The Missing 500 Million Years.

The trip to Clayton is always exciting because of the extraordinary types of rocks we cross over from Maine to New York. It is also a trip to deep time, not deep space time but deep earth time. I always like to have a geological puzzle to work on so this year I decided to find the missing 500 million years. To put that time interval in perspective, 500 million years is roughly the amount of time between the present and the first appearance of abundant fossils in the Cambrian period. My puzzle is 500 million years further back in time.

There is a little neck of ancient craton (crust) that lies between the Adirondack Mountains and the central portions of Ontario called the Frontenac Arch. This arch has brought up old basement rocks close to the surface and outcroppings of these ancient rocks in the Thousand Islands are common. In fact this arch is one of the reasons why there are so many islands. The older rocks are much harder than the younger rocks and resisted erosion. Further downstream on the St. Lawrence River and off of the arch there are far fewer islands because the softer types of rocks were washed away.

The arch pushed up rocks referred to as Grenville basement. These rocks formed from repeated episodes of volcanism, sedimentation, and metamorphism that formed an ancient continent called Rodinia about 1100 million years ago. Over the course of hundreds of millions of years the rocks were transformed to gneiss (dark striped rock) and granite (red granite of much use for building the famous castles on the islands) and many other related igneous and metamorphic rocks. We know that these types of rocks require tremendous heat (>600ยบC) and pressure (equivalent to being buried 50 to 60 kilometers deep). Likely a mountain range not unlike the Himalayas existed here in Rodinia. After Rodinia formed, the younger rocks would be very different looking in being much less deformed and metamorphosed.

During this period in the late proterozoic, there was no life on the surface. Mountains were exposed to unrelenting erosion by the elements. The primitive atmosphere probably had less oxygen than today so the style of chemical weathering was different too. Under these conditions rocks would be broken down completely to leave only the most durable minerals. Ignoring some rare minerals, quartz is the hardest chemically resistant mineral and is often the only one left after extreme weathering. My task was to find the location where rock composed mostly of quartz lies on top of the banded gneiss or red granite (geologically an unconformity, or in this specific instance a nonconformity).

 

Quartzite along the shore of Grindstone Island.

Since the island is covered with soil, trees, and bogs I decided that looking for the missing 500 million years would be easier by water. An early morning kayak while the sun was low made it easy to see features along the shore. Starting right next to the cottage I spotted a gray to white colored rock that was composed of nothing but quartz (quartzite). I could see fine lines in the rock that traced graceful curved shapes that likely were dunes once upon a time.

 

Original bedding preserved in the quartzite.

I followed the shore and the blocky gray rock to the northeast, towards the upper landing until I found something that looked like the Grenville basement rock. I found them on a tiny island- red granite with rafts of black minerals and more massive gneiss. These rocks have been folded and heated to the point of melting. So, somewhere between this tiny island and the big island was the missing time.

 

The little billion-year old island.

Grenville basement rocks (migmatite).

 

Gneiss and granite.

I searched along the shoreline and along the gray quartzite I spotted a darker rock just along the waterline.

 

Looking at the 500 million-year gap.

Paddling in closer I could see rounded cobbles of the red granite and other Grenville rocks. This conglomerate rock must have formed rather quickly because the cobbles are large, not very weathered, and poorly stuck together.

 

Old rock cobbles making up the new rock.

This unconformity is hard to find because it is easily eroded by the river. Over the cobbles lies a thicker section of very well-weathered quartz in a fine-grained sandstone or quartzite. This rock is so flat-lying and unchanged it could have been formed just recently. It is almost impossible to imagine all that has happened between when the red granite and gneiss were formed and the whole mass eroded down to sandstone (or now quartzite).

Comments

  1. polly says:

    Cool story!

  2. Mary says:

    Reading this post made me think about how unimaginably old most of the rocks are that we see every day–kind of a “stop and smell the roses” reminder.

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