DISCOVERING THE ANCESTRAL ROCKIES: A GUIDED FIELD INQUIRY FOR FIRST SEMESTER GEOLOGY STUDENTS

The foothills of the Front Range near Denver provide an excellent setting for first semester students to apply classroom knowledge to geologic problem solving and engage in the practice of deciphering geologic history.

Students in beginning geology classes learn that clastic sedimentary rocks result from four processes: a) weathering of rocks in a source area, b) transportation by an agent of transportation, c) deposition in an environment of deposition, and d) lithification of the sediment into rock. Students apply this classroom model on a field trip to interpret the geologic history of two locations in the foothills near Denver; one, a modern stream draining the granitic and metamorphic rocks of the Front Range, and two, an outcrop of the Pennsylvanian/Permian Fountain Formation, an arkosic sandstone and conglomerate of fluvial origin derived from the Ancestral Rockies during the late Paleozoic.

At the modern stream students observe channel patterns, bedforms and determine the size, shape and mineralogy of sediment grains collected from the channel. From these observations they speculate as to the types of rocks in the source area and the distance the sediment traveled. Their conclusions are easily confirmed by traveling a short distance upstream into the rocks of the source area.

Students make a similar analysis at nearby outcrops of the tilted Fountain Formation. They observe bedding characteristics, sedimentary structures, the textural and compositional maturity of the Formation and address questions on the lithology and proximity of the ancient source area, the tectonic setting, the agent of transportation and the environment of deposition. From this analysis they begin to piece together the picture of a nearby granitic mountain range antecedent to today’s mountains. Further discussion illustrates how similar studies throughout the state have resulted in a more complete paleogeography.

In this exercise beginning geology students relate classroom information to a field setting. They make geologic interpretations based on data gathered in the field and draw the connection between modern and ancient environments.