Color in Fine-Grained Rocks
For fine-grained rocks (siltstones, shales, interbedded sandstones and shales, mudstones, claystones, fireclays), the second digit in the Ferm code designates the color. In heterogeneous rocks, the color is assigned to the fine-grained part of the rock. Most fine-grained sedimentary rocks in this classification have colors in the gray range. The darkest color is a distinctive jet black, but most rocks are some shade of gray. Gray rocks are divided into those that are dark and those that are very light. Light gray rocks may have a tint of green, but not all loggers can see that subtle shade. In some areas, red and or green rocks may be found.
Although it is sometimes a common practice to wet rock core with a spray bottle during description, wetting fine-grained rocks (especially clay-rich rocks) can cause them to break down faster. Also, wetting core can intensify color in a rock and make dark gray shales look black, when they are really dark gray. All of the descriptions and photographs for color in this section are based on dry core unless otherwise noted.
Fine-grained rock colors are coded in the Ferm classification as:
- X1X Black
- X2X Dark Gray
- X3X Light Gray/Light Gray Green
- X4X Green
- X5X Red and Green
- X6X Red
Brightly colored fine-grained rocks do occur in certain parts of coal basins. These colors include different shades of red and vivid green, sometimes mixed together.
If the rock is homogeneous, proceed to the rock fabric page to determine the third digit.
If the rock contains limestone nodules, proceed to the rocks with limestond nodules page to determine the third digit.
If the rock contains sand, coal layers or streaks, proceed to the bedding page to determine the third digit.
Flint clays and ironstones exhibit colors ranging from buff and yellow to orange and brown. The color of these rocks, however, is not recorded in the numeric code. Go to the Ironstone and Flint Clay page to classify these rocks.