An Idyll on Grindstone Island Geology

Here it is the second day of spring 2015 and the wind chill is below zero, again. After experiencing the all-time coldest month in February and with March on track for being the coldest March on record, I decided to cast back in time to last summer and then a mere billion years to the proterozoic period. Last August we spent a pleasant week on Grindstone Island in the St. Lawrence River. The camp was located in a deep cove with splendid steep sided rock outcroppings. The intent was to go and unwind for a bit. It was a great spot to sit.

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Kayak.

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Swim.

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Eventually there came a time when I could not resist the allure of the outcrops near the camp. I knew that the geology in the area was underlain by rocks of great age and complex history.

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The precambrian rocks have been exposed thanks to the presence of the Frontenac Arch that pushed the basement upwards to form a nearly continuous geological linkage between the Adirondack massif to the south with the broader Grenville terrane to the north. Immediately behind the camp was a bluff of what appeared to be pink granite, similar to what can be seen in the road cuts along the stretch of I-81 on Wellesley Island. I was satisfied with that observation for a few days.

The terrain along this end of Grindstone Island has a series of parallel ridges that extend out into the river like stubby fingers. On the outside edges of the island to north and south lie the Cambrian sandstones that formed from the erosion of some ancient land mass. The remnants of that land is in part the pink granite. These landforms made me curious, what was the cause of the ridges and valleys? Was it because the rocks were folded or were they composed of softer and harder sections? I decided to explore around the area and headed up onto the ridge behind the camp.

The top of the ridge was covered with thin soils, pine trees (pitch pine maybe), and grassy glades. The rock outcroppings were very obvious as long linear features, maybe 1 to 2 meters wide. To my surprise the rocks were not pink granite, but quartzite. This is not the same rock as the Cambrian sandstones. Although these both are mostly made of quartz, the quartzites have been baked into a dense and very resistant rock. Also, the quartzites exhibited no bedding features like the more massive sandstones.

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So the geology already is a bit more complicated than just a massive amount of granite. I looked further and found a spot where the quartzite was cut by the pink granite. Using this simple observation it fair to say that the younger granite cuts across the older quartzite. The granite is the up and down pinkish feature to the right and left of the pocket knife.

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I decided to go and take a closer look at the granite along the bluffs. Now I discovered that the granite wasn’t quite the uniform and massive rock that it first appeared. Here we can see layers of light and dark colored minerals, as well as light-colored granite.

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And closer up we can see that this layered rock is a gneiss. The layers are well defined and swirl around in rather consistent patterns.

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And just like with the quartzite, the gneiss is also cut by the granite. In some places the granite is light colored and probably is different from the pink granite.

Quite a bit has happened to these rocks over time. Their history probably started over 1 billion (1,000 million) years ago. Even older rocks had to exist in order to be eroded into sand to form the quartzite and mud to form the gneiss. These sediments then had to be buried and transformed into the metamorphic rocks that were later intruded by the granites. Studies of the ages of these granites indicate that the igneous rocks formed more than 1100 million years ago, so the previous cycle of rock building and erosion must be much older.

There is much more to be learned about the geological history of Grindstone Island but it will have to wait for another visit.

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