Geology of the Lakes Basin and Haskell Peak

General Description:

This part of the “Lost Sierra” offers extremely varied geology, from Pleistocene glacial moraines-- not even rocks yet! --to twice-, even thrice-metamorphosed volcanic and sedimentary rocks formed in an ancient (Devonian) island arc, probably not far from the North American continental margin at that time. Rocks of intermediate, Tertiary age also are present, and include the Oligocene “ignimbrites” (ash-flow tuffs) and conglomerates on Haskell Peak. The tuffs are now well-dated and are correlated with tuffs in western Nevada that originated in calderas still farther east; we will spend 1 ½ days on Haskell Peak studying these and other rocks. One-half day will be devoted to investigation of Miocene “lahars” (volcanic mudflows) near Chilcoot and Clio.

The final three days of the workshop are set aside for study of the oldest, metamorphosed rocks in the area, those that comprise the Devonian island arc. On the first day, the Elwell Formation will be examined in the Packer Lake-Dugan Pond area, where wonderful exposures (“outcrops”) reveal the secrets of formation of the rare rock type “peperite”, found in association with phosphatic chert and andesitic sills. At Dugan Pond, we will search for ammonoids that have been baked by contact metamorphism adjoining a gabbro stock. The ammonoids are extremely important in arriving at the late Devonian age for the island arc. The next day will be spent circumnavigating beautiful Upper Salmon Lake, looking at pillow lavas, debris flows and turbidites in the Taylor Formation, and more chert, sills, and “dispersed” peperite in the Elwell Formation. Not to be neglected, we will see, in the oldest, Sierra Buttes Formation, pumiceous pyroclastic flows that were erupted onto the Devonian sea floor. The last day we will work our way “down-section” through the Taylor, Elwell, and Sierra Buttes formations along Big Bear, Silver, and Hellgrammite Lakes. Concentrating on the Taylor Fm., we will see more pillow lava, peperite with tuffaceaous (rather than chert) host. Arriving at the Elwell Fm., we will see that it has greatly “expanded” (thickened) by intrusion of basaltic sills, and still contains lots of peperite like that viewed the day before. At the end of the traverse, we will find monolithologic dactite breccia, perhaps representing a volcanic “dome” in the Sierra Buttes Fm.

Should many of these rock names be foreign to you, it is because the rocks are indeed rarely encountered and thus seldom described in the literature. Not to worry-- we will have fun looking at them and trying to determine how they formed anyway. All levels of experience are welcome, from none to the graduate level.


Elwood (Woody) Brooks is Emeritus Professor of Geology at California State University, East Bay, and currently is a Research Associate in the Geology Department at the University of California, Davis. His Ph.D. in geology was earned at the University of Wisconsin, Madison. He taught CSU, Hayward’s summer field geology course for 25 years, starting in 1970, in the Lakes Basin and on Haskell Peak.

Class Schedule

Plan to arrive at the Sierra Nevada Field Campus on Sunday. We will get acquainted after dinner that evening, and an introduction to the class will follow. All instruction will be in the field, “on the outcrop”. Be prepared for moderately strenuous hikes, on and off the trails, on the third, fourth, and last day of the workshop. Evenings are your own, for relaxation.

Supplies and Other Useful Items

Field Equipment

Camping gear

Helpful Reading