Take Interstate 8 east about 13 miles past the Boulevard exit to the viewpoint (now closed). We went on to the next exit, and turned left under the eastbound lanes of Interstate 8 to a pullout in fresh boulders.

The M-B facies of the La Posta pluton is exposed in the large outcrop adjacent to the overlook. Several features in this unit appear to set it apart from the remainder of the pluton. Despite being gradational into the S-B facies,

1. the An content of the plagioclase in the M-B facies drops abruptly from 35 to 25,
2. its Zr drops from around 500 ppm to around 200 ppm, and
3. Fe/(Fe+Mg) in biotite increases from 0.70 in the S-B facies to 0.82 in the M-B facies.

This has been interpreted to mean that the S-B facies is a hybrid or contaminated tail that intruded into the head of the ballooning La Posta diapir. In effect, its intrusion created a zoned magma chamber that crystallized inward to create the mapped zones but still preserved the original compositional differences between core and marginal zones.

This is the end (Doors tune plays...)