Blowing Rocks Nature Preserve

Sarah here-

Image 1: You can see where the tide usually reaches the highest point by how the rocks are much narrower. One of my students is taking notes about the weathering patterns she sees on the rocks.

I recently took my geology students on a field trip to Blowing Rocks Nature Preserve on the eastern coast of Florida near Jupiter Island. This class is my upper level Sedimentary Petrology class made up of mostly geology majors (we mostly study the formation and identification of different types of sedimentary rocks, like sandstone and limestone). I wanted to show you all what we saw!

Image 2: Here is a small sea arch (back of the image) created by wave energy constantly wearing parts of the limestone away. This image was taken at about 10-15 feet away from the rocks in the background. In the forefront, you can see a sea stack-this used to be a sea arch that over time has been worn down to the point that that other half of the rock has eroded completely. In the future, that sea stack will likely collapse from the constant weathering.

The rock that is shown here is the Anastasia Limestone, which was deposited in the late Pleistocene, which spanned about 2.5 million to 12,000 years ago. The ocean levels were much higher than they are currently, when this rock was made. We know this because the limestone that comprises the Anastasia was made underwater. Now, this limestone is exposed all along the eastern shore of Florida.

This limestone is really cool because once it was exposed, it began weathering in unique patterns. First, the energy of the waves is breaking the rocks down bit by bit. This is something we call mechanical or physical weathering. You can see evidence of this mechanical weathering by looking at how the rocks get narrower closer to the bottom-the waves usually only reach that point at high tide, so the rock above it isn’t nearly as affected (image 1). This mechanical weathering can make a few different types of features: sea arches (image 2) and sea stacks (image 2) are the kinds of things we can see here.

Image 3: Here we can see the dissolution pits from water sitting on top of the limestone. Limestone is easily eroded by chemical weathering, so in the future, these pits will continue to get much larger.

The cool geology doesn’t stop here though! Chemical weathering (i.e., breaking down the rock using chemicals-the most common one is water) also affects the rocks strongly here. Limestone is easily eroded away in the presence of acid, so any acidity in the ocean water or from rain above can wear away the rock in interesting patterns. Water splashes up on top of these rocks from regular wave action-that water slowly erodes the rock away, leaving small pits in the rock (image 3). However, what makes this place famous are the large pipes that are created from a mix of the chemical and mechanical weathering processes here. These pipes are quite literally large cylindrical tubes that have been worn out of the rock through hundreds of thousands of years (image 4). Water, when it comes in from waves, rushes up through these tubes and explodes out of the top! Sometimes, these can spray as high as 50 feet-hence the name of the park, Blowing Rocks (video 1)! As we go forward into the future, these pipes will continue to grow larger because they are continuously being worn down by wave energy.

Image 4: Here are some of the pipes created by the intense combination of chemical and mechanical weathering. At high tide, water explodes through these pipes and onto the surface!
Image 5: Here are some trace fossils showing ancient burrows of creatures that lived in this area! Some have interpreted them as mangrove tree roots, but this area was likely too high energy for mangroves to live.

There were some cool fossils on this trip, too! If you look closely, you can see lots of trace fossils from creatures who made burrows into the rock (image 5) and you can also see a lot of clam and snail fossils (mollusks!) Many of these fossils are broken up and the edges have been rounded-this is because of the higher energy waves constantly breaking them down (image 6). My students and I also found a living Portuguese man o’ war (image 7)- this isn’t a jellyfish because it isn’t a single organism, but it’s a closely related colonial organism. The man o’ war has long tentacles that can give humans very painful (but rarely fatal) stings. If you see one on the beach, don’t touch it! They are fairly common on the eastern coasts of south Florida, so be warned! All in all, my students had a great time on this trip, and they learned a lot about how rocks can change due to weathering over time. I hope you enjoyed it, too!

Image 6: Fossil bivalves (clams) are all throughout this area. Most of them are species that lived in fairly shallow and higher energy areas, which match the geologic interpretation of this area. The high energy of this area means that the shells are broken up and the edges have been rounded through constant mechanical wave action wearing down the edges!
Image 7: A Portuguese man o’ war that washed up on the beach! Definitely stay away from these critters-their stings can be pretty painful!

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