Monday, April 28, 2014
While Owen Beach was broad and stable and seemed well stocked with sediment, the situation changes as you walk north toward Point Defiance itself. Much of this stretch consists of relatively high sandy bluffs, eroding over the top of clay layers exposed at beach level. There's tilting to the beds, and some faulting, suggesting more structural activity than we typically see on Puget Sound bluffs. And as you approach the point the beach thins and there are dramatic ledges of older, more resistant materials outcropping at low tide. This stretch really deserves a post of its own, but this entry is actually about something else.
The best part is on the western side of the tip of Point Defiance, where a dramatic divot has been taken out of the beach. The beach simply vanishes -- dropping off into the head of a deep submarine ravine. Locals said that it had cut back significantly just this past winter. There wasn't much room to get around and there was something pretty spooky about this spot. I'd love to see some high resolution bathymetry of this place.
Rounding the point and heading back south along the Tacoma Narrows, the beach changes completely. There really isn't much beach on this side -- a combination, I suspect, of the steep topography/bathymetry and the strong tidal currents that probably carry most loose sediment off into deep water pretty rapidly.
Point Defiance is a distinctive feature in the geography of Puget Sound - something pretty special is going on here, most likely related to the underlying geology and its subsequent influence over glacial drainage patterns.
The shoreline of Point Defiance offers some interesting contrasts - as this and the next post will show. On the east side, Owen Beach is a broad gravel beach that shows little evidence of long-term erosion or accretion. Southeasterly winds blowing out of Commencement Bay probably wrap into this beach with enough energy to drive sediment northwestward. Northerlies coming down Colvos Passage do the opposite. The net result appears to be a fairly stable beach. Some new sediment may get here from the bluffs towards the Point, but I suspect most of that goes north and is lost over the edge (see next post).
The beach to the south is backed by a long seawall that supports the promenade that connects with the southern end of the park. At the northwest end, in front of the parking area, the wall ends and the sidewalk is stepped back far enough so that a modest berm and backshore remains, along with the normal drift logs. Again, it looks like a pretty stable beach, although I suppose on exceptionally high tides, waves can push debris back onto the lawn. But that's flooding, not erosion.
Farther north, concrete debris has been used to stabilize some old fill (on which a trail is built), and this is eroding (slowly) - but that's because the fill extended waterward of the natural beach profile. It would be easy to just pull it out.
Saturday, April 05, 2014
The Permian Sea that dominated this part of Pangaea 250 million years ago left the gypsum rich deposits that have now been recycled to form the dunes at White Sands (previous post). This sea (or some smaller connected seas) also gave rise to large carbonate reefs, one of which has been subsequently uplifted and exposed by erosion as the current Guadalupe Escarpment (Guadalupe Mountains National Park).
Guadalupe Peak is the highest point in Texas (8751'). El Capitan, the distinctive prow immediately to the south, marks the southern end of the Guadalupe Escarpment. Carlsbad Caverns was carved (dissolved, actually) into the core of the reef much later, aided by the presence of hydrocarbons in the adjacent basin that leaked hydrogen sulfide and produced sulfuric acid, a much more effective cave-forming agent than the more common carbonic acid.
The Capitan Reef faced eastward across the Delaware Sea (an arm of the larger Permian Sea). The Delaware Basin and the Midland Basin farther east are now both major oil and gas fields and driving south towards Carlsbad I was reminded of this by the pump jacks and well heads, the pipe yards, and the lines of red Halliburton trucks. I spent the fall of 1981 in Schlumberger's training program in Midland and spent weekends exploring west Texas in my new little pickup truck. That was a long time ago!
I suppose there may have been beaches on the shores of this Permian ocean - but the environment behind these reefs was probably pretty quiet and the shores were probably muddy and perhaps vegetated (although I suspect they didn't look like the mangroves or salt marshes you might find in similar settings today).
