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Sfakian terraces
02 July, 2012 21:58
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Sfakian coastlines: Marine terraces

In my ongoing project to better understand what I’m looking at when I contemplate Sfakian landscapes, my attention has drifted towards the Sfakian coastline. I at first wanted to understand what it means for a coastline to have risen over 600m duing the past 1-2 million years, thereby providing the height differential from which gorges were formed. But once I started looking, I also started noticing the large variation in the structure of the Sfakian coastline itself.

Consider the stretch of coastline bounded by the Sellouda escarpment in the west and the Aradena gorge in the east. In the following image the top photo shows the present-day section of coastline (looking east). You can see an obvious cliff-face at the east end which becomes increasingly buried under debris fans towards the west. The two very large fans in the center have their origins above the cliff-line. The bottom photo has the debris removed, to illustrate the scarp structure of this section of coast.



This is a pretty dramatic feature, and on a scale unique to this short stretch of coastline. I had initially wondered whether it was a fault scarp. However, it seems to be a good example of what is called a marine terrace. The idea is that this scarp is at least partly the result of wave erosion, but implies a sea level which is stable over a relatively long period of time. A wave notch implies a short interval, moving up to marine benches (bigger) and, on a much larger scale, marine terraces.

Such terraces are sometimes called ‘wave-cut terraces’, among other names. The term ‘marine terrace’ is more neutral because wave action is not the only factor in sculpting the cliff face. For example, the specific configuration of the strata and chemical composition of the rock may promote the erosion of near-vertical cliff faces, as the next photos illustrate. In this case, wave action serves to remove the fallen debris.



A series of such steps (of whatever size) thus implies the location of earlier sea-levels. In this case, we have a roughly current sea-level at the base of the scarp, and a much earlier long-interval sea-level at the top of the scarp. This particular configuration ends around Likos, as seen in the top photo of the next image. I’ve added a hypothetical earlier sea level in the bottom photo, to match the level of the older marrine terrace at the higher altitude.



The next photo is of the sea-level terrace near the Sellouda escarpment, where rolling wave action over the longer term has eroded the underlying rock to a flat surface. This, not incidentally, also provides an especially clear ‘map’ of the structure of the rock strata.



The next image illustrates the appearance of the terrace at ground/sea level, the top photo close to Marmara point and the bottom photo about half way to Sellouda. The finer detail of erosion means that the visible sea-level terrace was exposed during the most recent ‘paroxysm’.



Asymmetries
There are a number of differences between the west and east ends of this section of coastline, which are presumably clues to the tectonic process involved in it’s creation. For example, the degree of lift at the east end seems higher than at the west end. The size of the visible terrace at the west end near Sellouda is twice as broad as further east, and there’s no sea-level terrace at all close to Marmara. The sequence of the rock strata are also different at each end, which probably says something about the origins of how they were stacked and folded.

Implications
First, the size of marine terraces imply a relatively stable, longer-term sea-level. The concepts of time and change are very ambiguous in the geological literature, where ‘recent’ can mean 2-3 million years ago, and ‘rapid’ change can cover an interval of hundreds of thousands of years. Be that as it may, if we consider the lifespan of the Sfakian regional uplift, then the terraces imply some intervals of rapid rising and some intervals of relative stabilty – long enough to allow for the formation of the terraces at all. These time differences will also have something to say about gorge formation.

Second, the terraces themselves beg the question: why so obviously here, on such a scale and not elsewhere along the Sfakian coast or even the Cretan coast in general? Geologists do study notches and benches elsewhere on Crete and even on this section of coast, but for other reasons, and most often the smaller and recent wave notches. There’s probably nothing intrinsically mysterious about this section of coastline. I expect that geologists would say that local differences in rock type, strata geometry, and tectonic activity will explain it all. But of course, it’s precisely these details that I’m curious about.
Re: Sfakian terraces
03 July, 2012 02:20
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Excellent, excellent! Did I say excellent, Mike?

I stay curious to see you working out these details thumbs up .

All the best,

Erno
webmaster Sfakia-Crete.com
Re: Sfakian terraces
03 July, 2012 22:11

Fascinating photographs. I know little about geology and the various tectonic uplifts, but on the coastal path between Sougia and Koustogerako gorge regular bands of small, beach-washed pebbles and stones are exposed about 200 metres above the sea. Down below, distinct sea notches that show the extent of various recent uplifts, from around the time of the Byzantian paroxism, are visible.
Re: Sfakian terraces
04 July, 2012 15:14
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The problem with trying to detect high-altitude evidence is that the rocky landscape is changing so much that evidence is destroyed or covered up by even more recent events.

Here is a photo of the west end of Roumeli bay.


You can see the distinct wave notch at the base of the cliff, but there is also a section in the middle of the photo that has been covered up by landslides. These have had to have happened within the last 1500 years, which is not a long time ago.

Here's a photo of a boulder above the cliff face near Marmara, on the second terrace at about 120m altitude, where an old coastline might have been. It seems to bear the marks of water erosion.


But it's not so common to see things as obvious as this. All of the ground has been covered up by erosion and landslides from the cliffs above this point, which has been going on for tens of thousands of years. Limestone erodes very easily. Any of the exposed rock is itself breaking up and wearing away. Even inside the gorges, which have presumably been cut by water, there is rarely clear evidence of water erosion. Most of the visible surface of the gorge walls has been exposed by rock breaking off, or hidden by scree slopes.
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