This mountain range stands out in the landscape due to its horseshoe shape. Around it, there is a more or less flat area, formed by reddish-toned sand and clay rocks, 150 million years old. The mountain range has magmatic rocks, called dolerite, which are dark in colour and very hard, and are more recent than the previous rocks.
This magmatic rock is quite thick, over 60m thick, and is the remains of an ancient volcano and lava flow. Currently, this dolerite is mined to produce gravel, mainly used for road construction in the Lisbon and Tagus Valley region.
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