What does a triangle showing rocks falling down a slope mean?
There is a risk of falling or fallen rocks ahead.

A red-bordered triangle showing rocks falling down a slope.
What you must do
Watch for rocks or debris on the road and be ready to slow down or stop.
Where you will see it
On roads cut into hills or cliffs, common in upland and coastal areas.
Test yourself
The theory test asks you to recognise signs like this one. Try the road and traffic signs short test (with an easy and a hard set), or read the signs revision notes. Source: Know Your Traffic Signs.
Other common road signs
Meaning based on Know Your Traffic Signs and The Highway Code. The sign image is an official Department for Transport sign (Crown copyright) from Wikimedia Commons, reused under the Open Government Licence or the public domain. Always check the current Highway Code on gov.uk. Not affiliated with or endorsed by the DVSA.