Mountains formed when horizontal compressive forces, usually at convergent plate boundaries, buckle and crumple layers of sedimentary rock into a series of upfolds and downfolds.
- An upfold is an anticline (arch) and a downfold is a syncline (trough); the layers were originally laid down flat, often in shallow seas (geosynclines).
- Young fold mountains (formed in the Cenozoic, broadly the last 65 million years) include the Himalayas, the Alps, the Andes, and the Rockies; they are high, rugged, and seismically active.
- Old fold mountains (Palaeozoic, hundreds of millions of years old) are worn down and rounded, such as the Aravallis in India (among the oldest in the world), the Urals, and the Appalachians.
- The Himalayas formed from the collision of the Indian and Eurasian plates and are still rising.
- Fold mountains are distinct from block mountains (formed by faulting) and volcanic mountains (built by eruptions).
The anticline/syncline pair, the young versus old examples (Himalayas versus Aravallis), and the link to convergent plate boundaries are recurring map-and-fact items.
Anticline (upfold) versus syncline (downfold); young fold mountains (high, active, like the Himalayas) versus old fold mountains (worn, like the Aravallis); fold versus block (fault) mountains.
Mountains from compression and crumpling of rock layers into anticlines and synclines; the Himalayas are young, the Aravallis old.