Reading the Landscape Reveals Secrets About the Past and Present: Non-Native Boulders

Naturalists make an extra effort to "read the landscape" for all sorts of information related to habitats, environmental quality and history; both recent and long past. The list of clues presented to us is large and involves everything from trees to streams, animal tracks to the actual shape of the land. Sometimes a story about a particular place is held in just one clue. In other instances, the storyline is composed by the observer piecing together evidence of various sorts.

Pictured here, next to the boot, are clues that describe part of the character of the landscape in northwestern Pennsylvania; the common occurrence of rounded boulders composed of very hard igneous rocks. This photograph was taken along French Creek near the town of Venango, Crawford County, but could just as well have been taken in Erie, Mercer, or parts of Warren, Venango and Lawrence counties. These boulders can also be found along the Allegheny River, but not the Clarion River, nor Tionesta Creek. So what is the story?

The counties mentioned above were covered entirely, or in part, by periods of continental glaciation, the last of which ended approximately 12,000 years ago. Tremendously thick and massive ice-sheets originating in Canada moved slowly southward and carried along certain types of igneous rocks (e.g. granite) not native to the Pennsylvania counties mentioned. The stones and boulders were rounded off by the action of grinding, tumbling and polishing by the ice, debris-laden melt waters and the later action of flowing streams.

Although these types of rocks cover the land and stream beds of counties mentioned, they were also carried downstream into the Allegheny River, far south of the extent (terminus) of any glacial events. Other major watersheds, such as the Clarion and Tionesta, were not involved in glaciation and the rocks in these streams are less rounded, more often flat, angular and composed of native sandstones, conglomerates, silt stones and shales.

Can you think of a "Reading the Landscape" experience of your own?

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