A Brief Study of Pebbles or the Observation and Description of Grains

In today’s lab exercise you will examine some of the larger grains found in sediments and sedimentary rocks. You start out here because later you will apply some of the same ideas to much smaller or microscopic grains. In particular you will determine parent rock lithologies, look at the degree of abrasion or roundness of a grain, observe certain surface markings that resulted from sedimentation processes, and measure general shape of grains. Your objective here is to learn certain basic descriptive procedures for these grains and to explore just what one might be able to interpret from them. Through out this laboratory exercise and all subsequent ones keep in mind that it is very important to separate interpretation from description. Always observe, then describe, and finally then and only then interpret. Work swiftly for there is a lot to do here and a lot to discover!  This laboratory exercise is also your introduction to the use of a spreadsheet application program, mainly EXCEL.

 

Exercise 1: You will find a collection of pebbles in the wooden flat.. Separate the collection into piles representing the three rock types: sedimentary, igneous, and metamorphic. Create a separate pile for those which you are unable to place in one of the three types, these are the unknowns.  You will always have things which you can not place, these are the unknowns. Don’t fool yourself and think that you are fooling others by placing something that is unknown to you in a known category. Always be honest, that is the ethical way. Count the number of pebbles that fell into each of the four categories, sedimentary, igneous, metamorphic and unknown.  When you have made the counts and recorded those data, return the pebbles back to the box and thoroughly mix them up. Smooth out the collection so that it uniformly covers the bottom of the flat.

 

Exercise 2: A customary way to illustrate the results of such an examination is to construct a pie diagram where each slice of the pie is proportional to the relative percentage of each type plus the unknowns.  Construct such a diagram later after class as you do not have time now for that.

 

Pebble Counts

 

Pebble counting is a way of differentiating one river terrace deposit from another assuming that each terrace has a different source area.  It also could be used to ascertain changes in source area as new rock units are exhumed by erosion.  This is especially useful when dealing with areas of active mountain building.

 

The procedure is to mark out on a gravel bar or exposure surface an area one meter square; this is appropriate for pebbles and gives you roughly about 500 pebbles to count. For cobbles and boulders you need to increase the area proportionally to achieve a 500 count. You could physically mark the area with a string or rope or construct a rigid frame to lay over the surface.  You pick up each pebble that is sky visible at the start of the count, quickly identify it and deposit outside of the area. You do not skip any pebble but you also do not identify any pebble that was concealed below others at the start of the count.  Instead of building piles you have a ready made check list and simply put a mark next to each name on the list each time you pick up, identify and toss a pebble. Speed is important here as you have several hundred identifications to make for each sample site and a particular study may encompass dozens of sites.

 

Exercise 3: Make a list using the following categories: sandstone, shale, limestone, chert, basalt, granite, rhyolite, andesite, gabbro, diorite, gneiss, schist, phylite, slate, quartzite, marble, quartz and unknown. Assume that the wooden flat used in Exercise 1 is a marked out area on a gravel bar and proceed with a pebble count. Place a tick mark next to the appropriate category name and toss the identified pebble in the empty flat. Try to have fewer unknowns than your previous attempt. One trick in improving your identification is to wet the samples with water. Make a simple table with the results expressed by the number of pebbles in each category and the percentage of the population represented by each. 

 

The collection that you have been working with so far was made by me and represents pebbles that I have picked up over the years from all over North America. Does it represent a true distribution of pebbles to be found in North America, no. The sample set is biased.

