Questions  67-69 are based on the following passage.

    The geology of the Grand Canyon area exposes one of the most complete and studied sequences of rock on Earth. The nearly 40 major sedimentary rock layers exposed in the Grand Canyon and in the Grand Canyon National Park area range in age from about 200 million to nearly 2 billion years old. Most were deposited in warm, shallow seas and near ancient, long-gone seashores in western North America. Both marine and terrestrial sediments are represented, including fossilized sand dunes from an extinct desert. There are at least 14 known unconformities (breaks indicating different eras of sedimentary deposits) in the geologic record found in the Grand Canyon area.

    It is believed that the uplift of the region that eventually led to this dramatic exposure of rock started about 75 million years ago during the Laramide orogeny, a mountain-building event that is largely responsible for creating the Rocky Mountains. In total, the Colorado Plateau, where the Grand Canyon is located, was uplifted an estimated 2 miles (3.2 km). The adjacent Basin and Range province to the west started to form about 18 million years ago as the result of crustal stretching. A drainage system that flowed through what is today the eastern Grand Canyon emptied into the now lower Basin and Range province. The opening of the Gulf of California because of seismic activity around 6 million years ago enabled a large river to cut its way northeast from the gulf. The new river captured the older drainage to form the ancestral Colorado River, which in turn started to form the Grand Canyon.

    Wetter climates brought upon by ice ages starting 2 million years ago greatly increased excavation of the Grand Canyon, which was nearly as deep as it is now by 1.2 million years ago, when the river volume decreased at the end of that climatic period. Volcanic activity deposited lava over the area 1.8 million to 500,000 years ago. At least 13 lava dams blocked the Colorado River over that time period, forming lakes that were up to 2,000 feet (610 m) deep and slowing excavation of the canyon. The end of the last ice age and subsequent human activity has further reduced the ability of the Colorado River to deepen the canyon. It is unlikely that this amazing geological record will change substantially until major shifts in the river volume and/or direction take place.

All of the following are discussed in the passage, except ....