The igneous nature of the majority of the rocks in the Monument precludes a typical fossil record. The exception is the sedimentary rocks of the Mississippian age Copper Basin Formation found on the northern edge of the Monument. The Copper Basin Formation is made up of interbedded claystone, siltstone, sandstone, and minor conglomerate. The trace fossil Helminthoida (believed to be burrows of a marine worm) is found on some bedding surfaces of the Copper Basin Formation. Because of the relative youth of the volcanism that dominates the Monument, fossils found within the lava fields must have accumulated since the volcanic activity, and therefore, are Pleistocene to Recent in age. They are primarily unaltered remains and trace fossils like tree molds.

Figure 18. Vertical Tree Mold

Volcanic activity tends to destroy organic remains. However, tree molds are found in the lava flows of the ESRP. Tree molds are impressions in the solidified lava that form as trees are enveloped by the lava flows, begin to burn, release water and other vapors that quickly cool the surrounding lava, and leave behind a mold of the charred tree and occasionally some carbon residue. Generally, tree molds preserve impressions of the cracked, partly burnt wood but do not preserve bark or other textures that would aid in the identification of tree species. Tree molds can be both vertical (where the tree remained standing as it burned resulting in a columnar shaped hole in the lava) and horizontal (where the tree fell as it burned resulting in a linear mold in the lava). See figures 18 and 19. The deepest vertical tree mold mapped in the Monument to date is 2.9 meters (82 inches) and largest trunk width is 0.9 meters (35 inches). Some tree molds provide evidence of more than a dozen tree limbs. In the northern end of the Monument, more than 100 tree molds have been mapped. The two flows with the largest number of mapped tree molds are the Blue Dragon and Trench Mortar Flat flows.

Figure 19. Horizontal Tree Mold

When lacking abundant sedimentary deposits to preserve the flora and fauna, the organic remains must be protected in some other way in order to survive over time. Lava tubes are commonly used by animals as hibernation, roosting, and den sites. They often provide a source of water and an escape from high temperatures. Animal bones accumulate in the tubes as inhabitants die naturally or are hunted and killed in the caves. Bones are also introduced into the caves as a result of human or animal disposal. Once in the cave, wind blown sediments may bury the bones, helping in the preservation process.

Exploration of such deposits in the lava tubes of the Snake River Plain has revealed bones of extinct animals, such as mammoth and camel, as well as modern large animals such as bear, wolf, bison, elk and pronghorn (Miller, 1989). Small animals identified mainly from regurgitated owl pellets include birds, reptiles, amphibians, snails and fish.

Although these animals may not have occupied the caves in life, they do offer some information about surrounding paleoecology. It should be noted that paleontological exploration of lava tubes on the ESRP has not been systematic and few have been within the Monument boundaries. Those caves, which have been excavated, were commonly archeological sites.

In addition to lava tubes, lava blisters have also accumulated a faunal record. The openings to lava blisters are generally small and drop to a floor, which can be 8-10 feet below the surface or more. This creates and excellent trap for larger animals that fall in and cannot escape. Generally, these animals are carnivores that most likely were lured into the trap by smaller prey such as a rabbit or a squirrel. Carnivores found in these blister traps on the ESRP include the now extinct noble marten, as well as other animals no longer found in the area such as bison, wolverine, and Canada lynx (Miller, 1989). Although these traps contain a random collection of carnivores, they do not represent an accurate percentage of herbivores in relation to the carnivores, because herbivores are less likely to be lured into the trap.

A third type of unaltered fossil accumulation occurs in packrat nests. These nests, or middens, often contain twigs, leaves, pollen, cactus spines, porcupine quills and bones cemented by highly concentrated urine or "amber rat", which hardens and preserves the contents (Miller, 1989). These middens are an important contributor to the fossil record because of the ability to date the pollen and bone assemblages and relate that information to the paleoecology of the area.

In late summer of 2000, a Geoindicators Scoping Meeting was held at COM to determine what geologic processes are active within the Monument. Table 1 lists the geoindicators (a proxy for geologic processes) that are applicable to the Monument and indicates the relative ecological importance, human influence, and management significance of each geoindicator as rated by the scoping meeting work group. These geoindicators were adapted from Berger (1995). For the entire report see National Park Service, 2001.