Those Giant Boulders Around Cave Creek — Where Did They Come From?

Drive around Cave Creek and Carefree and you'll notice something unusual about the landscape.

Boulders. Really big boulders.

They appear stacked on hillsides, balanced on top of one another, and scattered across the desert as though someone dropped enormous rounded stones across the landscape.

Some look almost deliberately placed.

Then look toward Black Mountain.

It's rugged, dark and angular—a very different-looking mountain sitting in essentially the same neighborhood.

So what happened?

Why does one part of the landscape contain huge rounded boulders while Black Mountain looks so different?

The answer takes us millions of years underground.

The Boulders Started Below the Surface

Many of the large rounded boulders visible around Cave Creek and Carefree are granite.

But they didn't begin as boulders sitting on the surface.

Granite forms when molten rock—magma—cools slowly beneath the Earth's surface. Because it cools underground over a long period of time, relatively large mineral crystals have time to form.

Eventually, erosion removes the rock and soil above it, exposing the granite.

But that still doesn't explain those extraordinary shapes.

For that, we need cracks, water and a lot of time.

Nature Starts With a Giant Block of Granite

Large bodies of granite develop natural fractures called joints.

Imagine an enormous mass of granite underground divided by cracks into large blocks.

Water can penetrate those cracks.

Over long periods, chemical weathering begins attacking the rock, particularly along the fractures where water can reach it most easily.

The corners of a block are exposed to weathering from several directions. Edges are exposed from two directions. The flatter surfaces are exposed less.

So the corners disappear faster.

Then the edges.

Slowly, what started as a block becomes increasingly rounded.

Geologists call this process spheroidal weathering.

And it can begin while much of the granite is still underground.

Then the Desert Removes the Cover

Weathering creates the rounded forms, but something else has to reveal them.

Erosion gradually carries away the surrounding decomposed granite, soil and other material.

The harder pieces remain behind.

Eventually those enormous rounded rocks emerge at the surface.

That's why some of the boulder formations look almost impossible.

One giant rock may appear to have been carefully balanced on another.

In reality, nobody stacked them.

What we're seeing is what's left after an enormous amount of surrounding material disappeared.

The landscape was essentially uncovered around them.

Why Are Some Boulders So Huge?

Because the original granite mass was huge.

The spacing of fractures in the granite helped determine the size of the blocks that remained.

Closely spaced fractures could eventually produce smaller rocks.

Widely spaced fractures allowed enormous masses of granite to survive weathering and erosion.

That's why you'll encounter everything from ordinary granite rocks to formations that look large enough to be small buildings.

And once you understand how they formed, the hillsides begin to look different.

You're not simply looking at rocks scattered across a mountain.

You're looking at the remnants of rock that once existed beneath a much larger landscape.

Then There's Black Mountain

Now turn around and look at Black Mountain.

Something is clearly different.

Instead of pale, rounded granite boulders, much of Black Mountain appears darker, sharper and more fractured.

That's because Black Mountain isn't simply another pile of the same granite boulders.

The mountain contains different rock types and represents a more complicated geological story.

The western side is associated largely with ancient metamorphic rocks, while granite is much more prominent on the eastern side and in the surrounding Carefree foothills.

That difference in geology is one reason Black Mountain can appear almost divided when viewed from certain directions.

Different rocks weather differently.

Granite commonly develops the large rounded forms that make the boulder landscapes around this area so distinctive.

The metamorphic rocks exposed on Black Mountain tend to break and weather differently, contributing to the mountain's darker, rougher and more angular appearance.

They're Neighbors, But They Didn't Get Here the Same Way

That's what makes this landscape so interesting.

At first glance, Black Mountain and the giant boulder formations around it seem like they should have one simple explanation.

They don't.

The granite tells a story that begins with molten rock cooling deep underground, followed by fracturing, underground weathering, uplift and erosion.

Black Mountain exposes another part of the region's geological history, including rocks transformed by enormous heat and pressure.

Then erosion spent millions of years revealing both stories.

What we see today is simply the latest frame in a geological movie that has been running for an almost unimaginable amount of time.

Look More Closely Next Time

The next time you're driving through Cave Creek or Carefree, look at one of those enormous boulder piles.

Find a particularly round boulder.

Then imagine it underground.

Imagine cracks surrounding a huge block of granite.

Water slowly works its way through those cracks. Corners weather away. Edges become rounded. Material surrounding the harder granite gradually breaks down.

Eventually erosion carries that material away.

The giant rounded core remains.

Now look across at Black Mountain.

Suddenly the difference between the two isn't just scenery.

You're looking at different chapters of the Earth's history sitting almost side by side.

And once you know the story, those strange boulders scattered around Cave Creek don't seem quite so random anymore.