CTgeo

What to Observe

Where Water Meets the Land

 

Water waves and currents continually change the shape of Connecticut’s shoreline. Water beats upon the shore, in some places carrying away loose materials and gradually wearing down glacial debris and bedrock.  In other places water sweeps loose materials in the direction of its flow and deposits them elsewhere in a variety of patterns. Some of the sediments are eroded shoreline rock or shells; other sediments originated farther inland, were carried by coastal rivers and eventually dumped near the rivers’ mouths. Some inland sediments are from glacially transported rock and have a composition that is different from the local bedrock sediments.  

No Connecticut shoreline park can boast of the presence of all shoreline features, but each seems to have at least one or two classic examples of water sculpting the landscape.  

Following is a list of the more conspicuous shoreline features to search out when visiting a Connecticut beach, marsh, or shoreline wildlife refuge:  

·       Barrier island: A long island that is made of sand and that is parallel to the shoreline. Barrier islands produce protected waters on their inland side.  

·       Bay barrier: A sand bar that completely blocks the mouth of a bay. A bay barrier begins as a spit that keeps lengthening because of weak currents; eventually it connects to the other end of the bay.  

·       Beach: Wave-washed sediment along a coast that extends through the surf zone. Beaches can consist of shells, sand, or gravel. Winter beaches may be steeper and tend to be made of coarser particles. Beaches can change with the seasons because more erosion occurs in the winter when winds are stronger.  

·       Dune: A pile or ridge of sand deposited by wind.  

·       Headland: A rocky point, often with a steep cliff, jutting out into a large water body.  

·       Lagoon: A place of quiet water and fine sediment between the shoreline and a barrier island.  

·       Pocket beach: A small, frequently crescent-shaped beach between two headlands.  

·       Spit: A long ridge of sand that is connected to the mainland on one end but which ends in open water.  

·       Tombolo: A long ridge of sand that is connected to the mainland, like a spit, but which connects to an island at its other end.  

tombolo

 

 

Figure 1. A tombolo connects this beach with the offshore island at low tide at Sherwood Island State Park.

 

 

 

Scouting the Beach with the Eyes of a Geologist

Shoreline Puzzles

 

               How much information can you uncover about a beach simply by walking its length, observing its composition, and considering how the beach relates to everything that surrounds it, including the atmosphere? In this activity you are going to be a shoreline detective with a geologist’s hat! 

Materials

  • Topographic map of the beach

  • A USGS surficial map if possible

  • A geologic map of Connecticut

  • Notepad and pencil

  • Hand lens

  • Metric ruler

  • Measuring tape

 

  1. Use a topographic map to draw an accurately shaped map of the beach on graph paper, large enough to have room to label features.
  2. Walk the beach and sketch in any obvious large features, such as boulders or a jetty, in the appropriate location on the map.
  3. Walk to the highest part of the beach. At approximately equally spaced intervals, collect a sample of the sediment and measure its average diameter.
  4. Then observe the sediment carefully (You may need to use a hand lens to see very small            pieces and identify its composition. For larger sediments, use a rock name for identification. For sand sized particles, identify the particles using mineral names. If you are able to identify the minerals, you may also be able to figure out the name of its parent rock. 

  5. Design a legend for your map; then use appropriate symbols to record the size and composition information at each of the high points where you made measurements.  
  6. Walk to positions midway between the high points and the shoreline. Follow the same procedure, measuring the sediment size and its composition. 
  7. Record the midway position data on the map as you did with the high beach data. 
  8. Walk along the shoreline, next to the water. Stop at each interval you used to get high beach and midway beach data, and collect size and composition data along the water’s edge. 
  9. Record the waterline sediment data on the map using the symbols in your legend. 
  10. With a measuring tape, measure the width of the beach at each of the intervals where you worked.

Solving the mysteries………

1.     Are the sediment particles the same size at each of the three levels (high, midway, water line)? If not, what is the pattern and what do you think caused it? 

2.     Is the beach approximately the same width from one end to the other? If not, where is the beach wider? 

3.     Try to figure out what direction the water must come from most of the time in order for the beach to have its present shape. Be as specific as possible. (Example: from the south-southeast). 

4.  Is there any part of the beach that does not have any sand? If so, do you think this section of the beach may have contained sand at one time? What could have happened to the sand?  Explain. 

5.  Does the beach have any sand dunes? If so, why do you think the sand accumulated in those particular spots to make the dunes? 

6.   Compare the names of the rocks and minerals you identified on the beach with the rocks in the geologic map of Connecticut for this area. Do you think the beach sediments are eroded bedrock? Why? If you do not think the sediments came from local bedrock, what do you think their origin is? 

