STOP #1: Outcrop of Wissahickon Schist.
The small outcrop at the bend in the trail is classic Wissahickon Schist. Geologist name rocks to simply communication. The name Wissahickon Schist was first given to these rocks by Florence Bascom, the first woman geologist, founder of the Geology Department and Bryn Mawr College, and the person most responsible for a set of excellent geologic maps of the area between New York City and Baltimore drawn early in the 20th century. She named the rocks after the Wissahickon Creek because there are many excellent exposures of the rock in the area. The term schist is the name of a class of metamorphic rocks that have strong, usually thin and often irregular layering created by planar mineral grains that grew in the rock in response to high temperature and pressure. The Wissahickon Schist includes true schist -- a very mica-rich rock that often has a silvery appearance -- and layers that are very rich in quartz and so are called quartzite. In the outcrop photos below the quartzite which is more resistant to weathering, appears as a smoother often raised surface. It is especially clear in the second photo. The layers of schist are the other rock. Moss, lichen and dirt tend to collect on the more irregular surface of the schist and obscures its shiny character. Better examples of the schist can be seen at stop 2.
The mix of quartzite and schist is important because it reflects the original nature of the rocks before they were metamorphosed. Mica schists were original shales, fine grained sediments deposited in still quiet waters. Quartzites were once sandstones composed of quartz grains (sand). Because sand is heavier than the mud and clay minerals that form the shale, stronger currents are needed to carry the particles to where they are deposited. Mixing shales and sandstones, therefore, requires a special environment, one with periods of strong and weak flow. One place we find the mixing is in the sediments deposited by rivers flowing across generally flat areas. During floods, the stream has more energy and so is able to move material and particles of larger size. As the flood recedes, the larger particles, sand, are deposited first and then as the water slows even more, a layer of clay and mud will be deposited above the sand. However, these deposits are usually fairly small in extent -- the size of a river valley. The Wissahickon Schist and rocks like it are found all along the eastern edge of the Appalachians from Alabama to New York. An origin as river deposits is, therefore, unlikely.
Another area where one finds a mixing of sandstone and shales is in the deep ocean off the edge of continents. Sand and mud deposited by rivers entering the ocean collect on the continental slope. Filled with water, these sediments are very unstable on this gentle slope. Earthquakes or large storms can trigger submarine landslides that send the sediments flowing out across the ocean basin. As the flow loses speed, first the sand falls to the bottom and then the clay and mud. Over time you can build alternating layers sandstone and shale that reach a thickness of several kilometers, more near the mouths of very large rivers.