View to the southwest with the Roaring Twenties Rapids at the bottom edge and South Canyon entering from the top edge slightly to the right of center. The Fence Fault extends from the lower left corner diagonally upward to cross the river just this side of South Canyon. Strata on the southeast side (upper left quadrant) have been faulted downward by about 100 feet. Shinumo Wash enters from the left a short distance this side of South Canyon.

   It is interesting to note the two small washes that enter from the left edge and cross the fault to continue over the up thrown side of the fault before plunging down to the river. These washes had to be in place before the fault became active since they ignore a potentially easier path following the fault. Since the whole surface area here has been exposed only in the last few million years, the washes are probably less than 3 million years old. Movement on the fault didn’t occur until the paths for the washes became established. Thus movement on the fault has been confined to about the last 2 million years.

   The 1990 edition of “Grand Canyon Geology” has a table of recent earthquakes in the Grand Canyon area (page 441). In Jan. 1945 there was a magnitude 5.1 earthquake centered 2 miles up Shinumo Wash. (Note: While this epicenter sounds as if it were precisely located, there could easily be an error factor of several miles.)

   The fault crosses the river just before the river turns left, and continues up South Canyon. Erosion along the fault line has probably contributed to why South Canyon developed at this particular location. Also, as the river crosses the fault it increases the downstream gradient of the river. Canyon cutting has worked its way back upstream from the fault, and is most likely the cause of the steeper river gradient in the vicinity of the Roaring Twenties Rapids. (See Grand Canyon Gradient for details.)