The White Clay Creek landscape has been flooded, folded, heated, and eroded, creating the valley you see today. Some 600 million years ago this area was part of a large continental area that subsided and was covered by a shallow sea.
Through time, sediment composed of sand, silt, and mud spread over the sea floor. At various intervals, volcanoes poured lava onto these deposits. Gradually the sediments hardened into sedimentary rock. About 460 million years ago, an immense mountain-building episode folded and heated the rocks and completely changed their character. Rocks in this area ”cooked” at elevated temperatires and pressures for some 70 million years, long enough to form the new minerals. This process is called metamprphism. About 390 million years ago this area was uplifted and cooled which halted the metamorphism
The rocks in White Clay Creek are assigned to the Wissahickon Formation because they closely resemble the rocks along the Wissahickon Creek in Philadelphia. Because of metamorphism, the formation consists of schists, quartzose schists and amphibolite. Mudstone and siltstone containing much clay became schist, which is rich in flacky black mica (biotite) The schist also contains grains of feldspar, and quartz.
The sandstone initially contained more quartz and less mud. So it was changed into light gray quartzose schist. Much less biotite and feldspar are present.
The lava flows became very dark gray amphiboloes. Nearly black hornblende dominates these rocks. The initial sediments were layered and laminated, and this has been preserved despite the metamorohism
THE QUEST:
Park at the posted parking coordinates. Proceed down the trail about ¾ of a mile to the published coordinates. You will be walking through the valley at its narrowest and deepest point.
Location 1 (Cache doordinates) This rock outcrop is schist, consisting of alternating light and dark thin layers.
Location 2 Proceed further south to (39 D 44.398 75 D 46.391. You will see a large homgeneous dark rock which is amphibolite. There is exposed section of this rock that was cut away for the roadway. Look for the thin white quartz layers. These were introduced before the lava rock was metamorphosed.
Location 3 Further down the stream at (39 D 44.298 75 D 46.225) the rocks are quartzose schist.
You need to
(Optional) A. Take your picture with your GPS at Location 2, standing on the amphibolite rock that extends into the creek.
B. Answer the following questions:
1. How far and at what angle does the amphibolite extend into the stream?
2. Why do you think the stream bed is so different at this location?
3. Estimate how wide the valley floor is and how deep (to stream bed)do you think the valley is?