Boulder: A Rock Too Big to Lift
A boulder is a large rounded rock, worn smooth and usually moved there by ice or water. Geologists even have a size at which a stone becomes one.
A boulder is a large detached rock, typically rounded by being carried and tumbled. Geology draws the line at 256 millimetres across: anything bigger is a boulder, anything smaller is a cobble.
- A large rounded rock. *A boulder blocked the path.*
- A rock carried by a glacier. *Erratic boulders left on the moor.*
- A climbing problem's rock (bouldering). *He worked the boulder for an hour.*
Word forms
| Form | Word | Example |
|---|---|---|
| Noun | boulder | A granite boulder. |
| Plural | boulders | Boulders on the shore. |
| Activity | bouldering | Indoor bouldering. |
| Person | boulderer | An experienced boulderer. |
| Compound | boulder field | A boulder field below the cliff. |
How it is used in a sentence
- A boulder had rolled onto the road overnight.The everyday sense — large and immovable.
- Glacial erratics are boulders left far from their parent rock.Geological usage.
- Bouldering needs no rope, only a mat.Climbing short routes close to the ground.
Rock sizes, smallest to largest
- Sand — grains you can barely see individually
- Gravel — from grit up to about 6 cm
- Cobble — roughly 6 to 25 cm
- Boulder — above about 25 cm
- These are the standard geological grades, not just impressions of size
From a Scandinavian source — Swedish dialect *bullersten*, a noisy stone, from the sound of large rocks rolling in a stream. The earlier English form was *bulderstone*, later shortened.
Boulder or rock?
Rock is the general word and can mean the material itself. Boulder always means one large, detached, roughly rounded piece.
What is an erratic?
A boulder carried by a glacier and dropped far from where it formed, so its rock type does not match the ground around it.