NAVIGATION:  BACK TO MODULE FIVE INTRODUCTION

Eliot Blackwelder (1880-1969) and Lake Overflow re-proposed 1934

Eliot Blackwelder was once the chair of the geology department at Stanford University.  He also presided over the Geological Society of America.  His brilliant  insights litter the field of geomorphology and his work on the Hoover Dam stands as some of his most remembered.  Hoover Dam is considered one of the greatest engineering undertakings in modern times .  Standing 764 feet in height, the dam weighs over 6 million tons and built from nearly 4 million cubic yards of concrete--enough to build a highway from New York to San Francisco 16 feet wide.  Before construction could take place, geologists needed to scour the region and make sure the dam site was sound enough to support the massive structure, and that the surrounding area wouldn't suffer from the presence of Lake Mead.  While Blackwelder worked on the Hoover Dam, he came across an interesting piece evidence about the Colorado River's early history.      


Figure 1. Eliot Blackwelder and the Hoover Dam during construction.

Blackwelder described the Muddy Creek Formation, a deposit indicative of local fluvial deposits that drained into a closed basin roughly where Lake Mead sits today.  Deposition occurred between 16 and 6 million years ago.  A thin layer of limestone or travertine caps these deposits named the Hualipai Limestone.  Many think the Hualipai Limestone indicates a wetter climate or the arrival of spring fed Colorado River water from the Colorado Plateau.  The rapid arrival of the Colorado River into the Muddy Creek basin abruptly halted limestone deposition as the Colorado River filled and overspilled a series of basins around and downstream of Lake Mead.  Because the Colorado River eroded into the Hualipai and Muddy Creek Formations,  the river must be younger than these deposits and therefore younger than 6 million years.  In other words, the Muddy Creek and Hualipai Limestone needed to have  existed before  the Colorado River could have come along and cut a channel into them.  This key piece of information, termed the "Muddy Creek Problem", proved troublesome for the antecedence.




Figure 2.  Here is look at the Muddy Creek Formation about 10 miles south of Lake Mead.


Figure 3.  Hualipai Limestone (whitish rock capping the butte in the right foreground) with Lake Mead in the background.

The Kaibab Plateau uplifted during a period of regional compressive stress associated with the Rocky Mountains between 70 and 40 million years ago.   The Colorado Plateau lay between the Pacific Plate and the Rocky Mountains, and literally rafted and the slammed into the Rocky Mountains.  The compression created a series of upwarps and depressions across the Colorado Plateau.  It would similar to you putting your foot on a rug and gently pushing it across a smooth floor.  The rug would buckle and form little ridges and valleys, like the upwarps and depressions on the Colorado Plateau.  The Kaibab Plateau stands as one of the higher "ridges" on the rug of the Colorado Plateau.  

For antecedence to work, the Colorado River must be older than the Kaibab Plateau, not the other way around.  Well, Blackwelder's "Muddy Creek Problem" tells us that in the Lake Mead region, the Colorado River is much younger than the Kaibab Plateau.  This problem caused scientists to reconsider how the Grand Canyon formed.  Blackwelder collected more evidence he felt strongly supported Newberry's Overflow hypothesis, and adopted that idea.  Others remained unconvinced of overflow, and with antecedence "on the ropes", they looked for new ideas to explain Grand Canyon.  


  

Figure 4.  This image provides perspective on the position of the Kaibab Plateau with respect to the Lake Mead region.

  <Back>      <Next>