Seep Line Falls EarthCache

🌍 EarthCache: Seep Line Falls – Reading a Coastal Bluff

Along the drive to Port Renfrew, this small seasonal waterfall spills over a coastal bluff. At first glance, it may appear to be just a roadside trickle. But this waterfall reveals an important geological process — how groundwater moves through layered sediment.

This exposure provides a clear example of how water interacts with different sediment types and how erosion gradually reshapes coastal slopes over time.

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🪨 EARTH SCIENCE LESSON  

Why Is the Waterfall Here?

The bluff consists of layered sediments deposited during and after the last glaciation. Some layers are more permeable (water can pass through easily), while others are more compact and less permeable.

Rainwater infiltrates through the upper sediments. When it reaches a less permeable layer, it cannot easily move downward. Instead, the groundwater is forced to move sideways along that boundary.

Where this boundary intersects the bluff face, groundwater emerges as seepage — creating this waterfall.

The distinct horizontal line visible across the bluff marks this change in sediment type. This is known as a seep horizon.

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💧 SEASONAL FLOW

The amount of water flowing here depends on:

• Recent rainfall  

• Winter snowmelt  

• Summer dry conditions  

During wet seasons, the waterfall may be vigorous.  

In late summer, it may reduce to a small trickle or disappear entirely.

This is an example of seasonal groundwater discharge.

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📏 FIELD OBSERVATIONS

From a safe position at the base of the bluff, observe:

• The horizontal seep line  

• The material above and below it  

• The flow rate at the time of your visit  

• Signs of erosion  

⚠️ Safety Note:  

Do NOT climb the bluff. Loose sediment and rocks may fall without warning.

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📬 LOGGING REQUIREMENTS

Email your answers to the cache owner.  

Do not include answers in your online log.

1. Describe the horizontal seep line. Does it appear lighter or darker than the surrounding sediment? Does it look softer or more compact?

2. Walk along the visible horizontal layer and estimate its width using your steps. Approximately how many steps wide is the visible seep line?

3. Estimate the height of the seep line above the base of the bluff (in meters). You may use your own height for scale.

4. Is water emerging along the entire horizontal layer, or is it concentrated at one point?

5. Based on current conditions, would you describe the waterfall flow as:

   - Low (light trickle)

   - Moderate

   - High (vigorous discharge)

6. Which part of the bluff appears to be eroding more rapidly — the material above the seep line or below it? Explain why.

You may log your find once answers have been sent.

Photos are welcome but not required.