Purpose:  To enable students to construct a stratigraphic column of an outcrop and understand sedimentary depositional processes.

Grades:  7-12 Earth Science

A little background Information:

What is sediment?

            As rocks erode by chemical and physical weathering, they begin to breakdown into smaller pieces of rock and minerals.  This broken down material is transported by wind, water and ice to other areas where it is deposited as sediment.  Often this sediment is buried.  Fluids travelling through pore spaces carry particles that help cement the sediment and transform it into sedimentary rock.  This process is known as lithification.

What is an outcrop?

            Vast amounts of vegetation cover much of the land and conceal the underlying bedrock and unconsolidated material.  In some areas, the bedrock is exposed naturally due to tectonic forces (uplift) or erosional processes.  Often when engineers blast through mountains to build roads and mine for minerals, they uncover the hidden bedrock.  It is at these outcrops that the geologist is able to exam the various beds and construct a stratigraphic column.

What is a stratigraphic column?

            A stratigraphic column is designed to enable the geologist to describe and identify the layers of rock and sediment and determine what geological processes occurred in the area.   The codes and descriptions used are universal to most geologists, enabling others to decipher the data.  A sample column with definitions of terminology and codes can be linked to in order to assist the student.  A bed is the smallest division in a stratified column where material differs from the layers above and below it.   These beds contain various structural and sedimentary features.  The importance of constructing a stratigraphic column is to analyze these features.   Certain features (such as fossils) are good indicators of the age of the sediment.   Other features (such as ripple marks and clast orientation) reveal the flow direction of water, which deposited the sediment.  It is even possible to discover climatic conditions during deposition.

Water's role in transportation of sediment:

            Sediment is uplifted and carried by moving water.  The type of sediment in transport is dependent on the velocity of the flow.  A higher velocity creates the force necessary to move larger or denser particles.  As flow diminishes, sediment begins to drop out of the flow.  The larger particles are dropped first, progressing to the smaller particles.  Very fine sediment (such as clay and silt) will be held in suspension until the water becomes very calm or still.  This sequence of "fining up" is known as the Buoma Sequence and is common in stream deposits.  Water running into a lake (such as a delta) will deposit coarser material over the fine layer of mud at the bottom.  Looking at the beds you will see a "coarsening up" sequence.

            Often you will encounter a bed that appears to have no order to the grains deposited.  Very large boulders are mixed with smaller ones.  This can occur when a glacier suddenly drops it "load" in till deposits.  It can also occur in mud and debris flow, where a mass of material slips down hill very quickly.

Getting the big picture:

            When trying to reconstruct the events that occurred in this area, it is important to look at the "big picture" and not base your theories on an isolated spot.  In the field, a sedimentologist will dig several columns a distance from each other, using one as a reference column to compare to the others. For this project, students will only have to dig one column.  the purpose of this activity is for the student to learn how to dig and write up a stratigraphic column and decipher the data they uncover.

So, let's get started......

Note:  If you are conducting your research on a beach bluff, please be sure to obtain permission from the proper authorities. There are many areas that do not require special permission, but there are also many protected ones and you will need a permit.  Always bring lots of water, sun block and bug spray when working in the summer.  Watch out for ticks and poison ivy!

O.K., so you've found your spot...

Take a look around:

            Before you begin, take a good look around the area.  Is there anything that may give clues to the past?  If you are on a north shore beach, you want to look for evidence of glacial activity.  Ventifacts are rocks that have been cut by wind so one side has a smooth, flat surface almost at right angles to the adjacent side.  Striations are parallel scratches etched into a rock's surface by glacial transport.  Chattermarks appear as half moons dug into a rock.  Erratic boulders are boulders that differ in mineral composition from the native bedrock of an area.  These boulders can be as large as a house and were brought to the area by a glacier.

1. Locate a reference point:

            Geologic processes are constantly changing the landscape so when you return to an area it may appear completely different.  Unless you have a landmark, it may be difficult and often impossible to find the exact spot you were previously working at.  The important thing is to choose a mark that is not going to move for a number of years.  A good choice would be a large boulder, building or other semi-permanent feature.  I chose a large erratic on the beach that was situated close to my research spot.

                From the datum, pace off the distance to the spot you are working.  If you do not know what your pace is, simply walk comfortably for ten steps and mark the spot you stop at.   Measure the distance and divide by ten. Do this three times and get an average.   Knowing your pace can be quite helpful in the field when measuring distance.   Using a brunton (or other compass), you will also need to take a bearing from your datum to your working spot.  This will enable you (or someone not familiar with the area) to return and easily locate the area from the reference point.

