How did the rocks in Pennsylvania form?
There are three major categories of rocks, igneous, sedimentary, and metamorphic. Each of them formed in a different way. Igneous rocks formed from the cooling and crystallization of red-hot molten material called magma. Magma originates deep below the surface of the Earth. There are extensive deposits of diabase, a dark-colored igneous rock, in a belt through Bucks, Montgomery, Berks, Chester, Lancaster, Lebanon, Dauphin, York, and Adams Counties. The diabase formed about 200 million years ago. Other kinds of igneous rocks that are hundreds of millions of years older than the diabase are found in several places in Pennsylvania, including South Mountain in Cumberland, Adams, and Franklin Counties and near Jonestown in Lebanon County. Water, wind, or ice can carry small fragments of eroded rock to a new location, sometimes many miles from where they originated. Over time, the fragments can be cemented together to form sedimentary rocks. Most of the rocks that are seen at the surface in
