Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
As a general rule, if you think it might be a fossil, it probably isn't. If it's so clear even a novice can recognize it, it is.
|True petrified wood doesn't merely have a grain; it has a lot of detailed features of the wood. This is a piece of silicified wood.|
|The piece at right even has a knot.
The piece below is shown in side and end view. Note the annual rings in the end view at right, below.
|If you have any lingering doubts that coal is derived from wood, this carbonized tree limb from Puget Sound, Washington, should convince you.|
|Metamorphic rocks in particular can have very linear textures that result from the rock being stretched. These can easily be mistaken for petrified wood.
Extremely intensely sheared rocks, like the one at lower right, have an extremely straight, fine grain. They are called mylonites.
|The mylonite at right comes from the Norumbega Fault Zone in Holden, Maine. I attended the school not far from this spot as a kid, and I thought this stuff was petrified wood. Many years later, looking at it with the eye of a geologist, I can still see why a novice would mistake it for petrified wood.|
|That unusually round pebble is not a fossil egg. Real fossil eggs are found in sedimentary rock layers associated with fossil bones, very often in clusters or nests. Almost always the shells are fractured and often the skeletons of the unhatched young are still inside. Unless you have all those things, it's not a fossil egg. They are not found as isolated round pebbles.
The pebble at right is not a fossil egg. Real fossil eggs are shown below.
|Not the corned beef kind, but broken shells. If you have a rock with lots of small, flat or curving fragments in it, it's probably due to broken up shells.|
|The same rock viewed from the side, showing numerous broken shells in cross section.|
Elongated markings within rock layers are often burrows of aquatic organisms that were filled in by some other material.
|Organisms browsing the bottom may leave grooves in the sediment. Sometimes the overlying layer preserves a replica of the tracks and we see small ridges in relief.
These are called trace fossils and are indications of life that lacked hard body parts and did not leave fossils. Long before the Cambrian period brought abundant fossils, tracks were common and indicated that life was abundant.
|These are actual dinosaur tracks from Dinosaur Ridge just west of Denver, Colorado|
|Dinosaur tracks were often made in deep mud, subjected to erosion before they were buried, or subjected to erosion and weathering after exposure. They may be degraded. At least a few unscrupulous people have tried to pass them off as human tracks.
A roughly foot-shaped hole in a rock is not a track unless it's part of a regular trail of similar impressions, in sedimentary rocks, and especially if there are indisputable tracks nearby.
|Often mistaken for fossil moss or fern leaves. These are crystals of iron or manganese oxide something like the patterns frost crystals make on windowpanes. The ornamental variety of dendrites in quartz is actually called moss agate.|
|This feature looks almost like a fossil shell. It actually consists of very fine, radiating dendrites that formed on a hairline crack in the rock.|
|Geodes are cavities in volcanic or sedimentary rocks, lined with quartz. They're like Forrest Gump's box of chocolates: you never know what you're going to get. Some geodes have linings of crystalline quartz|
|Other geodes have concentric layers of agate.|
|Sometimes the inner surface is lined with rounded masses, a texture termed botryoidal.|
|Sometimes the silica settles out to form horizontal layers.|
|The outside of a geode (actually a double geode).|
|This geode has it all. The outer portion has concentric layers of silica. In the inner portion, the top is lined with silica crystals and the bottom has horizontal layering.|
|These occur in sandy soil where calcite or gypsum crystals enclose sand grains. They cement the sand grains together but the form of the sand preserves the form of the crystal.|
White chalky masses or gray glassy nodules often found in limestone and dolomite. They are often mistaken for fossil bones. Real fossil bones are usually dark brown, not white.
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Created 15 October 2009, Last Update