Do NOT cross the fence! These coordinates will take you to One of the largest glacial erratics in North Dakota. Glacial erratics are known all too well by most North Dakota farmers, who have laboriously piled them in the corners of their fields. I personally have done my fair share of picking these rocks in the field and transporting them to the “rock pile” as we called it. Early settlers used the stones for the foundations of their homes and farm buildings, while other people built whole structures with them. Locally, the rocks are usually referred to simply as field stones or boulders or just *!?/#! rocks, depending on one's point of view. Especially large rocks, too large to move out of the way, are sometimes called "leaverites" (as in "leave-er-right" there).
Geologists though, know the rocks as glacial erratics, rocks that differ in lithology from the surface on which they are found. The term "erratic," with reference to rocks, apparently dates to 1779, when de Saussure, a Swiss geologist, described granite boulders lying on top of limestone in the Jura Mountains in Switzerland. His term, "terrain erratique," is derived from the Latin erratus, "to wander," and means, literally, "ground that has wandered." Geologists most commonly use the term erratic to refer to boulders and cobbles left behind by glacial ice.
🧊 What Are Glacial Erratics?
Glacial erratics are rocks that were picked up and transported long distances by glaciers during the Ice Age and then dropped when the ice melted.
In North Dakota the ice was part of the Laurentide Ice Sheet which covered much of the state during the Pleistocene. Because of this transport:
- These rocks are often very different from the local bedrock (usually sedimentary in ND).
- Many come from far north in Canada, including granite, gneiss, and other ancient igneous/metamorphic rocks not native to North Dakota’s geology.
Map and information sourced from candiangeographic.ca
How did the glaciers move the boulders?
Glaciers moved huge boulders—glacial erratics—through a combination of freezing, dragging, and riding inside the ice. Here’s the process step by step, in plain terms:
🧊 1. Plucking (How boulders got into the ice)
At the glacier’s source area (often the Canadian Shield for North Dakota erratics):
- Meltwater seeped into cracks in bedrock.
- When temperatures dropped, the water froze and expanded, loosening blocks of rock.
- As the glacier flowed, it froze onto those loosened blocks and tore them away.
This is called glacial plucking, and it’s how massive granite boulders first became part of the glacier.
❄️ 2. Transport (How the boulders traveled long distances)
Once inside the glacier, boulders were moved in three main ways:
🪨 A. At the base (dragged along)
- Some boulders were embedded at the glacier’s bottom.
- As the ice flowed, these rocks were dragged over the ground, scraping and polishing both the rock and the bedrock beneath.
- This creates striations and grooves on rocks and surfaces.
🧊 B. Within the ice (carried like cargo)
- Many boulders became frozen inside the ice, sometimes hundreds of feet above the base.
- The glacier flowed slowly—often a few inches to a few feet per day—but over thousands of years this moved rocks hundreds of miles.
🏔️ C. On top of the ice
- Some rocks fell onto the glacier from valley walls.
- These rode on top until melting caused them to drop.
For North Dakota, most erratics were within or at the base of the Laurentide Ice Sheet, moving south from Canada.
🕰️ 3. Movement Was Slow but Powerful
Glaciers don’t slide fast like rivers:
- Typical speeds: tens to hundreds of feet per year
- Time involved: thousands to tens of thousands of years
- Weight didn’t matter much—ice behaves like a very thick, flowing solid
That’s why even multi-ton granite boulders could be transported intact.
🌡️ 4. Deposition (How the boulders were left behind)
When the climate warmed:
- The glacier thinned and melted.
- Ice could no longer support the rocks.
- Boulders were dropped in place, often far from any similar bedrock.
This left erratics scattered across places like North Dakota’s plains, fields, and old lake shorelines.
Diagram and information sourced from Antarticglaciers.org
🪨 Lithology at This Site
Lithology describes the observable physical characteristics of a rock, such as its color, texture, and crystal or grain size. These features help geologists determine how and where a rock formed.
The lithology of this boulder is noticeably different from the surrounding landscape. While most surface rocks and soils in this area are derived from sedimentary bedrock and glacial sediments, this erratic displays a crystalline texture typical of igneous or metamorphic rocks. This contrast in lithology provides important evidence that the boulder did not form here and was transported by glacial ice.
🧊 What This Boulder Tells Us About the Ice Here
This erratic is not just a transported rock—it is physical evidence that glacial ice once covered this exact location.
For a boulder of this size to be deposited here:
- Ice thickness must have been hundreds to thousands of feet thick
- Ice must have been moving southward from the Canadian Shield
- Melting must have occurred in place, not by water transport
Unlike river-transported rocks, glacial erratics:
- Are angular to sub-rounded, not smoothly rounded
- Can sit on uplands, not just valleys
- Appear isolated, without similar nearby bedrock
This boulder therefore acts as a marker of maximum glacial extent and ice dynamics in this part of North Dakota.
🧠 Why This Matters for North Dakota
- Granite boulders in ND prove the ice came from the north, since ND has almost no granite bedrock.
- The size and distribution of erratics help scientists map ice flow direction and ice limits.
- Scratches on boulders show which way the glacier moved.
Additional earthcache information sourced from www.dmr.nd.gov and www.britannica.com.
To log this EarthCache, send your answers to the questions. Do not post answers in your online log.
EarthCache Logging Questions
Question 1 – Lithology vs. Landscape (Observation + Comparison)
Observe the erratic and the surrounding ground.
Describe one visible characteristic of the erratic that clearly differs from the surrounding soil or nearby surface rocks.
Explain what this difference tells you about whether the rock formed locally or was transported.
Question 2 – Ice Thickness & Ice Presence (Reasoning)
This erratic sits above the surrounding terrain rather than in a stream bed.
Based on its position and size, explain why this rock indicates direct deposition from melting glacial ice rather than transport by flowing water.
What does this imply about how thick the ice must have been at this location?
Question 3 - Most glacial erratics in North Dakota came from the Canadian Shield.
Using:
- one observation from the erratic (rock type or texture), and
- one fact about North Dakota’s local sedimentary bedrock
Explain how this boulder supports evidence of southward ice movement by the Laurentide Ice Sheet.
📷 Question 4 -Photo Requirement:
Please include at least one photo with your log showing you or a personal item with the erratic.
Thank you to everyone who finds my earthcache! Happy ‘Cachin!