Geology of the Bay of Fundy

Sarah here –

Map of the Bay of Fundy. The reason why the tides are so high is because the bay gets very narrow, so all of the water going into the bay has to go vertically. Image from Bay of Fundy Tourism.
This post is a continuation of my first post, the geology of Acadia National Park. To recap for those of you who might not have read my first post, I documented all the geology I saw recently on a vacation my husband and I took to Maine and New Brunswick, Canada. This post will be all about the geology of the Bay of Fundy! Specifically, this will be about how glaciers have shaped the geology of the area.

The Bay of Fundy is an incredibly famous geologic area for a good reason-it has the highest tides on Earth, with the highest reaching nearly 56ft! The reason why the tides are so high here has to deal with the shape of the bay-the bay narrows quite a bit (as you can see from the map), so as all the water enters the bay, it’s forced to stack up on top of each other, making the tides reach these incredible heights.

The wave energy at this part of the Bay of Fundy is very high. We can tell because the sediment there is almost entirely very large rocks, as opposed to sand!
Glaciers have shaped a lot of the geology along the Bay of Fundy; as glaciers advance and retreat, they leave telltale signs. One of the best signs are when you see rocks called tillites. These rocks are made of glacial sediment. They’re fairly easy to recognize-often, you’ll see rocks with very large, poorly sorted clasts (meaning, all kinds of different sizes of sediment). These are left behind by glaciers! Here is an example of tillite along the coast of the Bay of Fundy. Look at all of the different sizes of clasts in there! This rock was found at the Irving Nature Center, St. John, New Brunswick. The wave energy in this particular area is very high, which you can tell by the lack of small sand grains and the prevalence of much larger clasts (pebbles-boulders). Another sign of glacial activity is the presence of striations on rocks. Striations are scratches in rocks that are caused by glacial ice moving over them. These glaciers can have lots of rock and sand debris within it, so as they move over rocks, it can cause a lot of surface damage to the rock. Check out this picture of striations on volcanic rocks, also from the Irving Nature Park!

Striations caused from glaciers scraping across a rock surface
Arguably, the most famous area of the Bay of Fundy is the site called Hopewell Rocks. I’ll discuss a lot more about the Hopewell Rocks in my final post, but for now, let’s talk about how glaciers shaped these famous rocks. As glaciers last retreated from these areas (meaning, the Earth warmed and glacial ice melted), the water from the glaciers filled into the ground and caused cracks to form along the coast. This is called chemical weathering. Water is a chemical and it’s the most common chemical that rocks come into contact with when we’re talking about chemical weathering. These cracks eventually caused these large rocks to be separated from the cliff line. This phenomenon might be more familiar to you when you’re driving- when roads (in colder areas, especially) have small cracks in them and water gets into those small cracks, that water can freeze, causing the crack to expand. After multiple rounds of freezing and melting, these cracks become a real problem to drivers!

Glacial melting caused the rocks along parts of the Bay of Fundy to crack (due to chemical weathering) and break off. Here’s a photo of some of these rocks at the famous Hopewell Rock site!
During our next (and final) piece about my trip through the Bay of Fundy, we’ll look at how these famous rocks are shaped by mechanical weathering, instead of chemical!

This rock, called a conglomerate because of the multiple large clasts within it, is indicative of a glacial environment.

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