Monday, July 07, 2014

Big Creek

I spent half an hour exploring the beach between the mouth of Big Creek and the much smaller creek at the Muriel Ponsler Wayside, a few miles north of Heceta Head.

The smaller creek emerges from a narrow ravine cut into the terrace (more later) and spreads out on the upper beach before cutting through a broad sand bar and escaping to the sea. Streams flowing across beaches are as instructive as they are ephemeral. This one exposed a cobbly substrate below several inches of beach sand.  The northerly wind was causing small dunes to cascade down the north bank of the stream. The stream channel was carving a steep cliff into the south bank. There were terraces and braided channels and small deltas.  But it will all be erased and completely rebuilt after the next high tide.

The elevation of the terrace is quite low here, apparently a result of the modest long-term uplift rates on this section of the coast. Highway 101 is built on the terrace, when it's not crossing Big Creek (Big Creek Bridge, for an explanation of bowstring arch bridges and a nice photo of this section of coast).

The modern shoreline has cut a small cliff into the Pleistocene alluvium of the terrace (it looks like the underlying wave-cut platform is below beach level). The cobble storm berm of the modern beach has pushed up against the terrace and modern sand dunes have formed on top.

The wind was blowing from the north, forming small dunes and a lot of other cool eolian features.

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