Last week we visited the New York State Museum in Albany. But before going in we were attracted by the building stone that surrounds the whole structure. That sort of thing that happens to geologists all the time. Building stone is mostly there to look good but we always stop and look not at it, but into it. There’s usually a good story – if-you know how to read it. Take a good look at our photo.
That slab of stone used to be sediment; most of it had been sand; now all of it is sandstone. But look at that surface. It’s covered with structures; not surprisingly, these are called sedimentary structures. Last week we learned that there are two types, physical and biological. Did plants or, more likely, animals produce them? Then they are biological. But if inanimate processes were responsible then they are physical. That takes us back to the museum walls. Take another look at our photo. These ones are called ripple marks.
It’s so easy to just walk past such things without taking notice, but not for us. We geologists always look at such things and drift off into the past. But this time it’s not the distant geological past, but the historical past. There must have been, centuries ago, an early naturalist who gazed at such features on the surface of bedrock and wondered about them. What were they? How had they formed? What did that person think? This must have been, back then, a nearly insoluble problem. But it is only human to wonder.
“Insoluble” until that person looked into a nearby body of water and saw what the flowing water could do. You see moving water is quite capable of picking up sand and shaping it into ripples, just the sort that you see here. There are two types of ripples. The first are the current ripples. That’s the product of currents flowing along the bottoms of oceans, lakes or rivers. Current ripples are asymmetric; the downstream slope is steeper than the upstream side. The other ones are the wave ripples. These are symmetrical; both slopes are equally steep.
We took a good look and decided that we were looking at symmetrical wave ripples. Our minds and our imaginations drifted off into the very distant past and we saw the shoreline of an ancient and sizable river. Waves were washing ashore and sculpting those ripples. We heard the unmistakable “lap-lap” sounds of that activity. And we looked out and saw the sand freshly sculpted into those ripples. They came to be buried by more sand and then a lot more - miles of it!
Centuries, millennia, and then hundreds of millions of years passed by and these ripples slowly hardened. What had been just a single moment in time petrified and became sandstone. They eventually came to be exposed in a quarry and then an architect spotted them and used them as building stones for the State Museum. You can now go to the museum and look at them yourselves. We don’t think that you will ever again walk past such features without noticing them – and more importantly - understanding them.
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