TITLE INTRODUCTION DETRITAL ROCKS CHEMICAL ROCKS
BIOCHEMICAL ROCKS ENVIRONMENTAL CLUES CREDITS
TRACKS AND TRAILS
graded bedding cross bedding ripples mud cracks
raindrop impressions bioturbation fossils
As animals move across unconsolidated they may leave a trace of their passage. Their tracks and trails are most readily preserved in wet sediment that becomes buried under more sediment soon after the animals pass by.
The trail on the left was made recently by a snail moving along the surface of a mud puddle. For scale, the stick is one inch in diameter. Snails methodically make a spiral pattern on the sediment as they consume organic matter on top of the mud surface. The image to the right shows similar patterns on sandstone surface that is approximately 400 million years old. Comparison of these trails illustrates the principle of uniformitarianism, the foundation of geology, that states that the present is the key to the past. So, the spiral patterns on the ancient rock were probably made by an animal similar to modern snails, in a similar natural environment.
The images below show a dinosaur pathway on steeply tilted sedimentary rock in Bolivia, just outside of Sucre. On the left is a distant view of the outcrop. The rounded tracks shown in the second and third images were made by a small herd of sauropods over 100 million years ago. The fourth image is of a smaller three-toed track. The image to the far right is of the same pathway, here showing the smaller three-toed tracks intersecting the larger tracks.
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If you don't want to journey to Bolivia to see dinosaur tracks, visit the Navajo Indian reservation in northern Arizona. A local guide can lead you to excellent tracks preserved in sandstone. Water poured onto the tracks highlights their features, but this will accelerate chemical weathering which may eventually ruin the tracks. The relatively small tracks on the left were made by plant-eating dinosaurs, but the large track to the right has been attributed to the king of carnivores, Tyrannosaurus rex. Note the size 11 sandals for scale.
