Three Sisters
Ben Oostdam story # 391


Preface: This morning, August 3, 2008, I listened to the Three Ahn Sisters performing "Lullaby for my Favorite Insomniac" live on NPR's "Echoes" with all-nighter John DiLiberto; these three sisters from Korea reminded me of another three sisters from Trinidad, but when I asked my wife Mercia if she remembered them, she did not, and that's why I'll get this story off my chest and relate them to the well-stacked coast of Columbus Bay - - the same Columbus who was mentioned next on the radio when Garrison Keillor commemorated that Columbus' three ships departed Spain on this very day in 1492 to go and discover America...
auspicious or ominous?

In the photograph above, you can discern four stacks, instead of the anticipated three named the "Three Sisters".
I am waiting now for a reply to my E-mail to IMA in an attempt to retrieve a scan of a Trinidad and Tobago stamp of an old painting which clearly shows three stacks - and hope to insert that scan later.
Since the time of painting, apparently one more sister was "born", or one additional stack appeared on the scene.
I challenge you to answer the question "Which one?" ...

In the mean time, let me try and explain some of the littoral (coast-line) processes leading to the formation of sea stacks - which can roughly be considered as pieces of bedrock sticking out above sea level. At their base, they are still connected to the now submerged ("drowned") remnants of a rocky headland consisting of more resistant rocks than those of the adjacent bay or bays.

The shape of a coast is mainly determined by:
(1) the type of its constituent materials (rocks or sediments)
(2) the response of these materials to various wave actions.
Erosion and deposition by waves tends to straighten out coast lines
and will form long stretches of uniform coast
when and where rocks and sediments are uniform.

Along a coast where segments of weaker materials alternate with more resistant rocks or sediments, the waves will shape the coast in the form of headlands and bays.
The offshore bathymetry (waterdepths distribution) will tend to have the depth contours correspond with and thus become parallel with the shape of the coastline - which is in fact the zero depth contour:

please click on images to enlarge them

Wave attack at points D and F (above figure) will start cutting sea caves on both sides of the headland.
The next phase - when caves join - is the development of a sea arch.
These are rather ephemeral geomorphs (landforms) and when they cave in (good verb!)
the (former) end of the headland is left detached from the rest and constitutes a stack.
These are well illustrated in some British geology papers, e.g. below, and in the photographs further down.

The illustrations below, from the excellent text [Sea Cliffs, Beaches, and Coastal Valleys of San Diego County: Some Amazing Histories and Some Horrifying Implications]by our former professor, Francis P. Shepard and my fellow classmate ">Gerald G. Kuhn dramatically shows the fast disappearance of three sea arches in La Jolla, California, the location of our alma mater, Scripps Institution of Oceanography.
Alligator Head Arch,
La Jolla, California

Cathedral Rock Arch
La Jolla, California

Wind and Sea Beach, La Jolla WindanseaBeachLaJolla1952ft0h4nb01z_00120.jpg

Some more photographs of Sea Arches from the Internet appear below,
including the "survivor", Bogenfels (German for 'Arch Rock') to which I dedicated this page

Ballestes, PERU


Bogenfels NAMIBIA
by McMorrow

Dorset UK chalk
Here are some more photographs of sea stacks selected from the Internet,
with due gratitude to the intrepid photographers :

Molokai, HI, USA by Basch - - SW-AUSTRALIA by Sam Abell - - Bandon, ORE, USA by Bill Simpson

Foula, Hebrides, UK

In conclusion, it is not only subaerial erosion which makes sea stacks disappear:
global sea level rise also contributes to their disappearance.
For example, here is a scan of a fathogram we took in 1982 during the IMA hydrographic survey of Columbus Bay, Trinidad
Going seaward into the Gulf of Paria while ligning up with the four stacks representing the Three Sisters, we did pass over a couple of "drowned" sisters ! . . .

BLO fecit 20080803_5 - stories