Bent and Broken EarthCache

THE YAKIMA FOLD BELT

To get to the earthcache coordinates and get a bird's-eye view of part of the Yakima Fold Belt, you will need to hike to the top of Badger Mountain. You can take either the Canyon Trail which starts at Trailhead Park on the northeast side of the mountain (1.4 mile hiking distance to top with 800 feet of elevation gain) or the less steep Skyline Trail which starts at the parking lot along Dallas Road on the west side of the mountain (2.0 mile hiking distance to the top with 700 feet of elevation gain). The terrain rating of 3.5 is for the hike to the top. Please stay on the trail. There is no need to leave the trail to view this earthcache.

In summer and warm weather, be sure to take plenty of water! In winter, there may be high winds at the top of the mountain which can be 2 to 3 times as strong as those at the base and frostbite can be a danger.

If you live in south central Washington, you are used to seeing the long mountain ridges that cross the area from east to west. In the Tri-Cities, this includes the Horse Heaven Hills and the mountains bordering the south and west of town including Jump Off Joe, Badger, Candy, Red, and Rattlesnake mountains. Farther northwest, they include the ridges around Yakima and south of Ellensburg including Umtanum and Manastash Ridges. To the north, they include the Saddle Mountains and the Frenchman Hills.

Generalized Map of Ridges in the
Yakima Fold Belt

These ridges belong to a geologic structural subdivision of the Columbia Basin called the Yakima Fold Belt (see figure above). The “folds” are the ridges which are arched or upwarped basalt layers. These arched folds are called anticlines while the downwarped layers forming the valleys in between are called synclines (see figure below).

The folds of the Yakima Fold Belt were formed about 10 million years ago (the Miocene epoch) near the end of the period of the volcanic eruptions that filled the Columbia Basin with basalt. These folds are a result of squeezing of the earth’s crust from a south to north direction. The rock layers were squeezed until they broke.

The break that formed in the rock layers is a type of fault called a thrust fault. Thrust faults are defined by the upper wall of the fault, called the “hanging wall”, moving upward relative to the lower wall of the fault, called the “footwall”. The opposite movement along a fault (i.e. the hanging wall moving down relative to the footwall) is called a normal fault (see figure below).

As the squeezing of the earth's crust continued, the layers on the south side of the fault rode up and over layers to the north. The nose of the overriding layers was dragged against the underlying layers until they were folded over to form the anticlines (see figure below).

Formation of an Anticlinal Fold
From the Thrust Fault

At the surface, the fault line of each thrust fault along each Yakima Fold Belt ridge is typically at the base of the north face of the ridge. There are sometimes secondary thrust faults that surface part way up the north face of the ridge and can form benches on the ridge face.

Each of the ridges in the Yakima Fold Belt were formed in this manner. The mountains and ridges you see today have been eroded so that the slopes are less steep. The north sides of the ridges commonly have landslide areas along the slopes and alluvial fans built up along their bases.

To get credit for this earthcache:

Send me an e-mail with the answers to the following questions. (NOTE: Do not post the answers in your log or it will be deleted.)

1. From the earthcache coordinates looking northwest you can see (from closer to farther away): Candy Mountain, Red Mountain and Rattlesnake Mountain. What observation can you make about their position relative to each other and Badger Mountain?
2. Looking at the profile of Rattlesnake Mountain from this location what is the apparent slope angle in degrees (from horizontal) of the southwest (left) side of Rattlesnake Mountain? What is the apparent slope angle in degrees (from horizontal) of the northeast (right) side of that mountain?
3. Looking at the northeast (right) side of Rattlesnake Mountain, do you think there is a secondary thrust fault? Why?
4. To the west and southwest on the south (left) side of the Yakima Valley, you can see Chandler Ridge. Do you see any similarities and/or differences between Chandler Ridge and Badger, Candy, Red and Rattlesnake Mountains?

References: Reidel, S. P., 2004, The Geologic Development of the Pasco Basin, South-Central Washington, Northwest Geological Society Field Trip Guide, October 16-17, 2004.

Congratulations to kiekefretter for the FTF!