7.  Is the beach primarily inorganic? If you did find a significant number of organisms (or their remains), is there one particular abundant animal represented? What is the animal? 

8.  If a beach made of shells is deeply buried someday, what kind of rock will it become? What kind of rock will the sandy beach become? What kind of rock will the gravelly beach become?

 

Beaches, where do they come from? Where do they go?

What are they made of?

 

Background: 

Everyone has probably been to a beach. We generally think of beaches as made of sand. But some are made of other materials. Water moves the beach materials around, but where does the water get them? Do the beaches remain the same all of the time? Why are some beaches made up of coarser materials than others are?

bouldery beach at Meigs Point

  

Figure 2. A bouldery beach at Meigs Point in Hammonasset Beach State Park, Madison, CT. Adult in center for scale.

 

 

 Geologists define sand as rounded rock or mineral grains between 0.074 and 4.76 mm in size. Or we can say sand ranges from fine to coarse. If the grains are larger than 4.76 mm the next coarser material is gravel, followed by cobbles (up to 256 mm), then boulders, like in the picture above. Boulders can be as large as houses. Some at Hammonasset are as large as Volkswagens. The finest sand is as small as grains of talcum powder. Grains smaller than 0.074 mm are too small to see individually and are called silt or clay, depending on what minerals they are made of. 

The longer sand has been moved around by water, the more times the grains have banged into other grains. This results in the points and edges being broken off. Grains tend to become smoother the more they move around. They don't necessarily become spherical, although this process is often termed "rounding".  

Water also sorts the grains, removing those that can be carried away easily, leaving behind the ones more difficult for water to move. Thus grains which have been in moving water for a long time become well sorted, meaning they are all about the same size. By looking at smoothness and sorting, you can get a good idea of how long grains have been moved around by water. 

Waves concentrate heavier grains in one area. On Connecticut beaches dirty areas on the sand are probably concentrations of black magnetite, an ore for iron, which comes mostly from our metamorphic rocks. In other areas you may see reddish areas on the beach, concentrations of garnets. These come from our metamorphic rocks and pegmatite. A magnet attracts magnetite. 

Objectives:

 In this exercise you will: 

1.     Look at the different minerals in several sand samples 

2.     Learn what grain rounding tells you about the time sand has been in the water 

3.     Figure out which beaches contain the longest traveled sand

Procedure: 

1.     Speculate on why the materials found on beaches range in size from talcum powder to Volkswagens in size. 

2.     Now you are going to examine sand samples from four beaches and one stream in Connecticut.  Collect samples from the five locations in five petri dishes. Also get a hand lens or other magnifier, and a magnet. A microscope can also be used.

3.   Examine 15 sand grains in each dish. Try to find as many different ones as possible. Record the information about each grain in Table 1. Use the diagram below to decide on the smoothness of the grains.

   

smoothness chart

 

Figure 3. Smoothness chart. Compare your sand grains to this chart to determine roundness.

 

4.      Also look at the sorting. A well-sorted sediment has grains all about the same size. A poorly sorted sediment has many different grain sizes. In between is moderately well sorted, and moderately poorly sorted. These are judgment calls on your part. Well-sorted sediments have been transported some distance, so that the smaller and larger grains have been left behind. Put all of your observations in the table below. 

5.     After looking at all five samples, answering the following questions. 

a.       Which, if any, samples contained magnetite? 

b.      Did you find red garnets in any samples? 

c.   Which sample was the best sorted? 

d.   Which sample had the smoothest grains? 

e.   Which sample had the most variety of different minerals? 

f.    Did any samples contain shells or other organic material? 

g.   Discuss the reasons for the differences you saw between the various sand samples. 

6.      The steepness of a beach (profile) generally influences the size of grains found there. Beach steepness varies from summer to winter, because winter tends to have stronger storms that produce higher waves. Visiting the same beach to measure the beach profile in summer and winter to see how it varies can be an interesting study. Examine the sediment sizes on the same part of the beach when you do the profiles. You can use the same equipment you used to measure the stream profile to do your beach profile. Also measure the width of the beach, checking to see that the tides are about the same level both times you are there. Tide tables are published in local newspapers, as well as various places on the Web. 

7.      Which beach profile had the largest sediments, the steeper profile or the less steep one?   

8.      What time of year did you find the profile steepest? Although beaches are generally steeper in winter, a good strong summer storm could remove enough sand to produce a steeper beach. 

9.   During what season was the beach widest?


Table 1

 

 

 

Sample

Smoothness

Sorting

Description of Grains

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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