Posted by Gravel Beach at Saturday, April 05, 2014
Labels: new mexico
Friday, April 04, 2014
I try to stick to a fairly standard theme in this blog. Posts are places (not issues or topics or events, and only occasionally people). They are built around photos I've taken myself (usually within days or sometimes weeks). The subject material is usually a blend of coasts and physical geography. Occasionally, I post photos of a shoreline with little geologic or geographic narrative. And sometimes I even post photos of something geologic with only a very tenuous connection to the coast.
A few days in New Mexico left me with a particularly tenuous connection to the coast and the next two posts are a bit of a stretch for Gravel Beach. My editor may object - but wait, this is a blog, there is no editor!
White Sands National Monument is a large dune field in the Tularosa Basin of south central New Mexico. The white sands are gypsum crystals, not the quartz sand we usually encounter in dune environments. The gypsum originated in a shallow Permian Sea that may have resembled the warm Persian Gulf of today. This 250-million year old Permian Sea is a recurring theme in southern New Mexico, one that gets visited again in the next post. Repeated drying up of the sea left thick deposits of gypsum.
These gypsum-rich rocks were buried for most of their history, but were then uplifted and exposed 70 million years ago during the Laramide Orogeny (formation of the Rocky Mountains) and erosion and chemical dissolution carried the gypsum into the isolated Tularosa Basin. The gradual drying up of Pleistocene Lake Otero further concentrated the gypsum and now the winds blowing across the dry lake beds of Lake Lucero and nearby playas carry the gypsum crystals up into the dunes.
Labels: new mexico
Venice Beach lies south of Santa Monica on a barrier beach built between the ocean and the large Ballonas wetlands to the east, the historic estuary of the Los Angeles River. There isn't much left to the wetlands now, since a large portion of them were excavated to create Marina del Rey in the 1960s. And the LA River hasn't flowed out to the Pacific this way since a flood in 1825 sent it south to San Pedro Bay (now Long Beach).
Historically, the river may have been a significant source of sediment for the beaches at the south end of the littoral cell (Dockweiler, Manhattan, Redondo). This source is long lost, but these beaches are now very wide as a result of the addition of large amounts of sand from leveling the El Segundo dunes, building LAX and the Hyperion Treatment Plant, and the dredging of Marina del Rey. The beaches have remained relatively stable, although eventually this material may end up at the south end where it gets sucked down into the submarine Redondo Canyon.
Like Santa Monica, Venice also has a breakwater, but unlike Santa Monica, the beach at Venice has built out and connected with this one (or at least is connected most of the time). Besides the breakwater, Venice also has a large groin farther south near the Venice Pier.
The Santa Monica littoral cell extends from Malibu south to the Palos Verdes Peninsula, shaping the western beaches of Los Angeles. As elsewhere on the southern California coast, sediment transport is generally from the northwest to the southeast.
Historically, the beach at Santa Monica was supplied by Malibu Creek and a number of other small streams in the northern part of the cell. These are streams dominated by big, but widely-spaced, flood events and sediment was probably delivered in big slugs. Development has altered runoff and more importantly, led to the construction of flood and debris control systems that trap sediment before it reaches the coast.
The Santa Monica Pier marks a significant geomorphic transition. To the north, the coast is backed by bluffs cut into a marine terrace composed of alternating marine and alluvial sediment. To the south, the bluffs disappear and the coast morphs into a long barrier beach (although this may be a little hard to see through the overprint of development). More on this in the next post on Venice Beach.
Early pictures of the bluffs show them eroding onto a relatively narrow beach. Perhaps at times in the past they were also a potential source of beach sediment? But the bluffs are now separated from the ocean by the Pacific Coast Highway and a wide beach created decades ago through some major nourishment projects.
An early breakwater - apparently part of a scheme to develop a boat basin (there have been some grand plans for the Santa Monica coastline that never materialized). The initial effect of the breakwater was for a salient (bulge) to develop on the beach behind it, but that feature is barely perceptible now that the breakwater has subsided and the beach has been widened so much by nourishment. The breakwater can still be seen in the aerial view and, if you look carefully, off the end of the pier in the photo below.