The Story of River Pebbles
 at the Falls of the Ohio

by Alan Goldstein, Interpretive Naturalist

The pebbles along the river bank seem ordinary. Most are small and smooth. Young children rub them and say they are "soft." Pick up a pebble and look at it. (Please leave your pebble here for others to enjoy.) Do you know the story of a river pebble at the Falls of the Ohio?

Each pebble has its own story. You will find pebbles that are orange, red, green, yellow, white, brown, gray, black and combinations of colors. Some may be mottled, banded, sparkling in the sunlight and others bisected by lines. Pebbles may contain fossils. Get your pebble wet. A wet pebble shows colors and patterns quite vividly. To understand a pebble's history, one must know what type of rock it is.

There are three types of rock and all can be found in river gravel:

Igneous rocks were once molten, with temperatures from 500 to 2,000 degrees Fahrenheit. Fast cooling rocks show tiny crystals which may be seen as they sparkle in the sun. Basalt is an example of a fast cooling rock formed from lava. Slow cooling rocks form larger intergrown crystals which are easy to see. Granite is one of the most well known igneous rocks, formed from slow cooling magma (lava deep within the earth).

Metamorphic rocks are created when mountains (like the Appalachians) are being formed.  As continents collide, the three types of rock can get mashed together. They can be pushed up to form towering mountains or shoved deep into the earth where temperatures can be thousands of degrees and the pressure can be thousands of pounds per square inch. This causes the rock's appearance to change - that is where the word metamorphic comes from. Two very common pebbles are gneiss (pronounced "nice") and quartzite. Gneiss was originally granite, but has been altered. Recognizable by its colorful  banded texture, it is often green, pink, red or orange. Quartzite was originally sandstone. Sand-sized quartz grains were fused together so it can be quite hard. Quartzite can be any color and contains a fine granular texture that may be seen under close examination.

Sedimentary rocks are formed by sediment (like mud or sand) that is compressed by the weight of overlying material. It may be cemented together with minerals. Rounded pebbles of limestone (gray to whitish color) are most common. The source of this rock may be tributaries entering the Ohio River or the channel cutting into the bedrock, upstream from the Falls. Look for fossils in some of these pebbles. They are 450 million years old, some 60 million years older than the fossils at the Falls. Chert is a hard rock formed from silica - quartz. (Flint is a dark gray variety of chert, agate is a banded variety.) It occurs as irregular blobs in the limestone at the Falls. It breaks into sharp-edged fragments. Rounded chert pebbles have been tumbled around for a long time. Sandstone is another sedimentary rock that can be found at the Falls. It is brown to orange in color and looks like it is made of grains of sand.

How is it that we can find the three different types of rock jumbled together so thoroughly on the river bank? There are no exposures of igneous or metamorphic rock along the Ohio River.  The answer can be found by rolling back the clock to the Ice Age. This is a time geologists call the Pleistocene. Between two million and 10,000 years ago there were a number of episodes when glaciers - huge masses of moving ice - came down from the Canada into southern Indiana. As the glacial ice bulldozed its way from the north, it plucked millions of tons of rock from the earth's surface and carried them along. The rocks were ground together almost to a polish. Fragments of harder rock became sand. Soft rocks were pulverized into silt and clay. When the glacial ice melted, the rocks and sediments were released. Many pebbles were moved by the streams of melt water and some of these found their way  into the Ohio River.

The Ohio River shifts around in the valley. Today the river we see is the channel of the last two thousand years. Before that, the river flowed across what would be downtown Louisville.  There are millions of tons of pebbles there today. Some are excavated as deep basements are built. Pebbles just like those on the river bank are dredged from the banks and bed the river on a regular basis. These pebbles may find their way into the local landscape or as aggregate in concrete.

            One pebble's story...

Two billion years ago, molten magma cooled about ten miles (16 km) underground. It took about a million years to harden into hard granite. Two hundred million years ago, continents collided. The granite rock which had moved to within a mile of the surface by the erosion and removal of surface rock was shoved a couple of miles down and put under tremendous pressure as continental plates ground together by tectonic action. The minerals in the granite became re-crystallized. The result was a hardened rock with many layers. It had become a metamorphic rock called gneiss.

About two million years ago, the rock was exposed on the surface of the landscape. Glaciers formed. Ice up to two miles thick crept along the surface. One day, about a million years ago, the glacial ice plucked our pebble (still attached to a giant boulder). As it was dragged along beneath a mile or more of ice, pieces flaked off and the large boulder become a number of smaller, rounder and smoother cobbles. Over a period of 4 or 5 glacial episodes, most cobbles were worn into smooth, small pebbles. Some had moved into southeastern Indiana about 10,000 years ago as the glaciers melted. Chilled water flowed away from the dying glacier slowly moving the stones. Within the last 10,000 years, our pebble has moved dozens of miles to the Falls. Today anyone can find a gneiss pebble on the river bank.

Updated March 16, 2010