Ocean Floor at Frear Park EarthCache
Ocean Floor at Frear Park
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An Earthcache that will take you on a walk through time and show you how some of the rocks you see everyday came to be. The initial point (IP) will bring you to a parking lot where you can proceed along the road/path to the coordinates for Stop 1. The coordinates for the various stops along this cache are provided in the table below.
So you don’t want to hike a mountain or go deep into the woods to learn some good rocknerd stuff…you really don’t need to. Often large boulders are used to deter or direct traffic in and around recreational areas. These boulders can help you learn how some rocks are formed just as well as if you saw them in their natural outcrop setting. Did you know you could look for fossils in Frear Park? You can also learn about sedimentary rocks.
Frear Park, located in Troy, New York is a 247 acre park that boasts an 18 hole golf course, ice rink, playgrounds, walking trails, tennis courts and a commanding view of the valley below. Additionally, within the park are some excellent examples of various rocks that represent different depositional environments.
WARNING – This is a more involved EC, not really hard, just lengthy. If you don’t like getting down close to rocks looking for small clues to their origin, this is not the EC for you. If you don’t like to read information and decipher it to find out what you are looking at, this EC REALLY isn’t for you. If you don’t mind getting a little dirty, want to take a stab at understanding how some of those cool fossils got here, or how the rocks with all those funny lines came to be, this EC is right up your alley. For this EC you will need to read the info below, then walk, bike or drive to TWO separate locations around Frear Park, examine the nearby boulders, take photos and answer questions. There are no signs to help you – so bring the cache page with you. Sedimentary rocks can be confusing and their many colors can be misleading. For that reason, the information here will be general. Before you begin – read the information below.
Depending upon the system you use, carbonate rocks typically consist of two parts – the large pieces (allochems) and the cement or matrix (interstitial) component. Allochems can be fossils, animal pellets, oolites (small, pearl-like spheres) or other rocks. Interstitial components can be clay, silt (carbonate) or calcite (siliciclastic). Sedimentary rocks come from three basic sources: Clay (fine grained, forms shale), Calcite, which as a sedimentary mineral exists as “micrite” (lime mud) or larger pieces of animal skeletons – these combine together to form limestone, and quartz sand – this forms from the weathering of parent rocks and is stable at Earth’s surface conditions and therefore changes very little to form sandstone.
So, the first step is the weathering process of parent rocks and transport to the ocean (quartz and clay) or the formation of limestone from micrite or animal skeletons. The difference in the rocks that are formed once all of the material is available in the ocean is based upon the depositional environment. If you have ever walked along a beach after a storm you may have noticed it is littered with shells and debris. This is because the stormy waters that were near the shore had enough energy to transport the larger particles and drop them at the shore. Depositional environments are very similar. As you head out into the ocean the heavier particles are dropped first (quartz sand grains) and the finer particles stay suspended in the water to be deposited further out and the calcium carbonate is carried in solution (dissolved in the water).
Sometimes the environment will change and this can be seen in the rock that is formed. For example, a drop in sea level, this causes the shoreline environment to migrate further out and all of the other depositional environments as well. So, where you once had clay dropping out of suspension forming shale’s, you now have heavier quartz sand grains being deposited which will form sandstone. In a cross section of a rock unit this would show shale’s with sandstone on top them. The change would be gradual with the grain sizes grading from smaller to larger in a nice uniform fashion – this being so because it occurred over a long period of time.
Another example would be a large storm that drops a tremendous amount of unsorted material further out into the sea and then calm seas return and the normal deposition sequence returns or a strong current that has the same affect (turbidity current). Rocks in this type of sequence may show unsorted materials, including shells, in haphazard fashion with no obvious gradation. The transition line from one depositional facies to another may appear as a very sharp contrast on the rock (see below) – an abrupt change from a micritic limestone to a sandy limestone or a micrite to a silty limestone with fossils.
Micrite , very commonly referred to as “limestone” is dull and very dense – really a “lime mud” that has been solidified. It is generally formed from deposits in quiet, calm water. Calm, quiet water can be deep water, where waves and currents can’t reach the bottom or tidal flats. Rocks can be 100% micrite or they can contain various allochems (fossils, etc) and then they are biomicritic. They can also have deposits of sandstone, limestone, etc (see below for storm deposition) within the unit - then they might be a micritic sandstone, etc.
