I was traveling a few backroads of Missouri when I spotted this little park outside of Arbela, Missouri. I saw a historical sign and actually stopped to check that out when I noticed this boulder not far from that sign. Once I checked it out and did a bit of research I knew exactlly what it was.
Questions to answer for your smiley :
1) From the lesson and your observation of GZ, which of the two sediment deposits would this boulder have been left by? Why?
2) What makes this rock an ERRATIC?
3) From the lesson we learned that this rock is Granite. Which mineral is most visible? What color of this mineral is most prevalent?
4) Where on the rock are the largest examples displayed?
5) As of June 2019, earthcaches now contain required photo logging tasks. Please provide a photo of yourself, your GPSr, or a personal item that proves that you have visited this site. You can use the park sign as your Backdrop.No Spoilers in that picture please.
EARTHCACHE REQUIREMENTS
Each cacher must send his/her own answers BEFORE logging a find. ... "Geocachers must complete the tasks before they log the EarthCache as found." (4.3. EarthCache logging tasks) if your answers are WAY off, I’ll contact you. Otherwise, please log your find after sending me your answers via email or messaging. Please indicate your geocaching name and include the names of other geocachers with you. Logging it as found without providing your answers AND not uploading a photo could result in the log being deleted without notice. According to the guidelines, sending the answers is a requirement, not a request.
The Lesson
Glacial Erratic
Just what is a Glacial Erratic? Well lets talk about that. A glacial erratic is a glacially deposited rock differing from the type of rock native to the area in which it rests. Erratics, which take their name from the Latin word errare (to wander), are carried by glacial ice, often over distances of hundreds of miles. Erratics can range in size from pebbles to large boulders, such as this one in the park. This rock, a gray and black bulk, about the size of a mini-cooper, sits out in the open along the roadside.
When glaciers move along, they bring with them pieces of rock , like what we see here. Then as the ice melts , they deposit the rocks and the sediments around them all in a mix called a "till". The oldest of such movements dates back about 2.4 million ago and is thought to be part of the oldest till in North America. The youngest in Missouri is about 600,000 years old.As the ice sheets began to melt , large amounts of sediment heavy meltwater flowed away from the movement of the glacier as it slid along. This type of sediment is known as an "outwash". ---- Spencer ( page 41) .
The difference between the two is the size of the sediments that they carried and where deposited and the presence of stratifed layers of the outwash. You can see this best in the two pictures I have posted.
Till
Outwash
The glacial movement that brought this erratic here flowed south to the Kansas River. It scoured the valley which became widened and deepened by the large amount of water moving along through it. This became the primary channel for meltwater for the remainder of the Pleistocene time. This filled this entire valley with sediment ; first with boulders and gravel then with sand and mud. Across northern Missouri this moving glacier left behind remnants of that movement like we have here.
So where did the boulder here come from? Well, according to Charles G. Spencer in Roadside Geology of Missouri , the most likely place , given the flow of the ice and the type of rock this is,is Minnesota.The rock here is granite, which is an Igneous rock.Granite typically has grains large enough to be visible with your naked eye. It is created from the slow crystallization of magma below Earth's surface. The large mineral crystals in granite are evidence that it cooled slowly from molten rock material. Here, with this example there are several " Veins" of one mineral in particular.
Granite is composed mainly of quartz and feldspar with minor amounts of mica. Here the main visible mineral is quartz. Quartz occurs in virtually every color. Common colors are clear, white, gray, purple, yellow, brown, black, pink, green, red. Another interesting feature , at least to me, here is just how smooth and rounded this erratic is. Much like in a rock-tumbler that will round off jagged edges , the route this rock took to get here, tumbling along the ice flow has done exactly the same thing. Where there was once sharp edges , like in the picture above, it is now smooth to the touch, though still maintaining that , grainy feel of quartz.
Resources Used :
A) Glacial erratic - Wikipedia
B) Roadside Geology of Missouri by Charles G. Spencer
C) Outwash | glacial deposition, sediment transport, fluvial processes | Britannica
