It's surprising to me how much earthcaches can teach you. Despite being an urban earthcache, the lesson you will learn here are about index fossils: archaeological tools that geologists use to determine relative antiqueness. 

Earthcache lesson: Index Fossils

Index Fossils

Simply put, index fossils are used by geologists to define and identify geological periods. However, not all fossils are index fossils. The listed below are certain characteristics of index fossils:

1. They must be short-lived. The species used as index fossils must not cover a large geological period, as this defeats the purpose of determining the ages of rocks. The shorter the the species lived, it's usefulness as an index fossil increases
2. Abundance in population. The species have to be found in large numbers in rock layers.
3. Wide geological distribution. The species must have lived in large parts around the world. An index fossil without this trait is called a zone fossil.
4. Uniqueness. The species must be easily distinguishable from the fossils.

Some of the best known index fossils are ammonites and trilobites. Both species are extremely easy to recognise.

Figure 1: Ammonite fossils

Figure 2: A trilobite fossil

Using Index Fossils

As mention before, geologists can immediately tell which rocks are younger (and by how much) based on the index fossils within them. Let's take a scenario in this case of a layered cake.

Imagine the bottom layer of a cake is has dried out compared to the top layer of the cake. We can deduce that the bottom layer was baked maybe 2-3 hours before the top layer was added.

This is essentially how relative antiqueness is determined by the index fossils.

What paleonthologists do is basically look at the layers, and see that Ammonitesare on top of Trilobites, therefore, based on those principles, they conclude that Trilobites must be older.

Now you have the relative antiqueness of those fossils. Trilobites are older than Ammonites.

What do we need that for? Exceptional cases where sedimentary layers end up torn, or at incredible distances where the sedimentary layers themseves don't appear to be exactly the same.

Figure 3: An exposed side of a mountain with distinct stratigraphic layers

Suppose a rock like figure 3 is discovered, and there's a Trilobite in the center and an Ammonite in the outmost layer, you know based on your previous inferences which layer is younger than the rest.

And going back to index fossils having a wide geographical area, if you find a rock with exactly the same fossil content as another in a distant country, you know that they belong to the same time period.

Logging Tasks

At the posted coordinates you will see an exceptionally large and detailed example of an index fossil. It is not hard to miss if you're on the constant lookout for it, but please let me know if you have problems searching for it. Don't look suspicious!

Send the answers to the following via my profile and log once you have sent them. I will check in with you if the answers are questionable.

1. What is the index fossil you have found at the posted coordinates?
2. Give an estimate of the diameter of the specimen.
3. Looking at just the building and nothing else but this listing, which characteristics of index fossils does the specimen possess?
4. In this context, how can you use the fossil you have found as an index fossil? Is it still possible?