Time Scavengers

Sarah here-

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!

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.

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.

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!