Jeita Grotto - Geology

Geology

The Jeita grotto is located within the Lower-Middle Jurassic strata of Keserouane which has a stratigraphic thickness of 1,000 metres (3,300 ft) and consists of dolostone and micritic limestone. The Keserouane formation was exposed to air by a local uplift during the Late Jurassic-Early Cretaceous. The Keserouane strata became karstified after its aerial exposure and then was buried during the Cretaceous. In the Neogene, upon the final uplift of Mount Lebanon, this early karstification phase was reactivated.

At the Nahr al-Kalb valley, the impervious Upper Jurassic volcanic rocks and Lower Cretaceous sand slant almost vertically forming a hydrogeological barrier and forcing the outlet of the Jeita underground river to the surface. This barrier could be the reason for the westernmost, large cave chambers with heights exceeding 60 metres (200 ft). Karstification of the Keserouane limestone was further intensified by the steep topography of the area and the volume of precipitation over the Lebanon (more than 1200mm).

A ratio of total slope gradient of 1/100 is calculated between the entrance of the cave and its inland extremity. This shows its smooth flat course even though it is sometimes interrupted by several small cascades and rapids. From the west, the Jeita cave starts with large halls and meanders. Through some rapids, the dimension becomes narrower. Going forth, it is spacious with the Thompson’s cavern (250 metres (820 ft) long and 60 metres (200 ft) wide), Grand Chaos (500 metres (1,600 ft) long) and Mroueh’s Hall (200 metres (660 ft) long and 50 metres (160 ft) wide). The latter two are floored with collapsed blocks. The cave ends with a Y-shaped pattern, where, each branch ends with a siphon.