We are viewing the "Great Unconformity" within Blacktail Canyon, a beautiful side canyon of the Colorado River, found in the Grand Canyon. Geology is totally preoccupied with time. Time’s passage is recorded by the presence of rock units, but not every geological event is preserved in stone. The absence of a record is of equal importance. Missing time results from rock units that have either eroded away or from periods of time in which no sediment was deposited. Such a gap in the temporal record is called an “unconformity”, and it's the geologist's business to decipher what happened during it. The immensity of this particular time gap has been recognized and singled out as the “GREAT UNCONFORMITY” written with capital letters. John Wesley Powell, the intrepid explorer, scientist and geologist of the American Southwest, first recognized the time gap in 1869.

Near the bottom of the Grand Canyon, a sandstone formation from the Cambrian age, called the Tapeats, rests upon metamorphosed schist from the Precambrian called the Vishnu. Mysteriously, the rock layers that once existed between the two layers is missing. You can run your fingers from the Vishnu Schist to the Tapeats Sandstone and literally touch the gap of lost time within the Great Unconformity. Where did over a billion years of time go? How can we attempt to comprehend its magnitude? Where did all the missing rock layers go? How were they removed? Were they ever there to begin with? What event or events caused this massive time-gap to form? Why did it happen at the end of the Precambrian? What happened during it? How do we know? Are the events global in their extent? Will it happen again? There are many questions. Regardless of the cause, the space of that paper-thin contact represents an enormous amount of time unaccounted for, far greater than all the other known unconformities of the Earth, on the order of being between 1.2 - 1.6 billion years. That’s more than a quarter of the age of the Earth -- Missing!

Blacktail canyon is a popular locale for literally touching the "Great Unconformity" in the serene setting of a beautiful side canyon. The regional westward dip in the strata has caused the Tapeats Sandstone to descend back down to river level making the details of the "Great Unconformity" well exposed to view along the river-polished walls of the canyon. In this image, the wide and flat jutting layers in the top half of the image represent the Tapeats sandstone layer, and suddenly changes in comparison to the metamorphosed Vishnu Shist below it.