The Sierra as observed today formed during the Paleogene and Neogene Periods (66 to 1.8 million years ago) from the collision of the African and Eurasian continental plates.

[1] On the southern side of the range, several long, narrow river valleys lead off towards the south-west, separated by a number of subsidiary ridges.

Geologically, the range is composed chiefly of soft micaceous schists, sloping steeply to the north, but more gradually to the south and south-east.

Above 2,500 metres (8,200 ft) the climate is subarctic (Dsc), due to the location's high elevation and low summer precipitation.

With June and September being around the threshold of 10 °C (50 °F) in mean temperature to avoid the subarctic classification, the climate at a slightly lower elevation is humid continental (Dsa/Dsb).