Welcome to my EarthCache! An EarthCache is a special type of geocache where visitors learn about a specific geological feature by observing it on site and answering questions, rather than finding a physical container.
At the posted coordinates, you will find a large dark grey rock with a feature that is pretty hard to miss. Embedded in the surface is a huge, almost circular pattern made up of rings and lines radiating outward from the centre. When I first saw it, I immediately thought it looked like the cross-section of a tree trunk. But appearances can be deceiving!
There are several things that can create surprisingly similar patterns in rock. For this EarthCache, we're going to look at three possibilities: rugose coral, stromatoporoid and petrified wood. Your job is to examine the evidence, use what you learn below and decide for yourself what you think we're looking at.
Logging Requirements
Please send me the answers to the following questions through my geocaching profile. You do not need to wait for a response before logging your find. I will contact you if there are any problems with your answers.
- Look at the entire circular feature and estimate its diameter at its widest point. You can use your hand, shoe or another object for scale. Is the overall shape mostly circular, oval or irregular?
- Look closely at the pattern inside it. You should be able to see both concentric bands and lines that appear to radiate outward. Which of these two patterns is more dominant? Describe what you see.
- Choose three or four of the most obvious concentric bands and follow them around the feature. Are they evenly spaced and approximately the same thickness, or do they widen, narrow, merge or become difficult to follow in places?
- Now concentrate on the lines radiating outward from the centre. Do they look like distinct individual spokes, or more like a fine texture crossing through the larger bands? Do they remain equally obvious from the centre to the outside?
- Now it's time to make the call! Based on your observations and what you learned below, what do you think the large circular feature is: a rugose coral, a stromatoporoid, petrified wood or something else? Give me at least two visible pieces of evidence from the rock that support your answer.
- Mandatory: Post a photograph of yourself or a personal item (such as your GPS, geocaching name, trackable, or another unique signature item) with the large circular feature visible in the background. Your face is not required, but the photograph must be unique to your visit.
Access Notes
This EarthCache examines one of the large landscaping rocks at the posted coordinates. Everything you need to answer the questions can be seen from the exposed surface of the rock.
There is no need to move, climb on, scratch, break or otherwise disturb the rock. Please leave it exactly as you found it.
So... What Is This Thing?
Sometimes geology gives us something that looks incredibly obvious until we start looking a little closer.
The feature in this rock has a roughly circular shape, several obvious rings and lines that seem to spread outward from the centre. If you saw the same pattern in a freshly cut tree stump, you probably wouldn't think twice about it.
But this isn't a tree stump. It's a pattern preserved in rock, and several completely different organisms can leave behind structures that look surprisingly similar when we're only seeing a cross-section through them.
That's an important part of identifying fossils. We can't always rely on the overall shape. Instead, we need to look at the internal structure, how the different lines and layers are arranged and what else is preserved in the surrounding rock.
For this feature, there are three possibilities worth considering.
Possibility #1: Rugose Coral
Rugose corals were marine animals that lived hundreds of millions of years ago. They are often called horn corals because many individual rugose corals grew in a shape resembling a horn, although some species formed colonies.
The important feature for us is what was happening inside the coral. A rugose coral contained thin internal walls called septa. These extended inward from the outer wall toward the centre of the coral. Imagine cutting across a horn coral and looking down at the cut end. Those septa can create a very noticeable pattern of lines radiating between the centre and the outside, a little like the spokes of a bicycle wheel.
Rugose corals also grew throughout their lives, so you may see curved or concentric growth features as well. However, when identifying one in cross-section, the presence of strong, organized radial septa is an important clue.
If this is a rugose coral, we should expect the radial pattern to be an important part of its internal structure.
Possibility #2: Stromatoporoid
Now we get to something that can look very different from a rugose coral while it is growing, but surprisingly similar once we cut through it.
Stromatoporoids were marine organisms that produced hard, calcium-carbonate skeletons. They were extremely important reef builders in ancient seas and are commonly associated with other marine fossils. Instead of building their skeleton around large radial septa, stromatoporoids generally grew by adding layer after layer of skeletal material. Over time, this could create large rounded, domed or irregular masses.
Cut through one of those masses and those layers may appear as a series of curved or roughly concentric bands. Depending on the direction of the cut and how well the fossil has been preserved, smaller structures may also cross between those layers.
This means a stromatoporoid cross-section can have both horizontal or concentric structures and finer lines running across them.
For this possibility, pay particular attention to whether the large feature seems to be built primarily from repeated layers, rather than from clearly defined spokes radiating from a central point.
Possibility #3: Petrified Wood
Petrified wood begins as actual wood. After the tree dies and is buried, mineral-rich groundwater can move through it. Over time, minerals fill spaces within the wood or replace some of its original material while preserving details of the tree's structure. When petrified wood is exposed in cross-section, it can preserve features that look remarkably similar to a modern tree stump. The most obvious are growth rings.
Depending on how well the wood has been preserved, you might also see wood grain, structures extending outward from the centre, differences between individual growth rings or even an outer edge resembling bark.
This creates an interesting problem for us. Both petrified wood and some marine fossils can produce a pattern of nested rings. So how can we tell them apart?
The Surrounding Rock Matters
The internal structure of the feature provides important clues to its identity, but it should not be considered in isolation. Geologists also examine the rock surrounding a fossil, because its composition and other fossils preserved within it can provide evidence about the environment in which the rock formed.
Look closely at the dark grey rock surrounding the large circular feature. Numerous lighter-coloured fragments and shapes are visible throughout it. Some appear to be pieces of shells and other fossil material. If these are remains of marine organisms, they indicate that the sediments making up this rock were deposited in an ancient marine environment.
This environmental context is particularly useful when considering the three possibilities discussed above. Rugose corals and stromatoporoids were marine organisms and are commonly found alongside other marine fossils. Petrified wood, on the other hand, originated as terrestrial plant material. Although wood can be transported into marine environments and eventually fossilized there, evidence of an ancient marine community would make a marine organism a more likely interpretation.
The surrounding rock cannot identify the feature on its own, but it provides another piece of evidence that can be combined with its internal structure.
Putting the Evidence Together
Fossil identification is rarely based on a single characteristic. Similar patterns can develop in very different organisms, particularly when we are looking at a two-dimensional cross-section through something that was originally three-dimensional.
For this feature, consider several characteristics together. Strong, regularly arranged radial septa would support an identification as rugose coral. A structure dominated by successive concentric or curved skeletal layers, particularly without well-defined septa, would be more consistent with a stromatoporoid. Growth rings accompanied by recognizable wood grain, rays or other preserved plant structures would provide stronger evidence for petrified wood.
Finally, compare those structural observations with the surrounding rock and the other fossils or fragments preserved within it. The best interpretation is the one that accounts for both the internal structure of the feature and its geological context, rather than simply relying on its overall circular appearance.
AI Content Disclosure - Some of the descriptive text and graphics were created with assistance from AI tools. All geological information has been reviewed and verified by the cache owner for accuracy.
This cache was placed by a PROUD Platinum EarthCache Master.
