The recreation road runs from south to north at Prickly Pear Canyon to Wolf Creek.
The marvelously picturesque canyon is cut in colorful red and green mudstones. Most of the rocks belong to the Precambrian Spokane Shale, a Belt formation named after the Spokane Hills east of Helena.
To log this Earthcache please e-mail the answers to the following questions.
(Please do not post answers in your log.)
Between the observation onsite and the material presented you should be able to answer the following questions.
- Take a close look at the formation in front of you. Do you see rocks that break into blocky pieces or are they broken into thin chips with roughly parallel tops and bottoms?
- Touching the rocks are they generally quite soft or hard and brittle?
- What is the primary color of the rocks? Why do you think they are that color.
Mudstone Overview:
Mudstone is an extremely fine-grained sedimentary rock consisting of a mixture of clay and silt-sized particles. These tiny particles are deposited in quiet low-energy environments like tidal flats, lakes, and the deep sea. When mudstone is buried beneath many layers of sediment deposited on top, it may be compressed to form shale, which breaks easily into thin flakes. Terms such as claystone and siltstone are often used in place of mudstone, although these refer to rocks whose grain size falls within much narrower ranges and under close examination these are often technically mudstones. Shale is often used to describe mudstones which are hard and fissile (break along bedding planes).
Mudstone can be separated into these categories:
Clastic sedimentary rocks are classified by size of the sediment particles making up the rock. Particle size descriptions like sand, silt, and clay have specific meaning in geology and engineering.
Shales, mudstones and claystones are rock types that are very similar to each other.
- Siltstone – more than half of the composition is silt-sized particles.
- Claystone – more than half of the composition is clay-sized particles.
- Mudstone – hardened mud; a mix of silt and clay sized particles.
The difference between mudstone and shale is that mudstones break into blocky pieces whereas shales break into thin chips with roughly parallel tops and bottoms.
The terms shale and claystone are sometimes used interchangeably.
Siltstone
As its name implies, it is primarily composed (greater than 2/3) of silt sized particles, defined as grains 2–62 mm- more than half of the composition is silt-sized particles.
Claystone
A claystone is lithified, and non-fissile mudrock. In order for a rock to be considered a claystone, it must consist of up to fifty percent clay, which measures less than 1/256 of a millimeter in particle size — more than half of the composition is clay-sized particles.
Mudstone
Mudstones and shales are made of silt- and clay-sized particles that are too small to see. The only difference between mudstone and shale is that mudstones break into blocky pieces whereas shales break into thin chips with roughly parallel tops and bottoms. Both are made of ancient mud.
Texture - clastic (only noticeable with a microscope).
Grain size - very fine-grained (< 0.06mm); clasts not visible to the naked eye.
Hardness - generally quite soft, but can be hard and brittle.
Colour - variable - black, white, grey, brown, red, green, blue etc.
Clasts - generally a mixture of clay minerals with any or all of quartz, feldspar ( orthoclase, plagioclase), mica ( biotite, chlorite, muscovite); can contain iron oxides (cause red or yellowish colouring); black colouring due to carbonaceous content and / or pyrite.
Other features - smooth to touch.