Rock Cities - Part I

Although our landscapes are blanketed by soils of various depths in most places, underlying bedrock is always the next layer down. Except in glaciated and outwash areas, the basic characteristics of the soil itself depends upon the original bedrock of an area, because the particles that compose a majority of the soils in many regions were derived by the erosion of bedrock eons ago.

In certain locales, the bedrock is not masked by an overburden of soil, but exists as an outcropping of rock – large or small. Bedrock can be exposed at the surface of the earth for a number of reasons, however the bottom line is that patterns of specific location (e.g. mountaintop), erosion (e.g. less weathering due to harder minerals in the rock) and soil formation (e.g. soil formed moves down slope away fro the bedrock) mean that certain bedrock outcrops exist in certain scattered locations in many parts of Pennsylvania.

Although bedrock outcrops can be in the form of various types of limestones or shales, etc., the most common outcropping bedrocks in our region are types of sandstones. One of the most erosion-resistant sandstones covering much of western and northcentral regions of the state are those known as the Pottsville Group, which were formed during the Pennsylvania Period of earth's geologic past.

Pictured today are Pottsville Sandstone outcrops that exist on Chestnut Ridge in Fayette County. In addition to cliffs (vertical outcrops still attached to the layer of bedrock below) there are also large “float-blocks,” which are chunks of bedrock that have broken off and are now independent pieces. Float-blocks can range in size from that of an automobile, to larger than a two-story house. In some areas, the combination of cliff and associated float-blocks creates the appearance of a rock city and provides a different set of environmental conditions that yield habitats and microhabitats that otherwise would not exist in a given area. Tomorrow, more on rock cities.

Today's photo is from the WPC library.

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