Oregon rocks: Rock pattern and texture caused by spheroidal weathering in cliffs at Smelt Sands State Park on the Oregon Coast in the U.S.A. as seen from Trail 804 (7)

Here is something geological that I have never noticed in my usual haunts in the UK. These strange patterns and textures were observed in cliff rocks at Smelt Sands State Park on the Oregon Coast in the U.S.A. They were visible from the famous Trail 804. At first I thought they represented some sort of turbulant conditions in the distant past that had disturbed the deposition of rock layers. However, a bit of research seems to indicate that they are examples of spheroidal weathering of the rocks. This would appear to be the physical and mechanical result of chemical decomposition processes in the rock.

The geological forms shown in these photographs seem to match the illustrations of spheroidal weathering in a textbook I picked up while over in Oregon. According to Harold L. Levin, author of Contemporary Physical Geology published by Saunders College Publishing, 1990, ISBN 0-03-031139-x

spheroidal weathering is a term used to describe the spalling away of concentric surficial shells of the rounded surface of a boulder or rock mass. Such onion skin weathering is believed to result primarily from the mechanical effects of chemical weathering. When feldspars decompose, the clay product has greater volume than the parent feldspar. The increase in volume disrupts the interlocking texture of mineral grains in the rock and causes breakage and separation of the layer of partially weathered rock near the surface.

On smaller rocks and boulders this process is known as spheroidal weathering while the loss of concentric plates from larger rock surfaces is termed exfoliation.

Again, I am hampered by my lack of geological knowledge. So if anyone reading this can correct any errors I may have made, I would be pleased to hear from them.

The baseline for me is that I like the way the rocks looked – the colours, patterns and textures that to my eye constitute wonderful natural abstract images.

View of the beach at Smelt Sands State Park on the Oregon Coast, U.S.A. from Trail 804, where the spheroidal weathering is visible in the iron-stained cliff rocks (8)

Rocks of the Oregon Coast: Rock pattern and texture caused by spheroidal weathering in cliffs at Smelt Sands State Park on the Oregon Coast in the U.S.A. as seen from Trail 804 (9)

Rocks of the Oregon Coast: Rock pattern and texture caused by spheroidal weathering in cliffs at Smelt Sands State Park on the Oregon Coast in the U.S.A. as seen from Trail 804 (10)

Rocks on the Oregon Coast: Rock pattern and texture caused by spheroidal weathering in cliffs at Smelt Sands State Park on the Oregon Coast in the U.S.A. as seen from Trail 804 (11)

Rock colour, pattern and texture in spheroidal weathering: Rock pattern and texture caused by spheroidal weathering in cliffs at Smelt Sands State Park on the Oregon Coast in the U.S.A. as seen from Trail 804 and showing colours of decomposing iron minerals (12)

COPYRIGHT JESSICA WINDER 2011

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