A cache by Kikekatze Message this owner

Hidden : 12/9/2025

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Size: Size: other (other)

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Geocache Description:

Please notice: As this is an Earthcache there is no container to be found! Just read the text below, answer the questions and send me the answers through my profile. You can log the cache immediately including the required picture. If I need further details, I will reach out to you. Logs without meeting both logging requirements will be deleted!

picture: kikekatze

Kolugljúfur is a narrow basalt canyon carved by the river Víðidalsá in northern Iceland. The river drops over several steps, forming the waterfalls known as Kolufossar, and then rushes through a deep, confined gorge.

When you arrive at the bridge and the viewpoints, you can see something unusual:
not only the main waterfall and its deep plunge pool, but also several round holes in the rock beside the present river course. Some of them sit higher than the current water level and look almost like small, quiet ponds perched at the canyon rim.

These features are not random. They are traces of very concentrated erosion – plunge pools and potholes – that record how the waterfall and the river have changed their position over time.

This EarthCache will help you explore:

how lava flows and basalt joints control the shape of the canyon,
how plunge pools form and grow,
how waterfalls slowly move upstream by headward erosion,
and why some “holes in the rock” are no longer directly connected to the present river.

Please stay on marked paths, keep a safe distance from the canyon edge and be very careful near the round water-filled holes: the ground can be wet, slippery and locally undercut.

Lava, basalt and natural weaknesses

Like many canyons in northern Iceland, Kolugljúfur is cut into stacked basaltic lava flows. Each flow started as liquid lava that spread across the landscape and then cooled.

When a basalt flow cools:

the outer surface solidifies first,
the interior remains hot for longer and contracts more slowly,
cracks (joints) form as the rock shrinks.

These joints often form vertical, column-like patterns, but also horizontal fractures along the boundaries between different flows.
The result is a rock body that is strong overall, but criss-crossed by natural planes of weakness.

Later, when the Víðidalsá began to cut down through these flows, it did not erode the rock evenly. Instead, it concentrated its energy where:

joints are dense,
boundaries between flows are weaker,
or older surfaces were already broken up.

This is where waterfalls and plunge pools tend to form.

What is a plunge pool?

A plunge pool is a deep, bowl-shaped hollow carved into bedrock at the base of a waterfall or a major rapid.

The process can be summarised like this:

Impact of falling water
Water dropping over the edge hits the base with high velocity. Its kinetic energy is focused on a relatively small area.
Turbulent swirling
The impact creates strong vortices. Water rotates in tight circles, sweeping along the rock floor and walls of the pool.
Tools of erosion
Sand, pebbles and even larger stones become trapped in the swirling current. They are repeatedly thrown against the bedrock, grinding and scouring it.
Deepening and widening
Over time, the combination of hydraulic pressure, turbulence and abrasion carves a smooth, rounded depression – the plunge pool (or kolk).

Potholes and “fossil” plunge pools – holes in the rock near the bridge

Near the bridge, you see almost circular holes in the basalt, some of them filled with still water. They are not in the centre of the present river channel, and some are perched a bit higher than today’s water level.

These are also products of strong, focused erosion:

Some are potholes formed by vortices along the margin of the main flow, especially during times of high discharge or flood events.
In these eddies, stones rotated in place and drilled into the bedrock, creating round, steep-sided holes.
Others can be interpreted as small plunge pools that were active when the waterfall or a smaller fall step was located slightly differently from today.

Once the river changed its position or cut deeper:

the old pothole or plunge pool was left “stranded” at a higher level,
the round hollow remained as a “fossil” record of former erosion.

From plunge pool to headward erosion – a moving waterfall

Plunge pools do not only deepen the riverbed. They also cause waterfalls to retreat upstream. This process is called headward erosion.

The steps are:

The plunge pool erodes the weaker, more fractured layers at the base of the waterfall.
The erosion cuts back under the harder caprock, creating an undercut or small cave behind the fall.
The overhanging basalt loses support and eventually collapses as blocks or slabs.
The waterfall lip jumps a short distance upstream.
The process starts again at the new position.

Over many cycles, this produces:

a longer canyon,
steep, near-vertical walls,
and several generations of abandoned potholes and plunge pools along the way.

At Kolugljúfur you can read this history in the landscape:

The main plunge pool at the current fall shows where erosion is active now.

picture: kikekatze

The round holes near the bridge show where intense vortices or small falls were eroding earlier, at a slightly different height or position.

picture: kikekatze

A simple sketch of waterfall retreat

Observations you can make on site

When you visit Kolugljúfur, try to connect the theory to what you see:

At the bridge area:
Look for the nearly circular water-filled holes in the basalt. Note their position relative to the present river and canyon edge.
At the waterfall viewpoint:
Observe the shape of the main plunge pool below the falls, and the cracks or overhangs in the rock just above it.

Together, these features allow you to reconstruct how the river has shifted and cut back through time.

Logging Tasks

1. Please answer the following questions and send your answers via Message Center or email (English or German are fine).
You may log immediately; if something is unclear, I will contact you.

a. The “holes in the rock” near the bridge

Stand near one of the circular water-filled holes.
Describe:

its approximate shape and size,
whether the inner walls are rough or rather smooth,
and whether it is directly connected to the flowing river or stands slightly apart.

What does this tell you about how this hole was formed?

b. Old versus active erosion

Compare one of these higher holes near the bridge with the main plunge pool beneath the current waterfall.

In what ways are they similar?
In what ways are they different (position, connection to the river, apparent activity)?

Explain how this comparison supports the idea that the river and the waterfall have changed their position over time.

c. Evidence for undercutting and headward erosion

From the waterfall viewpoint, look at the rock just above the plunge pool.

Describe at least two signs that the waterfall lip is being undercut or that blocks could break off in the future (for example: cracks, overhangs, recesses, sagging vegetation).
How do these observations fit into the process of headward erosion?

d. Your own explanation of waterfall retreat

In your own words, summarise how plunge pools and potholes help a waterfall like Kolugljúfur to migrate upstream over long timescales.
Mention both:

what happens at the base of the current fall, and
what the abandoned holes near the bridge tell you about past positions or stronger flow in the canyon.

2. Picture: Take a picture of you either at the coordinates or somewhere else at the waterfall. If you do not want to show your face, write your name and date on a piece of paper and take a picture of that before the waterfall.

picture: kikekatze

Safety

The canyon rim and the edges around the holes can be unstable and slippery.
Please stay on marked paths, avoid getting too close to overhanging edges, and keep a close eye on children.