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Brief Geological History

There are only a few rock types one would expect to find in the Juniata Watershed. These are all sedimentary rocks consisting of shales, sandstones, and carbonates. Unique in composition, each layer represents periods of deposition and erosion that have shaped the region.

Soil and Bedrock

Soils in the Juniata River basin are largely derived from underlying bedrock material of three main types: sandstone, shale, and limestone. Differences in erosion patterns and rates among these bedrock types enhanced the already-folded topography of the basin. Sandstone erodes the most slowly and thus makes up the majority of ridges in the region. Limestone weathers more quickly, eroding and dissolving to form wider, flatter valleys.
In Huntingdon County for example, 66 percent of the county consists of soils composed of residual material formed by the weathering of bedrock along the ridges and uplands. Twenty-two percent of the county’s soils are colluvium, the deposits left at the base of slopes resulting from long-term erosion processes such as gravity and rainfall. Only six percent of Huntingdon County’s land is stream-transported and deposited alluvial soil. (USDA, 1978). Although this information only pertains to one county in the region, soil surveys from the surrounding counties indicate that these proportions are similar across the Ridge and Valley Province.

Limitations
Common soil limitations for residential development include steep slopes, large stones or boulders, slow permeability, seasonal high water table, rapid seepage (which can lead to a hazard of ground water contamination from septic systems), sinkholes, flooding, and shallowness to bedrock. Factors that limit the growing of crops on certain soils include steep slopes, large stones, small surface stones or coarse fragments, shallowness to bedrock, wetness, flooding, limited available water capacity, and low natural fertility.
Residential development and crop cultivation are limited on many of the upland shale soils in the basin. Steep slopes, a shallow depth to bedrock, and shale fragments which limit the soil from holding much water, combine to leave these soils unsuitable for either cropland or development.
Hydric soils are also limiting to development or agriculture by being poorly drained and saturated much of the growing season. These soils exhibit ponding, and are generally high quality areas for wetland plants and wildlife. These are the soil series in the watershed that are considered hydric: Andover, Armagh, Atkins, Brinkerton, Carlisle, Dunning, Holly, Marks, Melvin, Newark, Nolo, Purdy, and Shelmadine.

Roadside Geology: The bedrock in the Juniata watershed has been subjected to three major mountain building events causing much folding and uplifting leaving many different rock layers exposed at the surface. Because of this, there is a relatively unique opportunity to drive around and view many different types of rock from different historical periods in just a short amount of time. For a map and the locations of some of the more noteworthy geologic sites within the area and a description of their significance click here.