Scientific fossil collecting involves recording precisely what layer each specimen is found in. This requires detailed work in stratigraphy, the measurement and description of rock units within a geologic formation. Field notes taken by paleontologists working at John Day Fossil Beds indicate which rock unit each fossil is collected from.

Fossils are almost always found in sedimentary rocks which form gradually over time as sediments accumulate and become cemented. Changes in sediment or patterns of deposition create natural boundaries between rock units.

GLOSSARY
sanidine - a type of feldspar
biotite - a type of mica
Ma - "Megannum", millions of years
tuff - a rock formed from the consolidation of volcanic ash

Dateable Layers
Volcanic layers form in a geologic instant, creating new mineral crystals with each volcanic event. The age of these layers can be determined by measuring the radioactive decay of potassium to argon in potassium-rich minerals such as sanidine or biotite. These layers provide the most accurate data about when a particular species lived. Volcanic layers usually do not contain fossils, but are used as age brackets for other layers.

Volcanic layers that have been dated in the Turtle Cove section include:

• 22.6 Ma Across the River Tuff
• 25.3 Ma Tin Roof Tuff
• 27.2 Ma Biotite Tuff
• 27.5 Ma Deep Creek Tuff
• 28.9 Ma Picture Gorge Ignimbrite
• 28.7 Ma Blue Basin Tuff
• 29.8 Ma A-B Tuff