If a rock has light-colored limestone masses or pebble-like nodules set in a matrix of fine-grained material that does not effervesce, then it is classified as a shale or sandy shale with limestone nodules. If the matrix material is smooth to the touch, it is a shale. If it is distinctly gritty or has visible sandy material, then it’s a sandy shale. The first digit refers to the major category and can be categorized as:

• 2XX Shale with limestone nodules
• 4XX Sandy shale with limestone nodules

The color of the matrix material is used to define the second digit, using the same criteria as that for shales and sandy shales:

• X2X Dark gray
• X3X Light gray/light gray green
• X4X Green
• X5X Red and green
• X6X Red

For the last digit, look for the presence of root structures or fossil shells, similar to the classification used for fine-grained siliclastic rocks.

The above properties combine to form to the six categories of this class:

• 2X4 Massive shales with limestone nodules
• 2X7 Fireclays with limestone nodules
• 2X9 Fossiliferous shales with limestone nodules
• 4X4 Massive sandy shales with limestone nodules
• 4X7 Sandy fireclays with limestone nodules
• 4X9 Fossiliferous shales with limestone nodules

 

 

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