Geological Exploration of Metro Cleveland (GEM-C)

The Geological Exploration of Metro Cleveland tour is a series of Earthcaches which help to illustrate some geology that is hidden within the urban downtown area of Cleveland. These urban earthcaches help to highlight a wide variety of geological topics that are hidden in plain sight - in fact thousands of people walk by these stones every day without so much as a second thought about their origins!

This tour does not need to be done in any particular order and each earthcache is a standalone lesson. As you wander through the streets of Cleveland take a moment to pause and reflect on the lessons you can learn in just a normal city block!

As with all earthcaches there is no container to find here, rather you must read through the description below and answer a few questions in order to log this cache as found. Please ensure that you send your answers to the CO via email or message in a timely fashion.

The Index Fossils of One Cleveland Center

Rocks and stones are old. The geologic processes that form the world around us create stones and rocks that are millions and billions of years old. When we're dealing with this scale of time, geologists often have to use a variety of tools to determine the age of a stone - one such tool is index fossils. 

Index fossils help determine the relative antiquity of different layers of sedimentary rocks. In order to classify as an index fossil, a fossil must meet the following criteria: 

• 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 species lived, the more useful it is as an index fossil. 
• Abundance in population. The species have to be found in large numbers in rock layers.
• Wide geological distribution. The species must have lived in large parts around the world.
• Uniqueness. The species must be easily distinguishable from other fossils.

Some of the best index fossils around originated as sea creatures - ammonites and trilobites for example. These species fill all four criteria above quite nicely, and moreover because each of these organisms had a wide variety of distinguishable species, it is very easy to narrow down a specific age range for a fossil and thus the surrounding stone.

Figure 1: Some Common Index Fossils and their relative ages

The earliest ammonites appeared in the fossil record during the Devonian era, and the last species died out during the Cretaceous–Paleogene extinction event. The earliest ammonites lived 415 million years ago. They moved by jet propulsion and had shells comprised of multiple linked chambers. Their tentacles extended outward from the head chamber to catch and devour their prey. As the ammonite grew, new chambers would grow behind the head. Fossilized ammonites often display what are called "sutures," intricate patterned details on the outer surface of the shell that mark where the chambers met the outer wall of the shell. Not all ammonites are round. Some are straight, but still display distinct chambers along the shell.

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Above Left: GZ and the area needed for answering question 1.

Above Right: An example of an ammonite fossil taken by the CO in Verona, Italy.

Logging Questions

1. Underneath the purple square look for a fossilized ammonite. Estimate its size (in inches) and provide a brief description of it.
2. Are there any other types of fossils in the sample of rock? Use figure (1) above to characterize them.
3. Based on the reading and the fossils you see, what is the maximum potential age of this rock? What is the minimum potential age?
4. Take a photo of yourself or a personal item at GZ and include it in your log! Please don't include any of the fossils in your photo!

References:

• https://geologyscience.com/geology-branches/paleontology/index-fossils/
• https://dam.assets.ohio.gov/image/upload/ohiodnr.gov/documents/geology/GB5_Hannibal_1992.pdf