2. Cutting the column:

           If you are working with unconsolidated sediment, it will be necessary to remove the overburden of erosional material from the surface to expose the original bedding.  A flat head shovel or a garden hoe can be used.  Start at the top of the cliff and begin removing sediment until the bedding is exposed.  At the top of the cliff, this should not be too deep.  You may have to dig a little deeper as you proceed down due to the material that has eroded from the top.  It is often helpful to proceed down the cliff at an angle to avoid having to remove the material you are cutting away.  It may be impossible to dig to the base of the cliff.  If this is the case, be sure to indicate that on your column.

                One column is not sufficient when trying to analyze a depositional environment.  Features that you uncover may be occurring in an isolated area only.  In order to get the "big picture", three columns should be dug, using one as a reference for the others.  Be sure they are not too close together.  For the purpose of this project, only one column needs to be dug.

3. Measure the beds:

                A bed in a stratigraphic column consists of material with similar features.  Smaller beds may be contained within larger ones, but if there is a repeating pattern, then those smaller beds should be counted as one.  Once you have distinguished the main beds, their thickness can be measured.

                    Starting at the base of the column, place a clinometer on the one meter mark of the jacob staff.   Look through the clinometer and level the staff by moving it and the clinometer together until the bubble is on the center line.  Again, look through the clinometer to see where on the section the one meter mark falls.  Mark this spot with flagging tape.   Continue in this manner until you reach the top of the section.  These marks will be reference guides for measuring the beds.  You will use your tape measure to get a more precise measurement.  Record these measurements onto the column sheet.   All measurements should be recorded in metric units.  When measuring lamina (which are <1cm), the number of lamina appearing in the bed are counted and that number is divided by the total thickness of the bed (providing lamina appear throughout the bed).

4. Describe the lithology:

                    A particular rock or sediment type differentiates each bed.  Some rock types would be limestone, sandstone, shale etc...Sediment types to note would be sand, clay, loess etc...Each type of lithology is indicated by coloring in the appropriate symbol on the column.  Be sure to include a key on your column that explains what each symbol stands for.  Standard symbols are used and obtained from most stratigraphy texts.   Check the sample column for the David Weld bluffs for correct symbols.  

5. Indicate grain size:

                    Next to the lithology column are marks that indicate mud, fine grain sands, medium grains and coarse.  When drawing the profile of your section, bring out the edge to the mark that represents the grain size of that bed.  There are standard guides that describe grain size.

6. Record sedimentary features:

                    Any special features that are found within the bed should be recorded in the "notes" section next to the column.  Note if the bedding is horizontal or dipping.  Is the sediment sorted? Is the sediment graded? Any structural features (such as folds and faults) should be indicated.  It would be helpful to take strikes and dips of these features.  Although color of rocks and minerals can be deceiving, it is a good idea to include this information also.

7. Collect samples:

                    If working with unconsolidated sediment, samples should be extracted to analyze grain size by sieving methods.  Always label your samples, indicating the bed it was drawn from.   Each bed can be labeled Unit 1, 2, 3 etc...and then subdivided as 1a, 1b, 1c etc...

8. Take pictures:

                    Always take lots of pictures of your area.  It is a good idea in your notes to indicate the frame number and brief description of the picture you are taking.  Place an object (of recognizable size) or a cm scale in the picture for scale.

9. Interpreting the data:

                    After you have recorded all the information on your column, it is time to interpret it.  It is best to first become familiar with the different environments that deposition occurs.   The various environments are referred to as  "facies models".    Some environments include: lake deposits, glacial, desert, coastal, alluvial and marine.  There are numerous texts with excellent examples and explanations of various facies models.

                    Structural features, such as faults and folds (if found over a large area) indicate tectonic activity that may have occurred.  They may also indicate slumps and deformation due to the overburden of material.

Putting it all together...

                Once all your data and samples have been collected, it is time to organize things.  Rewrite your column on a blank sheet of paper (click here to view a finished column).   You may want to enlarge this onto a poster for a display.  If you collected samples from each bed, they may be glued to the side of the column you drew on the poster, in the appropriate spot. 

                    Put your photos in plastic holders, carefully labeling and describing each one.  This can be done in book form, or mounted onto a poster board.  Be sure to draw in the direction of movement on the faults and the fold axis on the folds. 

                    Check the "Glossary of Sedimentary Terms" for an explanation of the features you are likely to encounter.  If you are unfamiliar with something...LOOK IT UP! The library is a valuable tool.

                    For your written report, state your objectives, methods and conclusion.  If you encountered any problems, explain what they were and how you rectified them.

                    Be bold with your theories, but always be prepared to back them up with evidence and facts!

Sample column:

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