Archive for December, 2009

The Knoxville Formation

27 December 2009

knoxville fm

As you go east along the south shore of Lake Chabot, the churned, oatmeal-colored volcanic rocks of the Leona Rhyolite give way to shale of the Knoxville Formation. The crust-building activity that created the Coast Range Ophiolite, of which the volcanic rocks of the Leona Quarry are the uppermost member, finally ended in the Late Jurassic some 150 million years ago, and ordinary seafloor mud and occasional sand began to cover the remains to form the Great Valley sequence. This long series of sedimentary rocks includes, bottom to top, the Knoxville Formation, Joaquin Miller Formation, Oakland Conglomerate, Shepard Creek Formation, Redwood Canyon Formation and Pinehurst Shale—a nearly uninterrupted sequence of rocks whose definitive exposures are in Oakland. The Knoxville is said to contain fossils of ammonites and Buchia, a mussel-like mollusk (here are some from farther north).

Lake Chabot

20 December 2009

Lake Chabot is a reservoir, but geologists still agree with laypeople that it qualifies as a lake.

lake chabot

That said, reservoirs differ from most lakes. They have steep banks and deep middles. This gives them a greater variety of habitats. They have jagged planforms on the map because they intrude up every little tributary valley instead of developing a nice rounded shape. And, of course, they’re temporary by any geological measure.

Around here, most lakes form as a result of tectonic activity or landslides. Lake Temescal started out as a sag pond on the Hayward fault. Clear Lake, farther north, formed when a landslide dammed a fork of the Russian River. It rose until it spilled eastward down Cache Creek, which has captured the watershed. Lake Chabot is about 50 feet short of that level; if Anthony Chabot had built his dam higher, the lake would now spill south into Castro Valley.

The valley of San Leandro Creek is so steep and its walls so high that I can easily picture it collapsing regularly, especially during big earthquakes, to have made ancient Lake Chabots in the past. So today’s lake isn’t so unusual for the region.


Get every new post delivered to your Inbox.

Join 9,469 other followers