How Did This Get Here?
To log this earthcache send me the answers to the following questions:
1) What kind of rock is this?: a) limestone, b) granite, c) gneiss, d) sandstone
2) What characteristics of the rock tell you this?
3) What about the rock tells you that it did not form in this location but was transported here?
4) Given your answers to the above questions, what is the most likely age of this rock?: a) Pleistocene, b) Jurassic, c) Mississippian, d) Archean
5) What is the most likely transportation medium that brought this rock to central Indiana?
6) Using the USGS map provided, what is the most likely provenance of this rock?
7) Please post a photo of yourself (face not required) or a personal item at the Earthcache location. Do NOT include a picture of the rock in the photo!
Rocks – We see them every day. They pave our streets and our driveways, line our property boundaries, fill our creeks, and sometimes even comprise the walls of our homes. But where do they all come from?
This cache brings you to a very large rock – known as a boulder – located along a public sidewalk in Speedway, Indiana. Although it may seem somewhat mundane, this rock reflects Deep Time and the myriad geological processes that have shaped our planet.
As you wonder across the Hoosier State, you will encounter hundreds of thousands of rocks, many of them looking very much like this one. Rocks native to the Hoosier State are primarily sedimentary in origin and date from the Ordovician (ca. 490 mya) to the Pennsylvanian (ca. 300 mya) time periods. Sedimentary rocks are composed of particles of other rocks or chemical precipitates and look very different from this rock, which is composed of relatively large crystals of quartz, feldspar, hornblende, and biotite that all formed under conditions of intense pressure and temperature deep within the Earth.
Two major kinds of rock have this combination of crystal size and mineral composition – igneous granite and metamorphic gneiss. Granite forms when magma cools slowly within the Earth. Gneiss can form from magma (or other kinds of rocks), often at tectonic plate boundaries, as large sections of crust smash into one another, resulting in the formation of mountains. While the mineral composition of granite and gneiss are similar, the differential pressure experienced during the formation of gneiss will result in the creation of distinct bands of alternating light and dark minerals that are not found in granite. The gneiss and granite rocks found in Indiana are hundreds of millions to billions of years older than our native sedimentary rocks!
If native Hoosier bedrock is primarily sedimentary in origin, then how did all these igneous and metamorphic rocks, like the large boulder you see before you, get here? Well, between about 500 and 300 million years ago, one of those tectonic plate collisions occurred at what is now the Atlantic Coast of North America, causing the uplift of a large section of the North American continental crust and forming the Appalachian Mountains. As a result of this, our native Hoosier bedrock was tilted and uplifted. Over the next nearly 300 million years, most sediments deposited in Indiana eroded away before they could be compacted and cemented into new sedimentary rocks, many through the erosive force of large masses of ice, known as glaciers, that grew and retreated across the state over the last million or so years.
As these glaciers grew, they scraped away the sediment and bedrock that was in their paths. That sediment, including massive boulders like this one, became entrained in the ice, with the rocks at the bottom of the ice acting like large masses of sandpaper, further contributing to the erosion. These masses of ice, known as glaciers, not only scraped away the rocks here in Indiana, they covered most of the northern half of the continent! Where did all that broken and eroded rock and sediment go!? Well, much of it was left behind in the form of a poorly sorted sediment known as glacial till. Till dating to the last glacial advance – known as the Wisconsin glaciation – was deposited in layers over 200 ft thick over most of northern Indiana and surrounding states between ca. 75,000 and 15,000 years ago. While the till itself is only a few 10s of thousands of years old, the rocks contained within are hundreds of millions to billions of years old! Maybe someday that till will be compacted and cemented to form a new native Hoosier sedimentary rock – tillite.
Sources:
1) Robert D. Hall. 1999. Geology of Indiana. IUPUI, Indiana.
2) Charles Plummer, Diane Carlson, and Lisa Hammersley. 2022. Physical Geology, 17th Edition. McGraw Hill
3) U.S. Geological Survey Geologic Names Committee. 2010. Divisions of Geologic Time - Major Chronostratigraphic and Geochronlogic Units. USGS Survey Fact Sheet 2010-3059. https://pubs.usgs.gov/fs/2010/3059/#:~:text=A%20geologic%20time%20scale%20is,revisions%20to%20geologic%20time%20scales.
4) Kate E. Barton, David G. Howell, and Jose F. Vigil. 2003. The North America Tapestry of Time and Terrain. USGS Investigations Series I-2781. https://pubs.usgs.gov/imap/i2781/