The Hajar (hajar is rocky in Arabic) Mountains run from the Musandam Peninsula, south-easterly to Ras al Hadd, the eastern tip of the Arabian Peninsula, a length of almost 600 kilometers along the coast of the sultanate. A natural passage, the Sumail gap, separates the mountains into eastern and a western sections. The highest summit, the Jabal Shams (3010meters) is situated in the Western Hajars Mountains and belongs to the Jabal Akhdar (Green Mountain) .
The rocks of this region are predominantly oceanic crust and ophiolite (rocks formed in the mantle layer of the earth), and pushed to the surface approximately 90 million years ago.
Rain falling on the high flanks of the mountains supplies an artificial system of irrigation canals, known as the aflaj, providing hundreds of mountain oases with water.
Until the latter part of the 20th century Oman was politically divided. The coastal area was ruled by a sultan in Muscat, while the interior was controlled by a religious leader (imam).
It is this region in the Western Hajars, including the cities of Nizwa, Bahla, Jabreen, Nakhl and Rostaq, where the temporary seat of government of the imams was located. Nizwa is still called today the secret capital of Oman