Erratics and native rocks in Manistee
The posted coordinates and waypoints will take you to three earthcache sites, all located on city property in Manistee, MI.
Geology Lesson: Comparing Erratics and Native rocks
"Erratics" take their name from the Latin word errare (to wander), and are carried, often by glacial ice, over distances of hundreds of miles. Erratics can range in size from pebbles to large boulders.
A glacial erratic is glacially deposited rock. An erratic in an area that is considered to be the path of a glacier is considered to be glacial, if it differs from the size and type of rock native to the area in which it rests. These rocks typically stand out. Manistee is considered to be in the glacier path that melted 10,000 years ago, and it's glacial erratics are considered to have come from the drumlins north of here in Leelanau county.
A nonglacial erratic is moved and deposited by a source other than glaciers, like rivers and large lakes. Michigan is full of nonglacial erratics that line our river beds and our lakes, including Lake Michigan. Winter ice and water erode our exposed bedrocks and carries the debris, depositing it down stream. These rocks blend in because they are typically the same size and type of rock that is native to the area in which it rests.
Rocks that have not been moved or deposited are considered native. One example of native rocks is bedrock, the hard, solid rock beneath surface materials such as soil and gravel. Manistee's bedrock is exposed at the public access by the 12th Street turnaround.
To identify the composition of the rocks, follow the steps below:
1. a) Rock has visible layers - go to #2
b) Rock does not have visible layers - go to #3
2. a) Rock has dull luster (The rock is SLATE)
b) Rock has visible minerals - go to #4
3. a) Rock has granular appearance (The rock is QUARTZITE)
b) Rock has shiny crystalline luster (The rock is MARBLE)
4. a) Rock has thick, dark bands alternating between lighter colored rock (The rock is GNEISS)
b) Rock has many flakes of platy minerals all pointing in the same direction (The rock is SCHIST)
LOG Requirements:
Send answers by message or email from profile page for the following questions:
1) Evaluate the erratics at posted coordinates. Identify their composition and compare this to the composition of the exposed, native bedrock on the cliff or bluff at 12th Street turnabout (waypoint #2). Are these glacial or nonglacial erratics and how did you determine this?
2) Evaluate some of the erratics at the beach (waypoint #1). Identify their composition and compare this to the composition of the exposed, native bedrock on the cliff or bluff at 12th Street turnabout (waypoint #2). Are these glacial or nonglacial erratics and how did you determine this?
3) Evaluate some of the native rocks on the cliff or bluff at 12th Street turnabout (waypoint #2) and identify their composition. Be sure to evaluate the native rocks and not the nonglacial erratics used for landscaping in the center of the turnabout. How are these similar or different from the erratics at the other sites?
(Thanks to the following for giving permission to use information from their website and/or placement of this cache: Wikipedia and City of Manistee)