December 17, 2007
Right there, midway between Desert Hot Springs and Palm Springs is Garnet Hill. We've all driven by it on Indian or I-10 a zillion times, but do we ever stop to smell the rocks? I did today. The main claim to fame of Garnet Hill is that, situated as it is in the middle of the eastern mouth of San Gorgonio Pass, it takes the full brunt of the wind and the occasional sand storms. As a result, it's rich with ventifacts; that is, rocks that are grooved and pitted by the sand. On Garnet Hill these grooves all run west to east, as does the wind.
I don't know if Garnet Hill is BLM land or private. There are no signs anywhere to give me a clue. There are a couple of gas pipelines running across it, and the pipeline company seems to maintain a couple of dirt roads, but there are lots of other Jeep tracks running all around.
You can see all my photos here, and these are some samples:
Garnet Hill is located west of Interstate 10 at the Indian Avenue exit. It is about one mile long, oriented parallel to I-10, and lies between the highway and the railroad tracks. This small hill is an eroded fold in recent rocks that have been upwarped along the north side of the Garnet Hill fault, with subsequent erosion.
The hill is unique for the number of stratigraphic units exposed. Uppermost is the Cabazon Fanglomerate of late Pleistocene age. The Cabazon is a poorly sorted sandstone with boulders of gneiss and granite derived from the San Bernardino Mountains to the east that have been brought to the valley floor by the Whitewater River. Also lying on the surface, however, are boulders of crystallized limestone and granodiorite, rock types originating from the San Jacinto Mountains to the west. This striking difference suggests very recent uplift of the hill along its bounding faults.
Underlying the Cabazon Fanglomerate and appearing in scattered exposures along the south margin is the marine Imperial Formation of late Pliocene time, seven to ten million years ago. The Imperial Formation is special to the valley since it is evidence of the only incursion of the Salton Trough by the marine waters of the Gulf of California. All this happened before the Colorado River blocked the sea from the valley by its delta. This arm of the sea extended into San Gorgonio Pass, filling the area with warm marine waters containing abundant shell fish, found today in the sandy portions of this formation.
The northwest slope of Garnet Hill faces San Gorgonio Pass. The hillside is covered with abundant granite boulders that are deeply grooved, pitted and polished by strong wind activity. Wind-polished boulders are called ventifacts. All the grooves are linear, and are oriented to the east entrance of San Gorgonio Pass. The amount and depth of the grooves in the granitic boulders suggests that the prevailing winds have been unchanged for centuries.
Very cool. Googled "Garnet Hill Fault" and found your site. Great photos!
Posted by: brian at Jan 9, 2008 6:32:30 PM