Basalt on the Beach at Strawberry Hill

The outcrops of basalt on the beach at Strawberry Hill on the Oregon Coast are the remnants of a wave-cut platform. The rocks were formed in Late Eocene times and the basalt can be very different in appearance from one place on the shore to another (Lund 1971). This is because the rocks represent a series of events not just one.  The basalt can be dense, hard, and uniform in structure or it can be fragmentary within a matrix – like a breccia. The basalt rocks come from volcanic eruption lava flows and pyroclastic flows. The rock can look different depending on whether it surfaced in the air or under the sea. In places the shore is traversed by lines of darker more angularly faceted basalt in dikes. Dikes were probably rifts or large cracks in the side walls of the volcano and the layers of volcanic deposits upon them. Dike are more readily formed in the fragmentary types of basalt. Molten lava oozed through the rifts and solidified relatively slowly.

The photographs in the gallery above show some of the different textures in the basalt on one particular beach. The images reflect the varying ways in which the basalts were formed and subsequently weathered. In The Pleistocene, when the sea was at a higher level than present, the rock was worn away and generally flattened by wave action to form a wave-cut platform or bench on which loose beach stones, pebbles, and sand accumulated. These loose materials (the beach deposits) were left high and dry on the platform as the sea level receded at a later date. These stranded beaches can now be seen in the sea cliffs and in the raised beaches or terraces at the top of beach in many places along that stretch of coast.

REFERENCE

Coastal landforms between Florence and Yachats, Oregon by Ernest H. Lund, February 1971, The ORE BIN, Volume 33, No. 2, pp 21-44.

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