Wednesday, June 20, 2007
Thrusting from Ice? or from Below?
The above photos were taken from parts of the cliffs, both south and north of Ebby's Landing- near Coupeville. This represents a portion of the island where melt water flowed west from Penn Cove glacier, created a drainage which remains today. Later, the sand which washed westward towards the present Admiralty Passage was picked up by the wind and deposited along the ancient beaches. This action allowed Dunes to form above the beachline (similar to that on the Oregon coast today), and to form coastal dunes just above the waterline. These dunes are now stabilized by vegetation, and can be seen by walking the trail above the Ebby's Preserve, where the stabilized sands "Lip Over' the farmland- remaining higher than the tilled soil.
Ignore the cross-bedding shown in the layers- which is old dune markings. Look for parallel fractures in the sands, where the beds have slid along the newer discontinuities. The fractures allow movement of both the sands and water which percolates through them. The tubular outlet indicates seepage from such fractures or faulting. A buckle, which occurs in one of the cross-beds, is unmistakenly a result of thrusting. This could be from the later glaciers sliding southward over the beds or the underlying bedrock sliding northward under them.
Coastal Dunes, after Glacial Melting
Ebby’s Landing is an excellent location to view the effects left by the last lobe of the Ice Age- where it traveled towards the west (probably from Glacier Peak to the east, through Penn Cove). This area is just west of the picturesque town of Coupeville, which sits next to Penn Cove- which was formed by the push by gravity of ice from mountains to the east about 14,000 years ago.
Coastal dunes form from wind-driven sand, which is delivered to the beaches by rivers as the ice melts. The sand is worked by the offshore currents, to be moved along the beach by daily tidal and storm currents. At some parts of the day, the sand is above the waterline, and subject to drying and aerial movement by onshore breezes. The winds are normally from the southwest, but heating of land during the day causes the air to ascend- bringing in wind from the beach and sea below the rising air. This sand-laden air dumps the sand as it climbs the beach, forming coastal dunes. These dunes are trapped by any hills or vegetation, forming wandering sand deposits which oscillate along the beaches. Consequently, there are many cross-bedded sand deposits seen in the now eroded cliffs. The deposits may be thin, since they form daily; varying direction winds create the cross-bedding as the temperature changes with the Sun’s elevation (nights bring off-shore breezes, when the seawater has warmer temperatures than the land). When one looks vertically, the dunes may have inter-dunal intervals which appear as discontinuities. These inter-dunal features may look like fractures, but they do not have paths for water flow similar to fractures- their permeability is usually less, since they have a fine layer of dust in the discontinuity.
Look at the accompanying photos, above and below this blog, to determine whether you can discriminate between fractures (at least two non-horizontal parallel lines in the sands, ignoring the bedding planes) and inter-dunal intervals.