I. Introduction
The coordinates will take you to an informational sign located along the Saucon Rail Trail (SRT).
As an EarthCache, no container is located at the coordinates. Instead you will have to learn some geology and make some observations about the surroundings. TO CLAIM CREDIT FOR THE CACHE YOU MUST EITHER E-MAIL OR MESSSAGE ANSWERS TO QUESTIONS PRESENTED BELOW. Do not put any answers or spoiler pictures in your logs.
The section of the trail containing the EarthCache is shown on the map below, and the EarthCache location is marked as the “The Great Unconformity”. Ample parking is available in the Saucon Township Community Park shown in the map’s inset.
Trail rules posted note: “The Saucon Rail Trail shall be open for use by the public every day of the year, but only between dawn and dusk of each day.”[1] Please abide by these rules. A full map of the trail system can be found on the SRT website.[2]
II. The Great Unconformity
Approximately 600 million years ago the area you are standing in was located below the equator and was nearly beachfront property. The rocks present and exposed in the area at this time were Byram Gneiss. Over the course of 100 million years this area was slowly covered by an encroaching sea. This ocean deposited a layer of quartz sand over the Byram Gneiss. The figure below shows a map created by Lehigh Universtiy of the area in both these time frames.[3]
The quartz sand that covered the Gneiss eventually had other layers of sediment cover it. Over millions of years under the geological pressure and heat created by its burial, the sand changed to the mineral quartzite. This layer became known as Hardyston Quartzite.
A geological survey map seen below shows the predominant rocks in the area.[4] The green arrow shows the location of the EarthCache.
The EarthCache is located at a unique location where the contact zone between the previously discussed rock layers (Byram Gneiss and Hardyston Quartzite) is exposed. A metal tab with a capital letter “G” placed in the rock by the SRT foundation shows the Gneiss face. A metal tab with the capital letter “Q” has been placed on the Quartzite. Both rock formations are on the East side of the trail.
The Byram Gneiss is a metamorphic rock composed of the minerals Hornblende, Orthoclase Feldspar, Quartz, Muscovite, and Biotite. Quartzite is a metamorphic rock as well. However, created from only quartz sand, it does not contain the same diversity of minerals as Gneiss.
The Hardyston Quartzite became a popular stone to quarry due to its attractiveness and tendency to break into well-formed blocks (ideal for walls and home foundations). The arrow in the geological survey map above emanating from the quartzite symbol points at one of the old quarries very nearby which harvested this material. The Gneiss was also quarried, however, it did not break into neat blocks, and was less desirable as a building material.
III. Signs of Early Life
The Byram Gneiss was already formed over 600 million years ago. Gneiss is a high grade metamorphic rock, meaning that it has been subjected to higher temperatures and pressures than Quartzite. Fossils rarely occur in metamorphic or igneous rocks. The heat and pressure required to change, or metamorphose, rocks usually destroys any fossils.
The Quartzite, however, does contain evidence of early life. On the West side of the trail is a rock formation labeled with a metal disc displaying the number “1”. This formation contains a fossil remain referred to as Skolithos.
Skolithos is one of the best known, globally distributed trace fossils, but it was first described from south central Pennsylvania.[5] It was originally defined in a paper by Samuel Stehman Haldeman in 1840 in which he described the Scolithus linearis, a new genus and species of fossil plant, the most ancient organic remains in Pennsylvania..
Those thinking of fossils typically envision the preserved form of some plant or animal. However, some fossils show the traces that creatures created, such as their footprints. Such fossils are referred to as “trace” fossils. Skolithos is one of the simplest trace fossils. They are dwelling traces that normally consist of a single, vertical, long, thin, unbranched, straight cylinder, with or without a funnel top, that are perpendicular to bedding and never cross adjacent tubes. Skolithos are most commonly developed in very shallow marine settings
In the Quartzite formation exposed, such fossils started out their existence as the home of some simple marine lifeform. This lifeform would have existed in the sand being deposited on top of the Gneiss layer as the oceans began to cover the area. Marine creatures would have burrowed into the sand to create dwellings, such as those depicted in the picture below.
To keep their tunnel in the sand from collapsing, many creatures create a permanent mucous‐cemented burrow that maintains its shape well after they die. Over time these tubes are typically filled with non‐laminated, finer‐grained sediment which create casts of the original burrow. The vertical continuity of the tubes, as well as the textural and compositional differences between the cast and the surrounding stone matrix (in this case Quartzite), make these fossils stick out. Some examples are seen in the picture below.
Environments with lower sedimentation rates and more frequent scour generally only preserve the bottom of the burrow (i.e., Skolithos). Maximum Skolithos tube lengths are typically reported in the 1‐2 meter (3-6 feet) range but this is hard to measure due to erosional truncation of the tube tops. Skolithos tube diameters range from 1‐30 millimeters (0.1 to 1 inch).
Droser and Bottjer (1989) developed a Skolithos Ichnofabric Index that ranges from 1 (Skolithos absent) to 5 (complete bioturbation of the original bedding by Skolithos).[6] Sandstone with a higher density of Skolithos is called piperock. The variation in density as related to index values is seen in the picture below.
QUESTIONS
1. The Hardyston Quartzite quarried around the Saucon Rail Trail came in a variety of colors ranging from blue-gray to brown-red. What color is the exposed quartzite at the cache location at Marker Q?
2. Both the Byram Gneiss and Hardyston Quartzite are exposed near Marker G. Is their more Gneiss or Quartzite exposed?
3. What type of rock was changed by heat and pressure to form the Gneiss present in the formation?
4. What average length and width are the Skolithos seen at Marker 1?
5. How would the density of Skolithos present rate on the Skolithos Ichnofabric Index?
References
1. http://sauconrailtrail.org/trail-rules/
2. http://sauconrailtrail.org/maps/
3. https://www.lehigh.edu/~fjp3/srt/EES_Website_Project_landis_mill/index.html
4. http://www.docs.dcnr.pa.gov/cs/groups/public/documents/document/dcnr_014381.zip
5. Marcus M. Key, Jr, Skolithos in the Lower Cambrian Antietam Formation at South Mountain, Pennsylvania, 79th Annual Field Conference of Pennsylvania Geologists, p. 13 (https://www.dickinson.edu/download/downloads/id/5075/1-key_2014apdf.pdf)
6. Droser, M.L., and Bottjer, D.J., Ichnofabric of Sandstones Deposited in High‐Energy Nearshore Environments: Measurement and Utilization: Palaios, v. 4, 1989, p. 598‐604.