Like so many, we are happy to hear of the reconstruction of the great Notre Dame Cathedral. Its near obliteration five years ago was a bad moment for all of western civilization. We certainly didn’t think that there was a Mountain Eagle column in that news. After all, we are the Catskill Geologists, not the Cathedral Geologists. But then we ran across a map view of the site. Take a look at our illustration, courtesy of Wikimedia Commons. Notre Dame lies on the westernmost of two stretched out islands that lie within the Seine River, right in the middle of Paris. Actually, island is not exactly the best word to use here. Technically, this is something that geologists call a longitudinal sand bar. In fact, both of these islands are sand bars.
We ask you, what sounds more permanent, sand bar or island? Yes, of course, we began to see that there was something here that we should learn more about. A longitudinal sand bar is an unusually large accumulation of sediment deposited in the middle of a stream. That happens when the river is carrying too much sediment. It “parks” that sediment temporarily in the form of a sizable bar. These are commonplace features in large rivers all over the world. We have a good one at Rogers Island. below the Rip Van Winkle Bridge where it spans the Hudson. The next time you are crossing the river look down and see it.
The word temporary is critical here. How long is that? What happens at the end of “temporary” and what brings temporary to an end? Will Notre Dame be destroyed by raging floodwaters and carried off, in bits, down the Seine? This is good, even great geoscience and, once we were on to it, we were eager to pursue the issue. We know Rogers Island quite well; we cross it frequently. The island is heavily forested. That means it has been there long enough for those trees to grow. The Notre Dame sandbar has been there for more than 800 years. No, these sandbars may be temporary, but they are long lasting. We don’t expect the flooding of Notre Dame any time soon so let’s assume that longitudinal bars do survive very long periods of time. Then what is it that does end those long lengths of time?
We, as scientists, have to do some hypothesizing here. Our first guess involves climate change. We are guessing that the longitudinal bars go back to a time when rainfall rates had been high but were slowing down. With less water in it, the Seine was also slowing down and losing its ability to carry sediment. It had to park its sands and silts right there, in the middle a future Paris. Another hypothesis is that sea levels had, a long, long time ago, been lower than they are now. There was a lot more downhill for the Seine and its steepness forced it to flow faster and carry more sediment. When the sea level rose, perhaps at the end of the Ice Age, then the river’s flow slowed down too, and it lost its ability to transport sediment. On both cases it will require a major climate change or a substantial sea level fall for the sandbar and its Cathedral to be destroyed.
If we were doing serious professional peer reviewed science we would go on and try to develop more hypotheses, but for today we can stop here. Those longitudinal bars are very old; they won’t last forever but they will last a very long time – by human standards. Fire continues as a far more immediate threat to Notre Dame.
Contact the authors at randjtitus@prodigy.net.” Join their facebook page “The Catskill Geologist.” Read their blogs at “thecatskillgeologist.com.”
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