Led by Mr. Randy Campbell, sponsor of the Geologic Excursion Club, a group of four students and three faculty members visited Graffiti Rock.  Students joining the trip were Ahmed Alshibi, Mohammed Albeik, Osama Abduelhadi and Qais Saleh.  Faculty members assisting Mr. Randy were Mr. Charles Cejka, Mr. Robert Deschamp.

Coral Reefs, Inselbergs and Kopjes (koh-pees)

The first stop of the day was atop the Tuwaiq Escarpment, where Excursion participants discovered indications that the area had once been submerged in shallow tropical seas.  Here were the fossilized remains of an ancient coral reef – dating, in fact, from the late Middle Jurassic Period, around 160 million years ago.

In the soft, powdered limestone trucked in to lay a track along the edge of the escarpment, teachers and students found fossilized examples of animals from another marine environment, bivalves and brachiopods that lived on an ancient sea floor.

The Excursion drove on to Quwayiyah, where the earliest sedimentary rocks of the Arabian Platform meet the much older metamorphic and igneous rocks of the Arabian Shield.

Beyond Quwayiyah, the landscape is dotted with low outcrops of granitic rock called kopjes.  Kopjes are great heaps of rounded boulders.  Before these outcrops became kopjes, they were rock domes called inselbergs, and before that, the granitic rock was magma (molten rock) cooling and crystallizing deep within the Earth.  The Excursion climbed a low kopje and collected fine examples of the quartz that is weathering out of the surrounding rock.

Graffiti Rock

The Excursion reached Graffiti Rock following a bit of off-roading highlighted by Mohammed Albeik’s expert descent of a rugged track down a 20-meter sandstone bench.

At this sandstone outcrop, oxides of iron and manganese have left a black coating on some of the yellow rock faces.  Prehistoric humans took advantage of this geology to chisel pictures onto the stone.  The figures are thought date from about 8,000 years ago, and include images of ibex, oryx, lions (or perhaps hyenas), dogs, scorpions, horned cattle, ostriches and people on camels.  Students and teachers on the Excursion found what appears to be a giraffe (though with hump like a camel!)

Sedimentary Formations

East of Quwaiyah, some of the first sedimentary rocks are the limestones and sandstones of the Khuff Formation.  Khuff Formation rock dates mostly from the Middle and Late Permian period, making it about 270-250 million years old.  Excursion participants stopped to clamber up the hills and look for traces of ancient life.

Nearer Riyadh, the excursion made another halt to explore Khashm adh Duwayban.  This hill is part of the Marrat Formation.  Like the Khuff Formation, the rock here is limestone with interbedded layers of sandstone.  The sediments were deposited in Early Jurassic seas, however, and so are less than 200 million years old.

Geological and archaeological images and information from:

Grainger, David.  The Geologic Evolution of Saudi Arabia:  A Voyage Through Space and Time. Saudi Geological Survey. Jeddah: 2007.