🌎 The Waterloo Alcove – Erosion of the Woodbine Formation

The cave you’re visiting is carved into rock from the Woodbine Formation, a Cretaceous-age geologic layer (~95 million years old) found throughout the Denison / Lake Texoma region.

This formation is made of:

• Sandstone

• Siltstone

• Clay-rich shale layers

• Iron-rich cemented zones

These layers erode at different speeds, a process called differential erosion.
That process is exactly what formed this shallow cave (alcove) at Waterloo.

🪨 What’s Unique About This Location?

Unlike limestone caverns found in other parts of Texas, the Waterloo cave is formed where:

• Softer Woodbine layers (shale & siltstone) eroded quickly from rain runoff,

• Harder sandstone layers above remained intact, creating an overhang,

• The results: a shallow alcove, not a deep cave.

The location clearly shows:

• A hard, resistant caprock on top

• Softer, crumbly layers beneath

• A horizontal notch cut by water flow

• Orange-brown iron staining typical of the Woodbine

These are all local, observable, site-specific features.

⚠️ Safety

• Stay at the entrance; you do not need to enter.

• Watch footing—mud and water may be present.

• No removing or breaking rock.

🧭 UPDATED LOGGING TASKS (Now directly tied to the lesson)

1️⃣ Identify the Differential Erosion at This Site

Look at the alcove wall. Describe the difference between the upper and lower rock zones. What makes the lower layer look more eroded than the upper one?

(Expected: upper = harder sandstone; lower = softer shale/siltstone eroded back more.)

2️⃣ Caprock Observation

Examine the ceiling/overhang.
Does the rock at the top appear:

• harder?

• more solid?

• less crumbly?

Explain how this harder caprock contributed to the formation of this alcove.

(Expected: the resistant upper sandstone prevented collapse, while lower layers washed out.)

3️⃣ Water Flow Interpretation

Look at the floor and rear wall of the alcove.
Based on what you see, describe how runoff water likely entered and carved this space.
Is there:

• a smooth channel?

• sediment build-up?

• undercutting?

• streaks/staining showing flow direction?

Explain what these signs tell you about how water shaped this exact spot.