Welcome to my Earthcache! An Earthcache is a special type of geocache where there is no container to find - instead you are looking for a unique geological feature of the area and need to answer questions, as well as posting a picture, in order to claim the find. This EarthCache takes you to the heart of Southern Ontario’s sedimentary history, where you’ll examine marl layers within limestone rocks that have been placed at GZ. The rocks at GZ were harvested just a few kilometers from here, and show a great example of marl layers within sedimentary limestone rock. 

EARTHCACHE REQUIREMENTS

As with all of my ECs, I am not looking for PhD thesis level responses, but I am hoping that you take some time to enjoy the area and learn something new. Please include a list of all cachers with your answer, if answering for more than one caching name. There is no need to send individual answers. 

To claim a 'find' for this Earthcache you must answer the following questions and send your answers in a message or email to the owner using the link at the top of the page. You can log your find with a photo at GZ. Send your answers to the tasks. I will be in contact if there is a problem, no need to wait for a response as long as the required photo is included in your log.

Observational Task

At GZ, you will be at the entrance to a small park off of Burke Dr in Caledonia ON. This earthcache is wheelchair accessible and all observations can be made from the paved sidewalk within the park.

Questions to Answer:

1. Describe the different layers you see. How thick are they? Do they look uniform or are there softer/weaker layers between harder ones?

2. What is the color and texture of one of the softer bands (hint, it's the top band of the rock at GZ)? Try rubbing it—does it feel powdery or gritty?

3. Using a weak acid, such as a vinegar packet, place a drop on the top layer (band) of the rock. Describe the reaction. Was it weak (marl) or strong (limestone)?

4. Based on your answers above, do you think you are observing marl? Justify your reasoning.

5.Mandatory: Include a photograph of yourself, your GPS, a signature item, thumbs up, etc at GZ. This can include the rocks, the park sign, or any other interesting feature you find at GZ. You do not need to show your face in the photo, but your photo must be unique to you. In the event of group photos, please have each member of your grpoup upload the photo. 

Geology Lesson:

This EarthCache lets you touch and see evidence of a tropical sea that existed hundreds of millions of years ago. Marl layers are more than just soft rock—they’re a time stamp of environmental change, showing moments when rivers brought more clay into the sea or water chemistry shifted.

The area around Caledonia sits within the Silurian and Devonian-aged bedrock of the Niagara Escarpment system, revealing a fascinating story of an ancient tropical sea. In this region, you're looking at Silurian dolostones and limestones, part of the Lockport and Guelph formations. These units frequently contain marly seams, particularly where ancient lagoons and reef systems once existed. The presence of marl suggests periods of reduced water circulation, which allowed more clay to settle out along with carbonate mud.

What Are Marls?

Marl is a sedimentary rock made from a mixture of clay and calcium carbonate (the same mineral found in shells and limestone). It forms in calm marine environments, where both fine clay particles (from land) and carbonate material (from marine organisms) settle out together. Marl lies between limestone and shale in composition:
More clay = marly shale
More carbonate = calcareous marl

In Southern Ontario, marls are often found interbedded with limestone, forming visible bands or softer layers between harder limestone beds. These beds can indicate changes in sea level or sediment influx during the Paleozoic Era (~400 million years ago).

If you examine the rocks at GZ closerly, you’ll likely see marl: As thin, softer bands between harder limestone beds that appear to be a completely different material than the surrounding limestone. These marls like like modelling clay that has been left out too long and is now rock solid - it has a much finer particle than typical limestone, and usually is a gray, tan, or greenish color. They typically appear to have a slightly dull finish, and look like they have been carved with a chisel rather than a flat cut, the way the limestone banding does. The marl bands crumble easily under pressure.

With a duller surface, compared to the crystalline appearance of pure limestone these bands, it should be fairly easy to tell the marl bands apart from the limestone. (hint: there are 2 distinct bands in the rock directly at GZ)

How to Tell Marl from Limestone

• If it's high in clay (more than ~35–45%), it’s marl.

• If it's almost pure calcium carbonate (with little to no clay), it’s limestone.

Limestone:

• Usually more solid, dense, and fine-grained.
• Can be white, gray, tan, or even bluish.
• Often shows fossil fragments or reacts strongly with acid.

Marl:

• Softer and often feels crumbly or earthy.
• Looks like a mix between clay and limestone.
• Can be pale gray, greenish, yellowish, or brownish.
• May appear more “dull” compared to the often cleaner look of limestone.
• does not react to acid, or has a very dull reaction

This cache was placed by a PROUD Platinum Earthcache Master.