Indian Rock
A large glacial erratic boulder of granite gneiss, formed in the Proterozoic era, 1.2 billion to 800 million years ago. It is estimated to weigh 17,300 tons. The source area for the boulder was nearby in the Ramapo Mountains-Hudson Highlands; it is difficult to know for certain exactly where it was picked up by the glacier, but most likely not more than 5 to 10 kilometers from its current location. The boulder rests upon glacial outwash which in turn lies atop Triassic sedimentary red beds (sandstone and shale) of the Newark Basin (circa 145 million years old). The rock was carried to its current location by the internal flow of the continental ice sheet during the last glacial maximum, circa 21,000 years ago. The base of the continental glacier scoured the bedrock terrain across which it moved, thus plucking large and small blocks of rock from their position in the Ramapo Mountains and Hudson Highlands. Indian Rock got as far as Rockland County before being liberated by the ice and deposited along with gravels shifted by glacial meltwater. Although Indian Rock may appear to be several rocks piled together, it actually originated as a single boulder (18 feet by 9 feet by 15 feet). Weaknesses within the rock caused by foliation and naturally occurring fractures serve as avenues for moisture infiltration. With repeated freeze-thaw cycles, this moisture expands to exert forces up to 20,000 lbs/inch along the planes of weakness, thus wedging the rock apart. Glacial polish, striations and grooves commonly found on erratics of this size have, for the most part, been effaced by the normal process of decomposition called weathering.
Gneiss
Gneiss is a metamorphic rock—one that was transformed from older rocks by heat and pressure within the Earth. Two continents collided 1.7 billion years ago, burying and heating a sandstone. Its minerals recrystallized and separated into alternating layers. Molten granite invaded this gneiss, forming a vein that was later deformed into a zigzag shape.
The Laurentide Ice Sheet
A massive sheet of ice that covered hundreds of thousands of square miles, including most of Canada and a large portion of the northern United States, multiple times during Quaternary glacial epochs. It last covered most of northern North America between circa 95,000 and circa 20,000 years before the present day. At times, its southern margin included the modern sites of New York City and Chicago, and then followed quite precisely the present course of the Missouri River up to the northern slopes of the Cypress Hills, beyond which it merged with the Cordilleran Ice Sheet. The ice coverage extended approximately as far south as 38 degrees latitude in the mid-continent.
The Proterozoic Eon
The period of Earth's history that began 2.5 billion years ago and ended 542 million years ago is known as the Proterozoic, which is subdivided into three eras: the Paleoproterozoic (2.5 to 1.6 billion years ago), Mesoproterozoic (1.6 to 1 billion years ago), and Neoproterozoic (1 billion to 542.0 million years ago). Many of the most exciting events in the history of the Earth and of life occurred during the Proterozoic Era — stable continents first appeared and began to accrete, a long process taking about a billion years. Also coming from this time are the first abundant fossils of living organisms, mostly bacteria and archaeans, but by about 1.8 billion years ago eukaryotic cells appear as fossils too.
With the beginning of the Mesoproterozoic comes the first evidence of oxygen build-up in the atmosphere. This global catastrophe spelled doom for many bacterial groups, but made possible the explosion of eukaryotic forms. These included multicellular algae, and toward the end of the Proterozoic, the first animals.
Logging requirement
To get credit for this EarthCache, simply answer the following six questions and e-mail them to me. When logging your find, please post a photograph of you (or your group) with the Indian Rock sign. The photograph is NOT required.
1. What two eras within the Proterozoic Eon was this rock formed?
2. What Ice Sheet did the Laurentide Ice Sheet merge with?
3. How wide is the largest crack (opening) on this rock?
4. How far did Indian Rock travel from its original location?
5. Do you see any glacial striations (lines in the rock) in Indian Rock?
6. When examining Indian Rock closely, what is the most predominate color that you see?
References: Britannica, rocksandminerals4u.com, geoscience.wisc.edu