Howdy Chris,

This is interesting because I have a 15 year old here who is getting hooked on geology. We went rock hounding for the last three days. We found a really good specimen of obsidian in a stream bed about 4800' elevation. We both knew that is was not formed up here in the PR. This whole area is punctuated by Julian schist, Stonewall quartz, San Marcos gabbro and Bonsall tonalite. There is one large pegamite dam about a mile from the house. So the question came up; How did the obsidian get up here?

Well, we have formed a hypothesis. We know that the local Indian population used obsidian to make arrow heads and spears. We also know from earlier anthroplogy studies that the local Kumyiyaa Indians traded with the Yuma Indians. The Yuma Indians inhabited the area of Obsidian Buttes, near the Salton Sea. We believe the obsidian was brought up here by an American Indian at some point in the past. Our best evidence is the condition of the specimen..

This piece of ryolite is about 5cm long, and is 3cm X 3cm in depth and width. Almost square. What is really interesting is that there are 3 well placed conchoidal fractures along the width of the specimen. Sure looks like flint knapping to me. We also agreed that because of the specific gravity of ryolitics, about 2.6, that the specimen is much too light to have been fractured by boulder blows during a heavy erosion event in the drainage. Because the sharpest edges of the specimen a lightly worn, we place the specimen here about 200 to 500 years ago. Any longer, it would be more rounded by erosive forces.

Any single rock tells a huge story. The clues are facinating.

This weekend, off to the desert and a search for evidence of the Coyote Creek Fault and the 1968 Borrego Mountain quake rupture. More coming. Mike is learning more during spring break than he ever does at school....(public type)

Glen