A HEARTBREAKING STREAM OF STAGGERING COMPLEXITY: WHAT THE PIONEER GEOLOGISTS KNEW AND DIDN'T KNOW; WHAT WE KNOW AND DON'T*

Pioneers J. W. Powell and Dutton said the Colorado River began in the Eocene and predated each structure it crossed, but they failed to convince Gilbert, Davis, and Walcott. In the 1930s, Blackwelder proposed that the Colorado may even be as young as Pleistocene. Colorado Plateau master Hunt continued to argue for an older age. Both drew on the work of Longwell; all three tried to understand the entire river—not just the Grand Canyon section. In 1936 Longwell, whose PhD thesis was on the Muddy Mountains and who also studied Boulder Canyon, summed up the possibilities in words that 69 years later still serve well:

"The original stream may have started cutting on a high-level surface that overtopped all of the present ranges (superposition); or, integrating basins that became connected along the course of the river (lake integration); or, a stream emptying into the Gulf of California may have extended its valley by headward erosion, starting the cutting of the canyons and finally integrating with drainage from the Rocky Mountains (headward erosion)."

Longwell doubted that the Colorado has always been a single, integrated stream flowing west across the Kaibab and debouching at Grand Wash Cliffs. His thinking influenced attendees at the 1964 McKee Symposium to espouse headward erosion east across the Kaibab to intersect and reverse the drainage on a southward-flowing ancestral Colorado.

Geologists at the 2000 Colorado River Symposium wrestled with the same questions that puzzled their forebears, but thanks to radiometric dating and other techniques unknown to the pioneers, have been able to draw the boundaries of possible theories more tightly. This talk reviews the history of Colorado River and Grand Canyon geologizing, emphasizing the major questions that remain. Paraphrasing Watergate, the essential question is “Where did the Colorado flow and when did it flow there?” Plateau geologists have the daunting task of trying to understand a river that by “devouring its progeny” has created its own gaps in the tape recording of geologic history. But who could ask for a more intriguing puzzle in a more sublime setting?

*Apologies to Dave Eggers for the title