Landforms of Coasts. Erosional and Depositional

Transcription

1 Landforms of Coasts Erosional and Depositional Unless otherwise noted the artwork and photographs in this slide show are original and by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States.

2 Depositional (Barrier) Coasts

3 Cape Henlopen, DE St Joseph Spit, FL Hatteras Is, NC Assateague Is, MD

4 St Simons Sound Jekyll Island, GA is a good example of a barrier island. It stands several miles off the mainland, separated by a n area of primarily tidal marsh with some tidal channels or rivers. The island itself is a little over 8 miles long and two miles wide, and made of sand. The muddy marsh makes it wider at some points, but are nowhere above sea level at spring high tide. Image from USGS 7.5 Jekyll Island, GA map, Marsh to Mainland ~1 mile St Andrews Sound St Simons and St Andrews Sounds are tidal inlets or simply inlets. The first separates Jekyll from St Simons Island and the second from Cumberland Island. Tidal channels branch off from them and move water onto and off of the marshes. As with most of the islands on the Georgia coast, the north and south ends have very different characters, as the next slides show.

5 North Jekyll Island Much of the northern end of the island is undergoing erosion. this is the up-drift end of the longshore transport system and the waves do not bring sand across St Simons Sound with then, so they get it off of this end of the island. The beach is littered with dead trees (and their stumps) that usde to grow where they now lie (above), and the waves are undercutting the live forest as well (left).

6 South Jekyll Island The south end of the island is a different story. Here the beach is very wide, and growing wider. The line of vegetation in the upper picture was at the shore in the 1950 s. A boat that sank well offshore in the middle 1990 s has now been engulfed by the prograding sand, and is now onshore even at normal high tide (left). All in all the island has grown by over ½ mile since This rate is much higher than the erosional rate on the north end of the island.

7 The southern end of Jekyll Island, like the southern ends of most of the Atlantic barrier islands, has a hooked shape. This occus because the waves that transport sand southward along the front of the island refract into the inlet. This drives longshore transport around the end of the island and onto the back side for a short distance, as the arrow indicates. This feature is called a recurved spit or just a spit.

8 St George Island on the Gulf Coast of Florida is different from Jekyll Island. It is much longer (about 30 miles) and much narrower (the sand island is nowhere wider than about 1/3 mile). The inlets separating it from the adjacent Islands (St Vincent Island to the west and Dog Island to the east) are narrower. Finally, between the island and the mainland is an open body of water (Apalachicola Bay) rather than tidal marsh. Notice also that there are spits on both ends, though the western one is more pronounced than the eastern. * Image from Google Maps 2 miles

9 The conventional wisdom for why these differences occur has to do with the relative strengths of the waves and tides as sediment movers. On the Georgia coast the spring tide rangwe is upwards of 3m (10 ) and so this coast is called a mesotidal coast or tide-dominated barrier system. The longshore transport is trumped, if you will, by the need to move that amount of water into and out of the inlets twice a day. The Florida Gulf coast, in contrast, has a normal spring tide range of less than a meter (1-2 ) and in general there is only one tide cycle per day. This type of coast is called microtidal or a wave-dominated barrier system. Less water movement by tides requires fewer, narrower inlets, and little sediment is transported into the back-barrier bay for constructing marsh.

10 Just west of St Vincent Island (visible at the east edge of the map) is the St Joseph Peninsula or St Joe Spit. It is a recurved spit, but instead of being on the down-drift end of an island it is a peninsula built out from, and still connected to, the mainland. Like the south end of Jekyll Island is continues to prograde. Also like Jekyll Island it would be an island if a huge boulder seawall were not maintained just north of Cape San Blas (arrow). That point would long ago have eroded through without human interference. Image from Google Maps 2 miles

11 There is another interesting depositional feature of coasts that can occur even on what are generally erosional coasts like this one in New England. Nahant is a northern suburb of Boston and occupies what are generally called Nahant Island and Little Nahant Island (arrow) though, in fact, the entire landform is a peninsula. The narrow connectors between the high, rocky islands and the mainland were constructed from sand brought into place by a northern longshore transport and a southern one created by waves refracting around the islands This is a tombolo. Image from Google Maps

12 2 miles It should be clear from earlier slides that the longshore drift system tends to prograde the down-drift ends of islands and spits pretty aggressively. On microtidal coasts the mouths of some bays can be entirely closed off, like this one in southern Delaware. There is an artificial cut for navigation (just under the k in Park ), but the bays would be cut off from the sea if the cut were not actively maintained. Image from Google Maps This is called a baymouth bar.

13 This slide schematically summarizes all the landforms of depositional (barrier) coasts.

14 Erosional Coasts

15 Other coasts, such as this one in southern Maine, are very much different from the barrier island coasts of the southeast. They are very rugged and rocky and are rarely straight for any significant distance. There are projections of rocy land into the sea called headlands. Between them are coves of various sizes, often (but not always) with a small pocket beach at their heads. There are also offshore rocks of various sizes, some connected to the mainland by tombolos (below) Images from Google Maps

16 The ground-level view reinforces the rugged terrain of these coasts. View northward across a pocket beach from one headland to another near Biarritz, France, on the Atlantic. People for scale. View northward across a pocket beach toward a headland in Olympic National Park, WA on the Pacific. People for scale. (Photograph by Dr. Sam Peavy, used by permission.)

17 Various of the landforms characteristic of erosional coasts are obvious in Dr. Peavy s photograph. Waves, lacking sediment supplied to them otherwise, wear away at the base of a seacliff, creating a wave-cut notch. The cliff is then maintained by mass-wasting as the notch saps its support. Relatively easily erodable parts of the cliff may be eroded right through, leaving a sea arch. When the arch collapses its outer support will remain as an offshore sea stack. Sea Stack Sea Cliff Sea Arch Wave-cut notch (Photograph by Dr. Sam Peavy, used by permission.)

18 The retreat of the seacliffs leaves behind a relatively level surface at about wave-base depth (or the depth at which they can effectively erode) called a wave-cut platform, which may be visible at spring low tide. Deep erosion into a broad headland can create a seacave. Wave-cut platform Sea cave (Photograph at Point Arena N.S. by Thomas H. Johnson, Jr. Used by permission.) View from inside a seacave at Biarritz, France

19 The western coast of the U.S. is erosional because it is also emergent. The dashed lines in Rev. Johnson s photograph indicate three different levels of raise wave-cut platforms. These are called marine terraces or wave-cut terraces.

20 This slide schematically summarizes the landforms of erosional coasts. Unfortunate Pirate of the Caribbean Black Pearl