Archive for November 27th, 2009

A bit of the Jurassic in southern California

November 27th, 2009

Waterfall (mostly dry) over Jurassic rocks in the Los Peñasquitos Canyon Preserve in San Diego County (N32.92712°, W117.17757°).

Waterfall (mostly dry) over Jurassic rocks in the Los Peñasquitos Canyon Preserve in San Diego County (N32.92712°, W117.17757°).

SAN DIEGO, CALIFORNIA–A nice geological interlude for this Wooster geologist during a family Thanksgiving.  This afternoon we visited Los Peñasquitos (meaning little cliffs) Canyon Preserve in San Diego County.  We walked about two miles along a trail to an exposure of Jurassic metaconglomerate.  The rock is interesting for several reasons.  The clasts are either highly angular (meaning parts are a breccia) or very well rounded; the clasts are volcanic in origin; and the matrix includes recrystallized belemnites.  The composition suggests that the main sediment source (if not the only one) was a set of offshore volcanic islands.

Note the very rounded and very anglar clasts in this metaconglomerate.

Note the very rounded and very angular clasts in this metaconglomerate.

This area has connections to Old California history.  Rancho Los Peñasquitos was where General Stephen Watts Kearny and his Army of the West rested after the Battle of San Pasqual in 1846.