Gneiss (pronounced nice) is a foliated or banded metamorphic rock composed mostly of granulated minerals like quartz. The materials could be of a very small grain the size of beach sand or larger fused pieces like quartz crystals. Foliation means that the rock is composed of materials aligned in layers that run all the way through the rock. Metamorphosis describes rock that changes from its original form because of extreme heat, pressure, chemical action or some combination of all three. While the amount of pressure required to cause metamorphosis is up for debate, the amount of heat is generally accepted to be 212 to 392 degrees Fahrenheit (if you come here on most summer days, you might know what that feels like!). The raw material that formed the gneiss was aligned in bands or layers by the process of metamorphosis.
Gneiss is often mistaken for granite. Although granite can become gneiss through metamorphosis, granite is an igneous rock in its original form. Plentiful in Maryland, you will occasionally see gneiss used as filler material around drains and water run-off areas. Here, with smaller pieces of gneiss (but still the size of a bowling ball), you will see that it resembles the famous Smith Island Cake of eastern Maryland with thin layers running throughout the rock (or cake)!
Gneiss can be almost any color but it is common in grey, black and white as it is seen here. It is a seven on Moh’s hardness scale meaning that it is harder than most minerals. Gneiss is opaque but can have bands of translucent quartz.
Gneiss formations are often named after a geologic region where they occur. This particular gneiss, the Baltimore Gneiss, was formed over 1.1 billion years ago. It is some of the oldest rock in this area. It was named after the Baltimore terrane. The word terrane makes you think of terrain which geocachers use to describe the lay of the land and how hard it is get to a geocache. But the word terrane actually refers to pieces of the earth’s crust which break off from one tectonic plate and become attached to another tectonic plate as the plates collide and rub together. In breaking off from one tectonic plate and attaching to another plate a “chunk” of terrane may “travel” thousands of miles from its original plate. This traveling action was discovered in the 1970s through the study of plate tectonics and it significantly advanced and clarified the science of geology. The Baltimore terrane likely broke off of the Laurentian continental plate and then became re-attached at some later point.
McKeldin Falls is a significant feature of this section of South Patapsco River. It drops approximately 12 feet in a 60 foot run into a large pool (NO SWIMMING ALLOWED! per park rules). A waterfall is usually a geologic feature formed by water eroding sediment (loose dirt and softer rock) down to an erosion-resistant rock such as the gneiss.
Gneiss is also a type of basement rock. Basement rock is defined as rock covered by sediment. These sediments can be unconsolidated sand and dirt or formed into a rock like shale. Mudrock or shale deposits are very common in the western parts of Maryland. This basement rock is really the top layer of a tectonic plate but it lies below the sedimentary deposits.
At waypoint 1 you can see the action of the South Patapsco River as it has covered and uncovered the Baltimore Gneiss. This sediment dates from hundreds to millions of years old. Even if the sediment is 100 million years old, it would be 10 times younger than the gneiss. In the pool at the bottom of the falls you can see sediment that has accumulated and settled. Sometimes storms such as Hurricane Irene in August 2011 will carry large volumes of unconsolidated sediment and leave it to rest on this basement rock. At the same time, the river’s normal action pulls away the sediment, carrying it eventually to the Chesapeake Bay, revealing the gneiss you can see at waypoint 2.
In order to claim this EarthCache as a find, please send me answers to the following questions through my profile above. Failure to provide the answers will result in log deletion.
1) Describe the current water flow over the falls in a few short sentences. Do you think the current flow is depositing or removing sediment from waypoint 1?
2) Describe the texture of the gneiss at waypoint 2.
3) When you approach waypoint 2 there is a section of rock embedded in the gneiss that doesn’t have the banded appearance of gneiss but instead appears as a crystalline formation (located about halfway up the gneiss formation from the river and about 25 feet from the wood line as you approach from waypoint 1 to waypoint 2). What are the approximate measurements of this section?
Although it isn’t required to log the cache, please also post a photo of yourself and your GPSr at waypoint 1 or 2. Cache coordinates are for the primary parking at the McKeldin Rapids Overlook parking lot. This lot has only a few spots but there is a much larger lot at the alternate parking coordinates. Take the McKeldin Rapids Trail to waypoint 1 at the bottom of the overlook. From waypoint 1 it is about .25 miles to waypoint 2 over a fairly good trail.
The park website has a warning about snakes in the area: “The exposed rocks in this area of the river are a favored sunning spot for several species of snakes, including the northern water snake and an occasional copperhead, one of only two venomous species in Maryland.” I have personally seen Black Rat Snakes here so be on the lookout.
There is a nominal fee for park entrance and it is open 9 AM to Sunset. The fee is two dollars per car on weekdays and three dollars per person on the weekends and holidays. They also offer a military discount. Hours and fees may vary with the seasons. Verify the current fees and hours at Patapsco Valley State Park - McKeldin Area