Tom Eblen: Down in CentrePointe's pit, a fascinating story told by limestone

Herald-Leader ColumnistJune 7, 2014 

When most of us think about the CentrePointe block's history, we focus on its role as a center of Lexington commerce for two centuries.

But over the past 12 weeks, as more than 60,000 cubic yards of dirt and rock have been blasted, dug and hauled off the block to make way for CentrePointe's huge underground parking garage, some people have expressed a deeper curiosity.

So I called Frank Ettensohn, a University of Kentucky geology professor, and asked him to take developer Dudley Webb and me on a sedimental journey. We walked around the bottom of CentrePit as the geologist described a much older history.

"Each of these layers is like a page in a book," said Ettensohn, a specialist in the sedimentary rock layers of the Inner Bluegrass region known as the Lexington Limestone. "If you know how to read the pages you can see all sorts of things going on here."

After digging out about 10 feet of dirt and clay, Hunt Construction's excavation crews hit solid rock, which they dislodged with nearly 50 blasts, said project manager Tim Linde. By mid-June, they will finish removing all of that material to a depth of nearly 40 feet. As many as 475 dump trucks a day took dirt to R.J. Corman's railroad yard, while truckloads of rock went to C&R Asphalt for recycling.

With a rock hammer in hand, Ettensohn walked us around the bottom perimeter of the pit and explained how the layers of limestone above us were formed during what geologists call the Middle Ordovician period. That was about 450 million years ago, give or take a few million years.

Central Kentucky was then part of a continent called Laurentia, which now forms the core of North America. The East and West Coasts weren't there yet, and neither was much of the Southeast. Florida was still part of Africa.

"This area was a very shallow sea, maybe 60 feet deep, much like what we see in the Bahamas today," Ettensohn said. It was a sub-tropical region, because Central Kentucky was about 20 degrees south of the Equator, instead of 38 degrees north of it, as it is today.

"These plates move all over, and they're moving now as we stand here," he said. "But it's a very, very slow process."

The Lexington Limestone is between 200 feet and 320 feet thick. It is made up of 11 different types, each named for a place where there is a notable example.

Ettensohn said CentrePointe has two types. Excavation exposed the top of a Grier layer, which may extend another 200 feet below the ground. It is named for the Grier's Creek area of Woodford County. Above the Grier is a thin Brannon layer, named for the area around Brannon Road near the Jessamine-Fayette county line.

The layers are different, he said, because a mountain-building event on the east side of the continent sent sea water and sediment rushing this way, eventually forming the Brannon.

Along the pit's wall below Limestone Street, Ettensohn pointed out a brown stripe of bentonite — a thin layer of volcanic ash from a prehistoric eruption. He also saw evidence of ancient earthquakes. "We know there were at least three major earthquake events that gave rise to the deformation in the Brannon," he said.

Fine-grained limestone indicates eras of deep water, he said, while coarse-grained limestone was formed in shallow water. Ettensohn pointed to areas of coarse rock that would have been giant dunes migrating with water flows along the sea bottom.

Hurricane-like storms helped form many of the limestone layers, he said. Thin layers of muddy shale between them are evidence of calmer periods of geological history.

Limestone is made mostly of calcium carbonate, the remains of small creatures that lived at the bottom of these shallow seas. "They died and their shells were reworked by storms," he said.

The most common creatures whose fossil fragments are still visible in the limestone were crinoids and bryozoans, which looked more like small, twiggy bushes than animals, and brachiopods, which resemble clams.

Ettensohn picked up a small rock and pointed to tiny sparkling specks, the pulverized remains of ancient star fish and sea urchins. Then he found a fossil fragment of a trilobite, a long-extinct animal that looked something like a crab.

"They're not particularly good," he said of the fossils, "because they've been shattered to heck and back."

But these billions upon billions of crushed sea creatures left a sturdy foundation for Lexington, whose existence will be no more than a blip in geological history.

Tom Eblen: (859) 231-1415. Email: teblen@herald-leader.com. Twitter: @tomeblen. Blog: tomeblen.bloginky.com

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