ISSUE IN THE QUESTION: A tributary to a main river (like Bright Angel Creek is a tributary that flows into the Colorado River) does several geomorphic tasks as the main river incises (cuts down): e The tributary also incises to "keep up" with the main river » The tributary extends its length in the headward direction (up hill). e The tributary transports the rocks that it eroding (both incising downward and extending headward). QUESTION: What rock type is the headward end of Bright Angel Creek currently eroding? What rock type did the Bright Angel Creek start to erode as the Colorado River started incising into the Grand Canyon some 4.8 million years ago? HINT: It is the same rock type. It has a light blue color, and it is the top of the Paleozoic strata in the Grand Canyon area. Precambrian rocks Vishnu Schist m Kaibab Limestone Proterozoic granite The reason why the Kaibab Limestone is the correct answer is that it is the top layer in the Grand Canyon area. As the Colorado River was starting to incise, it had to erode through the Kaibab Limestone first. Hence, so did its tributaries. However, the tributaries are still working on extending headward, and this rock layer is still what they have to erode first as the tiniest of trickle of a stream is starting to flow off the rim.
