Garnet- and sillimanite-bearing quartzofeldspathic gneiss and granofels form a septum at McClures Beach between the tonalite (Kgd) of Tomales Point and the granodiorite and granite of Inverness Ridge (Kgr) (Ross, 1977, 1978 referenced in Clark and Blake). There are text-book examples of cross-cutting relationships on bedrock exposures exposed in the beach south of the path down to the beach and abundant examples of large boulders with a wide variety of examples running from granitics to gneisses and almost everything in between. These tend to be best exposed during the winter period.
The Granitics (from Clark and Blake)
(Kgd) Tonalite of Tomales Point -- Hornblende-biotite tonalite that contains dark diorite inclusions forms the basement of Tomales Point. This rock is lithologically similar to the tonalite that crops out at Bodega Head about 10 km to the north, and both are probably part of the same granitic mass (Ross, 1978). K-Ar dating of hornblende in the tonalite of Bodega Head yields an age of 94.3 Ma (Late Cretaceous) (Evernden and Kistler, 1970; recalculated using new constants in Dalrymple, 1979).Other McClures Beach items of interest
(Kgr) Granodiorite and granite of Inverness Ridge -- Granodiorite and granite are exposed along Inverness Ridge, where dikes and masses of aplite and alaskite are locally abundant. The granodiorite has 2 to 12 percent biotite and hornblende, about 30 percent quartz, 25 to 35 percent K-feldspar, and the remainder plagioclase. Chemically, it is about 70 percent Si02, 15 percent Al203, 2.6 percent Fe0, 2.8 percent Ca0, 3.3 percent Na20, and 3.4 percent K20 (from Ross in Clark and others, 1984; and Ross, 1984). K-Ar dating of biotite in this granodiorite gives an age of 82.0 Ma (Late Cretaceous) (Curtis and others, 1958; recalculated using new constants in Dalrymple, 1979), and fisson-track dating of sphene from this rock yields ages of 82.7 ± 6.9 and 86.8 ± 7.4 Ma (Late Cretaceous) (Naeser and Ross, 1976).
The Quaternary dune deposits are visible from the path to the beach. Slope modifications by gopher activity is very evident as is the impact on erosion being generated by the expanding Tule Elk population in this area. Winter wave concentration of black sands can be common on the beach. The sands here apppear to be almost entirely locally derived and can contrast dramatically to the Franciscan chert-rich beaches to the south.Back to Pt Reyes Metamorphics Page
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