 

Bias: “A purposeful or accidental distortion of observations, data or calculations in a nonrandom manner.” (Bates and Jackson, 1980, Glossary of Geology)

 

As a scientist you always strive to eliminate bias. The collection came from eight different sites, three in North Carolina, one in Indiana, one in Wisconsin, one in Arizona, and one in Colorado so it might be biased toward what might be found in North Carolina. Actually it is biased for Indiana since over half of the pebbles came from a single collection in Indiana. Also I didn’t usually just pick up everything in a specified area but picked up those pebbles that caught my eye, so it is biased for those that caught my eye.  To minimize bias in pebble counts (1) use a uniform sample size so that you can compare one set of pebbles to the next set of pebbles, (2) use a large sample size several hundred items, and (3) identify all the pebbles, not just the easy ones or the pretty ones, or those that catch your eye. Finally you always document the procedures that you followed at each site. Documentation is a full and detailed explanation of what you did, done in detail enough so that someone who was not present could replicate your determinations and verify your results.

 

Exercise 4: Tonight write a documentation of what you did in Exercise 3.

 

Conglomerate is nothing more than pebbles that are cemented together, sometimes with some matrix tossed in. To do a pebble count on a conglomerate you have two choices (1) collect a sample, bring it back to the lab, disaggregate it (break it up to free the pebbles) and do the count; or (2) find an exposed surface of sufficient area, identify each exposed pebble but instead of tossing it you mark it with a ‘x’ using a water soluble marker (I really detest marked up outcrops). In either situation you may have trouble maintaining the ideal sample size. That is all right as long as you document what you did. Remember some information is better than no information.

 

Roundness

Roundness "The degree of abrasion of a clastic particle as shown by the sharpness of its edges and corners, expressed by Wadell (1932) as the ratio of the average radius of curvature of the several edges or corners of the particle to the radius of curvature of the maximum inscribed sphere (or to one-half the nominal diameter of the particle.)" Bates and Jackson p. 546.

For descriptive purposes we can take the entire spectrum of roundness and break it up in to a small number of divisions each referred to as a Roundness Class ["An arbitrarily defined range of roundness values for the classification of sedimentary particles." Bates and Jackson, p. 547]. Below are verbal descriptions of the usual roundness classes as commonly applied         :

Well-rounded: original faces, edges, and corners have been destroyed by abrasion and whose entire surface consists of broad curves without any flat areas. Roundness value between 0.60 and 1.00. (See Pettijohn, 1957, p. 59 for further details if you are so inclined)

Rounded: Round or curving in shape; original edges and corners have been smoothed of to rather broad curves and whose original faces are almost completely removed by abrasion. Some flat areas may remain. Roundness value between 0.40 and 0.60.

Subrounded: Partially rounded, showing considerable but not complete abrasion, original form still evident but the edges and corners are rounded to smooth curves. Reduced area of original faces. Roundness value between 0.25 and 0.40.

Subangular: Somewhat angular, free from sharp edges but not smoothly rounded, showing signs of slight abrasion but retaining original form. Faces untouched while edges and corners are rounded off to some extent. Roundness value between 0.15 and 0.25.

Angular: Sharp edges and corners, little or no evidence of abrasion. Roundness value between 0.0 and 0.15.

Very Angular: Powers (1953) used this as a class similar to that of Angular of Pettijohn (1957) and with a roundness value of 0.10 to 0.17. I would suggest to reserve this term for those few particles whose edges and corners are so sharp that they could cut you.

Subangular-subrounded: A term sometimes used when one cannot decide which to choose as is generally the case when you are working with granular or smaller sized particles.

Some text and reference books use cartoon illustrations of these various classes (see below) and some geologists will actually carry a reference card with them with these same illustrations. This, I think, is a little silly. Similarly it is a bit silly to spend the day measuring circles and what not to get a numerical value for roundness. It is best to train your eye to recognize the various roundness classes.

Modified after Powers, M . C., 1953, Journal of Sedimentary Petrology, v. 23, p. 118.

Exercise 5: Take a good long look at the reference samples for roundness. Compare the sample to the above verbal descriptions of roundness class. Make sure you understand what each class represents and how to identify it.

Exercise 6: Examine each of the eight roundness samples and determine their roundness class.  Do not compare them to the reference samples; do not even have the reference samples near by to tempt you. Remember: in real life you will have to do this without carrying around a box of reference rocks. Record your answers in your notebook.

