Geologic Eras
The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. Below is a table summary of geologic eras in the Earth's history. These eras are further broken down into geologic periods (which have shorter time scales).
Timespan of Geologic Eras
| Name |
Time Span |
Duration (million years) |
| Cenozoic |
66 to 0 million years ago |
66 |
| Mesozoic |
251.9 to 66 million years ago |
185.902 |
| Paleozoic |
538.8 to 251.9 million years ago |
286.898 |
| Neoproterozoic |
1,000 to 538.8 million years ago |
461.2 |
| Mesoproterozoic |
1,600 to 1,000 million years ago |
600 |
| Paleoproterozoic |
2,500 to 1,600 million years ago |
900 |
| Neoarchean |
2,800 to 2,500 million years ago |
300 |
| Mesoarchean |
3,200 to 2,800 million years ago |
400 |
| Paleoarchean |
3,600 to 3,200 million years ago |
400 |
| Eoarchean |
4,000 to 3,600 million years ago |
400 |
For this Earthache, we will focus on the the Neoproterozoic Era. It is the unit of geologic time from 1 billion to 538.8 million years ago. Lots of interesting things happened during this era. The earliest fossils of complex multicellular life are found in the Ediacaran Period, at the end of the Neoproterozoic era. These organisms include the oldest definitive animals in the fossil record.
At the onset of the Neoproterozoic the supercontinent Rodinia, which had assembled during the late Mesoproterozoic, straddled the equator. During the Tonian era (the beginning of the Neoproterozoic), rifting commenced which broke Rodinia into a number of individual land masses.
But why are we focusing on this era in particular? You guessed it! The majority of rock formations in this area of southern Idaho were formed during this era, including most of the hilly/mountainous formations you can see to the north and west from the posted and parking coordinates.
Rock Formations Near City Creek
There are many formations made during the Proterozoic era, and in particular the Neoproterozoic. A very small sample of these, which all can be found in the area surrounding Pocatello is listed below. (Note, there are quite a number of subcategories of these formations that aren't listed. If you're interested, read the references section, to get more information!)
Neoproterozoic Formations Near Pocatello
| Name |
Description |
| Inkom Formation |
Quartzite and pebble conglomerate with minor shale. Medium- to thick-bedded quartzite is pink, red, or maroon, medium-grained to gritty, and poorly sorted |
| Mutual Formation |
Slate and phyllite with minor mediumgrained quartzite. Olive-green, laminated to thin-bedded slate is typical; maroon slate and fine-grained quartzite with thin, normally graded beds, ripples, and flame structures are present in the uppermost 15-50 feet |
| Caddy Canyon Formation |
Quartzite, minor conglomerate, and rare slate and marble. Divided into two informal members, and upper and lower. |
| Pocatello Formation |
Interbedded quartzite, metavolcanic rocks, metadiamictite, phyllite, and rare marble. Divided into three members: the informal upper member, the Scout Mountain Member (itself subdivided into four informal units), and the Bannock Volcanic Member |
Since majority of the rocks found onsite are metamorphic rocks, the following chart can be helpful in identifying them (from opengeology.org).
Questions
Please submit the answers to the following questions via the "message owner" link. Do this before posting a "found it" log, as any log without accompanying answers will be periodically deleted. Group answers are acceptable, but please be explicit about who is in your group when sending your answers.
Examining samples of rocks piles at the posted location (and the other piles in the vicinity):
- Determine what the most abundant type of sample. Given the rock formation chart and the metamorphic rock identification chart, what type of rock do you think it is? Of this rock type, what is the most abundant color?
- Determine the second most abundant type of rock in the immediate area. Can you identify it from the chart above? (Hint: the flatty ones.) Of this rock type, what is the most abundant color?
- Given your answers to #1 and #2, which of the four formations do you think is the one at GZ?
- With your log, please post a picture at the posted coordinates with one of the (two) trees, including something unique in the picture (your GPS/paper with your caching name/your blankie). You do not need to show your face in the picture.
Location Access
You can parking at the posted parking location, driving along City Creek Road. This is accessible with a 2WD when there's no snow, but drive slow and watch for other vehicles. If you're fortunate enough to have a 4x4 or an ATV, then you might be able to take "the high road", and give yourself a considerably shorter hike... but would that be as much fun?
References
https://en.wikipedia.org/wiki/Neoproterozoic
https://en.wikipedia.org/wiki/Geologic_time_scale
https://www.idahogeology.org/pub/Maps/Geologic_Maps/PDF/GM-26-m.pdf
https://opengeology.org/textbook/6-metamorphic-rocks/