THE HOLBROOK CRACKS

The Holbrook Cracks is an area located in the grasslands about 15 miles southwest of Joseph City, Arizona.  It is locally known as The Cracks, or Holbrook Cracks or even the Joseph City Fissures. Whatever you call it the place is fascinating.

Coconino Sandstone is the white rock that is fracturing at the Cracks and is likely about 700 feet thick. Below that layer is the Schnebly Hill Sandstone visible as the red cliffs surrounding Sedona, Arizona.  Midway into the Schnebly Hill Sandstone is a relatively thin layer of limestone known as the Fort Apache Limestone.  It is approximately one-quarter mile (1,320 feet) beneath the surface.

The Fort Apache Limestone layer was deposited by an extension off of the Paleo-Pacific Ocean known as the Pedregosa Sea.  It withdrew and evaporated over 260 million years ago leaving behind the limestone layer of marine organisms up to one hundred feet thick at The Cracks.  In the red cliffs at Sedona it is about 20 feet thick and is visible as a gray band and shelf in many locations because it is harder than the Schnebly Hill Sandstone and acts as a capstone that retards erosion.

This limestone layer is believed to be the cause of the surface cracking in conjunction with an anticline and ground water.

Anticline

As the limestone layer is dissolved by acidic ground water the sandstone layers above become unsupported and try to slide off of the peak of the anticline.  The cracks are opening in the Coconino Sandstone at the peak of the anticline.  They range in size from tightly closed fractures in the rock to fissures 10 feet wide that likely extend down 1,300 feet to the limestone layer below.  A 10 foot wide crack at the surface narrows quickly with depth and is quickly filled with rocks sloughing off the walls and windblown sand.  The deepest one I know of so far is about 150 feet.

The cracks form in complex patterns over at least a four square mile area. Surrounding the area of the cracks are encroaching sand dunes where cracks also appear in inconvenient places, like across the road.

The question I have is why would there be an anticline at this location?  Anticlines are generally formed by compressional tectonic forces on the land that create long bows and folds in the earth.   Yet the Colorado Plateau is known for its stability and lack of such features.  Standing on the hill where the Big Crack is found and looking out over the area of the cracks, it is a domed area rising above the surrounding terrain like a giant zit.

An anticline is a domed feature if looked at in a two dimensional cross-section but they are three dimensional features, (refer to the illustration above).  I can only think of two reasons for a two dimensional dome to occur; a salt dome pushing up from below or a plutonic intrusion from far below.  It's just food for thought.
  

There are also sinkholes forming around the vicinity of the cracks.  On the dirt road approaching from the north I drove through several natural shallow depressions where water flows in but there weren't any outflow channels.  This is a tell-tale sign of karst topography and limestone caverns below.

So there is some complex geology going on at The Cracks on the surface and underground: surface fracturing, domed hills, a possible anticline, karst topography, caverns and sinkholes.  It makes interpretation difficult.

Google Earth satellite photo of just a small area of The Cracks.

In one of the larger cracks, Arizona Public Service, the states electrical power company, dumped construction debris into it when the transmission towers from the Cholla Power Plant were raised to deliver electricity to southern Arizona.  The power lines pass directly over the area of the cracks and a dirt maintenance road follows the power lines.

Straping, wire, insulators, etc. 

I found that when I drove under the high voltage power lines the electrically charged air also charged the metal frame of my quad.  If I touched any part of the frame it would shock me like a 9 volt battery held to the tongue. The quad would also periodically just shut itself off and I would have to restart it.

Crack next to the road.

Ryan looks for the bottom.

Crack 6

Ryan's Hangover

To dark and deep to see the bottom.

The Big Crack is about 10 feet at its widest point.

This is not a place to take children, pets or to travel at night.

In December Mike Eilertsen from Snowflake, Arizona contacted me about taking his eight grade  STEM (Science, Technology, Engineering, Math) class to The Cracks.  On February 7th, 2020 I met them near the cracks as an unofficial guide and driver.  Mike with nine students, one parent and a drone driver, Mathan Tenney, showed up and spent the day taking drone videos and lowering a remote control vehicle (RCV) with lights and a video camera into two different cracks.  The RVC was not purchased but designed and built by the class.

It was a project to be entered into a yearly national STEM competition.  They had won the contest three years ago and were making another attempt.  I greatly enjoyed being of help and observing their project.

White Mountain Independent newspaper article

Class Video

School teacher Mike Eilertsen

Junior High School STEM class of the Snowflake Unified School District

The Remote Controlled Vehicle (RCV)

Lowering the RCV via a pulley and parachute cord.

The RCV has an umbilical supplying power from a generator.

Mathen Tenney and his drone

Microbial soil is abundant throughout the area of the cracks so avoid walking on it as much as possible and stay on the rock. Microbial soil is a crusty mat of bacteria and fungi that helps to hold the soil in place and retards erosion. You cannot see it but if the soil crunches when you step on it then it is there.

Lichen growing on colorful sandstone.

Blue, yellow and green lichens are growing on many of the rocks so try not to step on them either. Lichen is a plant that grows about 1 millimeter per year and it is sometimes used by archaeologists to date recent human activity.

Desert varnish can also be found on some rocks in the area. It is a red to black patina found on exposed rock surfaces in arid regions. Composed of clay minerals, iron and manganese oxides as well as other trace elements, it is deposited slowly over time by moisture and wind blown dust. This layer is the thickest I have ever seen and it looks as if the rock had been clad in iron, which indeed it has. This deposit is sloughing off in plates as it is sandblasted and undermined by strong winds. 

Desert Varnish

 The Cracks cover an area about 4 square miles.

I've mapped the roads and locations of some cracks. The green line is the primary dirt road through the area that follows dual high voltage power lines.  The yellow lines are side roads to other areas of the cracks and to a third power line.  The brown line is a hike up a dry, sandy wash.

Finding The Cracks in not easy and getting there is even harder.  There are 15 miles of unmarked dirt roads and deep, loose sand and rocky terrain once you arrive there.  The dirt bike bogged down in Big Sandy Wash but the four wheel drive quad was able to keep going.  A HIGH CLEARANCE FOUR WHEEL DRIVE VEHICLE IS ABSOLUTELY NECESSARY AT THE CRACKS.  BE PREPARED TO SELF RESCUE FROM THE SAND!

The cracks are on the Rock Art Ranch but they allow access.  Leave any gates as found to keep cattle where they belong.

The GPS location is:     34˚ 49'  18''  N          110˚  27'  03''  W