his day.
The book was about,
HOW DO YOU GET A PEBBLE INSIDE OF A ROCK (part of what we now call a conglomerate)?
HOW DO YOU GET LAYERS BETWEEN LAYERS (horizontality and superposition, see below)?
and, HOW DO YOU GET A SHELL OR BONE INSIDE OF A ROCK (what we call a fossil!)?

THE RULES

Original Horizontality

The idea here is pretty simple.  As a consequence of gravity, most sediments are deposited in horizontal layers.

Superposition

Again, not too complex!  Those sediments that were deposited first are at the lower levels of a sequence, and hence the oldest.  Those deposited last are at the upper levels and are the youngest.  Note:  These rules generally apply to sediments deposited out of water, but they could include air-deposition (volcanic ash fall) or lava flows.

But of course, once deposited and turned to rock (something we’ll learn about later, with sedimentary rocks, and called “lithification), THEN those layers may be tilted or otherwise deformed.
Check out the climbers in this picture–

The Yellow Door 5.13a/b Seal Rock Phil Gruber Lynn Hill Sport Climbing Rock Boulder Colorado Flatirons,, Climbing Magazine, Sept 2018

Here, the layers have been tilted (uplifted, like shingles on a roof), but not overturned.  Therefore, the rock layers to the right of the climbers are older than those to their left!

Lateral Continuity

Lastly, regarding layering of sediments– geologists often refer to an un-deformed sequence that has been merely eroded (not deformed or uplifted) as having lateral continuity.  This just means that we can recognize the same layers across a region that has been eroded into valleys or hills.

Principle of Inclusion

Another fundamental relative dating tool– included fragments, within an existing rock, are older than that rock.
This is similar to the idea that fossils (included fragments within a rock) were in existence before the formation of the rock.
Alternatively, a conglomerate contains pebbles that had to have come from somewhere (hence older pieces of rock).

from John Merck,
https://www.geol.umd.edu/~jmerck/geol100/images.jpg

Cross Cutting

Imagine sticking a big fat spoon or fork or knife INTO a cake.
In order for that entity to be in your cake, it had to come after the cake was made.
Things that CROSS-CUT existing layers are considered to have come after the layers were made.
Classic cross-cuts would include either a dike (igneous intrusion, or “squirt” of magma through rock layers) or a fault (a breaking surface within a group of rocks).

from John Merck,
https://www.geol.umd.edu/~jmerck/geol100/images.jpg

Unconformities

Finally, we come to the subject of MISSING TIME.  Layers or sequences of rocks are rarely a record of all that has past!
Unconformities refer to missing time.
Missing time occurs due to erosion (or associated “non-deposition,” i.e. no sediments being lain down).
In Figure “a” below, an igneous or metamorphic rock, formed, was uplifted and eroded, and then had sedimentary layers deposited atop.
This is a NONCONFORMITY.

In figure “b” below, a series of sedimentary rock has been tilted and then eroded (the dark line is the erosion surface) and then had sedimentary layers deposited atop.
This is an ANGULAR UNCONFORMITY.

In figure “c” below, a series of sedimentary rock has two layers that have a lengthy separation in time, marked by the lower layer having undergone a lengthy period of erosion (indicated by the wavy dark line).  Then sedimentary layers were deposited atop.
This is a DISCONFORMITY.

In figure “d” below, we see a disconformity that is not well represented by an wavy erosion surface.
This is a PARACONFORMITY.

NOTE– both disconformities and paraconformities are quite difficult to discern in the field, where techniques/tools to derive ages of rocks above and below are unavailable.

Diagrams from Steven Earle, Physical Geology

INDEX FOSSILS

Index fossils are those that were relatively wide-ranging in a geographic sense, relatively easy to identify, and represent an “existence period” that is relatively short-lived.

In the previous section (titled–  Age of the Earth, Early Efforts) we learned that the early geologist Will Smith used the concept of Faunal Succession to recognize rock layers that were the same age, or of different ages.  This is really all about index fossils, in that these fossils seem to be relegated to a particular period of time past!

Not all fossils are index fossils.
In the diagram below, we see animals and plants that had enormous time spans– these “groups” would make terrible index fossils.
But within each group (fish or mammals or ferns or grasses) there will be SPECIES that represent shorter periods of existence and would therefore make good index fossils.

Once “dated” using isotopic techniques, index fossils can be a tip-off as to the age of sedimentary rocks.

Examples of index fossils from the USGS website,, see link,

https://pubs.usgs.gov/gip/geotime/fossils.html