Mudstone (commonly referred to as shale) formed from clay, it consists of grains that are too small to distinguish with your eyes and feels smooth. It can come in a variety of colors including red, brown, white, black and gray.
Siltstone formed from silts, it consists of grains larger than clay but smaller than sand. Hmmm, how do I field test that? In a siltstone you can’t SEE the grains but you can FEEL them. It also comes in a variety of colors.
Sandstone formed from sand, it consists of sand sized grains, commonly quartz and other minerals and feels like sandpaper. Like most sedimentary rocks, it can come in a variety of colors.
As with most rocks - very few things fit nicely into a definition. The rocks above may be pure or may be a combination of several types of rock.
Quartzite A typical specimen is dense, very hard, has uniform texture, and it is made up of fused quartz sand grains. The rock is usually light in color, the grains are very visible and the overall rock often has a luster (shine) to it. Unlike all of the rocks described so far, quartzite is metamorphic – that is, it WAS a sandstone and then it was exposed to heat and pressure and it was changed to quartzite.
Limestone is a rock of biochemical origin that is dense with a uniform, fine grained appearance. It is made up primarily of calcium carbonate (typically from the shells of dead organisms) and varying degrees of other minerals and sand. Because of this biogenic origin, limestone can be a great source for fossils and can come in many forms. Limestone is a great indicator of life at the time of deposition because it commonly contains fossils and other indicators that demonstrate the amount of energy in the environment at the time. Animal skeletons that were buried in the lime and clay mud, hardened into limestone. During burial, minerals would precipitate out of the groundwater fill the pore spaces and replace portions of the original organism. White fossils are composed of the minerals quartz and calcite.
Slickenslides a point where a rock breaks is called a fracture, if the fracture has associated movement it is called a fault. In a fault zone there is movement of one side relative to the other. As this occurs friction develops and as the two sides of the fault scrape past one another they scratch and polish the surface revealing smooth, shiny parallel grooves.
Precipitated Vein In some instances a fracture or crack will devlop in a rock. During this time groundwater will invade the fracture. Once in the fracture, minerals, such as quartz and calcite, may begin to precipitate out of solution and grow in the fractured area. These veins can be very minute to hundreds of feet in length and appear as a line of white crystals (if calcite or quartz) within the rock.
Stop 1, take a walk around and check out the boulders that line the road and do the following:
Take a photo of a fossil, one evidencing mineral replacement would be great.
Several rocks show evidence of a change in depositional environment, one rock shows this VERY well – take a photo that shows this change on one of the rocks AND line your GPS up with the “line” that you think shows when this change occurred. Now, look at the rock and using the descriptions above - try to determine what type of rock (or combination) it is. Several rocks contain fractures that have been filled by a mineral precipitate (mostly quartz) – take a picture of one of these veins. Email the answer and post the photo.
Great Work!! Now off to the next stop.
Stop 2 – this is another parking area lined with cool boulders. In the corner closest to the waypoint given there is a rock that looks different from everything else. Using the discussion above, determine what type of rock it is, describe how it looks different and what you think may have given it its shape. Email answer
Stop 3 (other side of parking lot) – several of these rocks (especially the one to the left of the blocked road) have a significant amount of fossils – take a look at it one and try to determine its depositional setting (beach, just off shore, deep water, turbulent setting, evidence of storm deposits, etc). Email answer
Now walk up the row of boulders closest to the tree line towards the road. One of these boulders has striations, they appear to be eroded slickenslides and not glacial in origin – find them and post your photo of them with your GPS aligned in the same direction as the slickenslides – nice and close so we can see the striations.
That’s it!! Stand up, stretch and clean the dirt off your knees. You just walked through several million years of life on Earth. Next time you are at the beach remember this EC and think about what is forming where the waves are crashing, where you are wading and what is happening really far out on the ocean floor. Don’t forget to email your answers and upload your pictures – otherwise we will have no choice but to delete your smiley and we don’t want that.
That being said – we aren’t looking to delete smiley’s for wrong answers – we want to be able to see that you read the material, gave it your best guess and most of all - HAVE FUN!!. Although we read through everyone's answers on all of our EC's we are not able to respond to each one individually. We hope you enjoy this Earthcache and maybe even learn something new about the world around us.
Additional Hints
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