Recognizing rocks

One of the skills that geology students learn in the field is how to recognize different rock formations.  Technically, "formations" are simply mappable units of rock.  In other words, a rock layer that has distinctive properties which allow it to be recognized and correlated over a region.  Formations are assigned formal names generally based on their location and lithology (rock type).

Some examples of formations include the Kaibab Limestone, Coconino Sandstone, and Bright Angel Shale in the Grand Canyon or the Manlius Limestone, Binnewater Sandstone, and Mount Marion Shale right here in Ulster County.  While most people just see them as gray rocks on the side of the road, there are over a dozen different limestone formations here in the Mid-Hudson Valley and locally-trained geologists (if they have any field experience) can recognize them on sight because they each have distinctive bedding, grain size, color, fossils, etc.

Contact (at quarter) between two different limestone formations

A few days ago I was with my family near the top of a well-known waterfall in the Catskills - Kaaterskill Falls.  A favorite subject of Hudson River School artists like Thomas Cole, it's protected State land today and a beautiful place to visit.

 

Kaaterskill Falls (1826) by Thomas Cole

We sat on the top of the falls for a bit and then wandered upstream (it's called Spruce Creek and is a tributary of Kaaterskill Creek further downstream).

 

My wife, posing in the middle of Spruce Creek above Kaaterskill Falls

As my wife and kid jumped and balanced their ways across the rocky stream, I noticed something interesting. A beautifully-sculpted, fist-sized cobble.

Isn't it pretty?  There's a crack running completely around the middle but it's well-cemented enough so that it doesn't come apart.  The neat thing about being a geologist is that I can appreciate this rock for more than only its natural beauty.  I immediately recognized it as a piece of the Manlius Limestone Formation.

Here's a close-up of the Manlius Limestone at a roadside outcrop on Route 199 in Kingston, NY.

It's characterized by being a light-gray, fine-grained (carbonate mud) limestone with brown clay-rich layers.  The cobble from Kaaterskill Creek is weathered piece of this limestone.

OK, so it's a piece of Manlius Limestone, so what?  Well it was found up in the Catskills at an elevation of around 2,000 feet.  The Manlius Limestone is not found up in the Catskills, it's found at lower elevations, down around 200 feet above sea level, near the Hudson River to the east.  It's present at Kaaterskill Falls, but 1,800 feet below the surface and nowhere exposed at the surface.

So, where did this weathered stream cobble come from?  Unless somewhat physically carried it up into the mountains and dumped it in the stream (unlikely), it had to have been carried here by glaciers - it's a glacial erratic.  Further north, west of Albany, the Manlius Limestone is well exposed along the cliffs at Thacher State Park.

Manlius Limestone at Thacher State Park

During the last ice age, glaciers picked up a piece of this rock, carried it as far south as the area west of Catskill, and then dumped it at the top of the mountains when the glaciers melted starting around 12,000 years ago.  At some point, this rock found its way into Spruce Creek where it tumbled and weathered into its present shape.  A few days ago, it caught the eye of a wandering geologist.