Devils Gate Enigma EarthCache

East of the Sierra Nevada, the road from Big Pine east to Death Valley passes through a narrow gap in a ridge of hard dolomite/quartzite which has been cut by a creek. This gap is known as Devils Gate. Why is this gap here? What is some of the evidence for its formation and for the forces which shaped this ridge?

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At Big Pine turn east from US 395 on to California 168.  After about 2.5 miles turn right on Death Valley Road (also known as Waucoba Rd. or Big Pine Rd. on some maps or books).  Informational signs will warn you that the pavement ends in 30 miles and that the road may be closed at that point. But have no fear. You need only travel about 8 miles on good, paved 2-lane blacktop to reach the cache.

As you drive east along this road, the white bands are the lake deposits of 3 million year old Waucobi Lake. Notice how these deposits are overlain with grayish to buff colored alluvium. Alluvium is unconsolidated gravel, sand and finer rock debris deposited by moving water in time long past. Carefully observe the alluvium as you will need to answer a question about its presence or absence when you reach Devils Gate.

The road slowly rises toward the ridge and then suddenly the road enters a narrow defile called Devils Gate. Park on the left at the parking coordinates and walk back down the road about 100 ft to ground zero.

The ridge here is composed of a formation called Reed Dolomite. Dolomite is like limestone, but some of the calcium has been replaced by magnesium giving a hard calcium magnesium carbonate. The dolomite here also has considerable quartzite mixed in. Note the surface color of the ridge. Pick up a piece of broken rock. Note the color difference between the freshly broken surface vs. the unbroken surface. Also note the wildly contorted layers of the rock beds. One of the laws of geology says that most rock layers were originally straight and horizontal. But they don't look that way here. The extreme bending is evidence of the compressional forces that originally formed this ridge. Geologists describe the folding seen here as either an anticline if it bends upward like an arch or a syncline if it dips downward like an inverted arch. Also, notice the faults coursing through the rock layers. This is more evidence of forces that pushed up this ridge.

Now for why this gap is here. If you could see over the ridge just to the north, you could see that the ridge simply ends a short distance away. But the stream that has carved this canyon cut through the hard bedrock even though alluvium, which is much more easily eroded, is available at a convenient level just beyond the ridge. Why didn’t the stream follow the path of least resistance and simply go around the ridge? The most likely answer seems to be that this ridge was originally buried under the alluvium when the stream flowed here. Once it had established its course, the stream had no choice but to keep cutting down into the hard rock as the alluvium eroded away.

LOGGING THE CACHE

To log this cache e-mail the answers to the following questions to me. DO NOT post your answers in your log. If I do not receive an e-mail from you, I will delete your log.

1. Do you see any alluvium here in the gap? If not, what would explain its absence?
2. What is the color of the Reed Dolomite? Speculate on what causes this color.
3. Is the folding of the rock layers at GZ a syncline or anticline?

REFERENCE

Sharp, Robert P. and Glazner, Allen F,, Geology Underfoot in Death Valley and Owens Valley, 1997.