Region 3: The Northern Rocky Mountains

The region that is now northeastern Washington State has few unmetamorphosed sedimentary deposits. Stevens and Pend Oreille Counties in Washington contain some Cambrian rocks that hold trilobites (Figure 3.21) and brachiopods. On rare occasions, these rocks also yield sponges. There are also Ordovician slates containing fossil graptolites and conodonts, indicating that a deep- water marine community was present during at least part of this time.

Trilobites

Trilobites are iconic Paleozoic fossils, but were more common in the Cambrian and Ordovician than in later periods. They were arthropods, and had well-defined head, tail, and thoracic (leg-bearing) segments. Most had large compound eyes, often with lenses visible to the naked eye. In life, they had antennae like many other arthropods, but since these were not mineralized, they only fossilize under exceptional circumstances. Many could roll up for protection, and several species also had large spines.

Figure 3.21: Cambrian trilobite <em class='sp'>Olenellus</em>. These and similar forms occur in Cambrian rocks in California and Nevada. They are typically 5 - 10 centimeters (3 - 6 inches) long.

Figure 3.21: Cambrian trilobite Olenellus. These and similar forms occur in Cambrian rocks in California and Nevada. They are typically 5 - 10 centimeters (3 - 6 inches) long.