Ecosystems and Changes: Performance Assessment

This performance Assessment revolves around the concept that current surface layers of the Earth will subsequently be buried under newer deposits and eventually turn into rock under the pressure of the newer layers above it. Thus, plants and animals that died at any given age will be found in the rock layer formed at that particular time.

It is through this understanding that, in general, the deeper a paleontologist digs down into the ground, the further back into time they travel. The layering effect of ages of sedimentation is vividly seen on rock cliff faces, where one can often view hundreds of millions of years of sediment accumulation that encase plant and animal fossils that died when each layer was formed on what was once the surface ofthe ground. Paleontologists use the depth that fossils are found as well as other criteria to date the age at which fossils were formed.

For many students, the most common way to see such geological striations of rock layers is at road cuts. These are locations where a road or highway project is forced to cut through solid rock to build a road. Thus, some automobile trips take students and their families literally back into time as they drive through neatly stacked layers of sedimentary rock laid down over many millions of years! A brief stop (at a marked pull-over) allows students to place their hands directly on the cut rock wall that was once the surface of the Earth, perhaps hundreds of millions of years ago. During such family excursions, one must consider that there is the potential danger that a rock(s) from high above the road level may become loose and fall. In areas where the rock face are loose, we often see road signs warning us of “Falling Rocks”.

This performance Assessment is designed to impress upon students that the striated nature of sedimentary rock formations and the relative age of the fossils found in each layer, is a powerful tool for scientists trying to understand the natural history of various locations.

By using a graduated cylinder to add sand or dirt with various “fossils” included at various depths, a fairly clear and memorable concept is vividly demonstrated. That is, the deeper below the present surface of the Earth you go, the further back in time we “travel”. An example of a road cut at Sideling Hill on I-68 in Washington County Maryland. Notice how once flat rock layers can be distorted and bent due to the enormous pressures exerted by the Earth. However, the striated nature of the geological at this and many thousands more road cut across the country is a spectacular and highly educational sight to behold.