Before I finish this series of columns on geologic curiosities and wonders, I have one more geologic curiosity to talk about – your aggregate operation. And even though I probably have not visited your specific pit or quarry, believe me when I say: “There’s something curious going on there.”
There were obvious geologic curiosities in some places I visited, like the gnarly mastodon teeth found in a gravel pit near Denver, Colorado, the cute kindchen found in a quarry near Saint Joseph, Missouri, or the beautiful crystals from a stone quarry at Mont Saint-Hilaire, Québec, Canada.
But most aggregate operations I have visited are not blessed with such rare curiosities. You might be thinking: “Yah, that’s my boring place.”
Boring? Humbug! Take, for example, a sand and gravel operation near Cañon City, Colorado. Generally the top size of the gravel was about 15 to 20cm. But there were piles of VW Bug-sized boulders all over the place. Some of those boulders were sold for landscaping applications. Where did those bizarre boulders come from, and how did they get there?
The answer may lie in the fact the operation was in a sand and gravel deposit of glacial origin.
How about the beautiful banded rocks in the Helidon Sandstone quarry near Toowoomba, Queensland, Australia? Does the superheated water and gas from ancient volcanic activity have anything to do with why they make such beautiful stone slabs?
While studying a gravel pit in Phoenix, Arizona, near my home, I saw curious black splotchy areas (manganese dioxide) where all the cobbles were weak and rotten. Hmmm … those were areas to avoid when making hi-spec aggregate. How curious!
I visited a number of limestone quarries near Saint Joseph, Missouri. Just below the root zone near the land surface was a tan silty soil (the loess that contained the kindchen). Immediately below that was a strange, thin zone of soil containing pinkish-purple gravel and cobbles. Those stones were collected and sold as premium landscape rock.
But where did these curious stones come from? Hint, hint: a glacier that extended from Canada into Missouri once covered the area. Along the way the glacier passed over an area of pinkish-purple quartzite rock near Sioux Falls, South Dakota.
Let’s get to a really boring pit – like many I have seen, and maybe even like yours. Nothing special, just monotonous layers of sand and gravel occasionally interrupted with lenses of clay. The clay causes processing problems, so nobody likes it. Seriously, nobody likes the clay!
Well, geologists love the clay. There are a number of reasons why it might be there. One possibility is that the sand and gravel was laid down by an ancient river or stream. Occasionally the river would overflow its banks and the clay would settle out in quiet backwater areas. Pollen would also settle out in the same area, and would be preserved in the clay.
The science of interpreting pollen is called palynology, from the Greek, meaning ‘the study of scattered dust’.
Palynologists can look at the pollen in the clay and tell you what the climate was like at the time, and maybe even how long ago the clay was deposited. Now, how cool is that?
So never fear, the next time you go to your pit or quarry, just look around. I guarantee there’s something curious going on there!