Creating cobbles - Oregon Historical Markers on Waymarking.com

This sign is located at the Yaquina Head Lighthouse parking lot

Marker Name: Creating cobbles
Marker Text: Cobbles, pebbles, and sand are the result of boiling lava meeting the cold ocean, followed by 14 million years of weather and erosion.
Fragments of ancient lava
Hot basalt exploded upon contact with cold sea water and intermixed with quickly chilled volcanic glass to form a “breccia.” The cobbles on this beach are weathered remains of these exploded fragments.
Sea water alters the glass surrounding the basalt fragments into a mineral colled palagonite which is easily washed away. The water then attacks the fragment causing its corners to become more and more rounded. The cobbles are further rounded and made smaller by beach wave action.
In the summer there is some sand on these beaches, but winter waves wash most of the sand out to sea leaving almost all cobbles on the beach.
Between tides
Tidal forces shape the shoreline and bring life to the intertidal area.
Tides are caused by the gravitational pull of the sun and moon. Here and elsewhere in the coastal Pacific Northwest, there are almost always two high and two low tides daily.
The tides would be easy to predict if they occurred exactly every six hours. However, because the tides follow a cycle that is slightly more than six hours long, the whole sequence is repeated later each day.
Mean (average) lowest low tide is used as the base level for most coastal charts and tide tables. When ships enter or leave harbor, the depth of water at mean low tide is more important to them than mean sea level.

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