The tempting interpretation of roundness is that the greater degree of rounding the "older" or longer amount of time a grain has been involved in the weathering process. This is not exactly true for it assumes that the weathering environment and the abrasion process act hand-in-hand, they do not; it assumes that the abrasion rate is constant, it isn’t; and it assumes that all stones resists abrasion to the same degree, they don’t. At most all that one can infer from the roundness of a single stone is that the more rounded it is the greater the amount of abrasion is has been subjected to relative to other stones of a similar lithologic character (mineralogy, texture). The roundness of a single stone will tell you nothing, but the roundness character of a population of stones can provide some reasonably valuable data from which one could make a valid interpretation though not always a correct interpretation.

Let me clarify something at this point; a correct interpretation is one where if you were to make a time machine and go back and witness the deposition event then you could correctly identify the process. A valid interpretation, on the other hand, is one based on evaluation of all the possible data available and logically analyzed by the interpreter; it may not be correct. In this class I am only interested in valid interpretations.

If you examine a population (two or hopefully many more) of stones then you will "time-average" the abrasion process that set them into various roundness classes and thus allow for an interpretation of the environment that the stones were subjected to. Alluvial or streambed gravel progress from an angular condition at their source area to greater degrees of rounding down stream. A population of rounded grains should be at some distance from their source, but you can not quantify that distance in terms of miles or kilometers. Similarly, diluvial gravel (those involved in glacial processes) are angular at their source site where the flowing ice plucked them up and become progressively more rounded down stream. Beach gravel tend as a population to become more rounded the longer they linger on the wave swept beach. A beach gravel deposit consisting almost entirely of well rounded stones would have the valid interpretation of having been actively involved in the beach process for a lengthy period of time, but you can not quantify the time in terms of years. A shore with big waves all the time does the job swiftly whereas a beach with dinky waves is no fun to surf and also takes a great deal of years to do anything to a stone. Also soft stones on a big wave beach get turned to dust in a matter of seasons while durable stones may take millennia to acquire the same roundness class. Have you observed any of these? Have you observed other relationships of roundness? Learn to draw upon your own life experiences to aid in your interpretations. The more you see in this world the more correct your valid interpretations will become.

Surface Features

The process of rounding results in damage to the surface of a grain. Most of the time all you get is general wear and tear giving a dull surface. However, there are some surface features that you should be familiar with.

Percussion Mark: "A concentric scar produced on a hard, dense pebble (esp. one of chert or quartzite) by a sharp blow, as by the violent impact of one pebble on another; it may be indicative of high velocity flow." Bates and Jackson, p. 465.

Striation: "One of multiple scratches or minute lines, generally parallel, inscribed on a rock surface by a geologic agent, i.e. glaciers (glacial striation)" Bates and Jackson, p. 617.

Impression Mark: A depression into a grain, generally associated with fracturing where the grain has been mechanically damaged by adjacent grains being pushed against it. Frequently found in gravel beds caught up in a tight fold.

Frosting: "A lusterless ground-glass or mat surface on rounded mineral grains, esp. of quartz. It may result from innumerable impacts of other grains during wind action, or from deposition of many microscopic crystals" Bates and Jackson p. 248.

Polish: "A attribute of surface texture of a rock or particle, characterized by high luster and strong reflected light, produced by various agents; e.g. desert polish, glacial polish, of the coating formed on a gastrolith. Syn. Gloss" Bates and Jackson, p. 488.

Differential Weathering: "Weathering that occurs at different rates, as a result of variations in composition and resistance of a rock or differences in intensity of weathering, and usually resulting in an uneven surface where more resistant material stands higher or protrudes above softer or less resistant parts." Bates and Jackson, p. 174

The above are to be taken as descriptive terms without implication as to how the feature formed. Most of the features have multiple methods of formation so therefore you can not say that such and such feature proves that it was deposited in a river, or was transported by a glacier, or was subjected to the action of wind. You can say that they suggest such an agent of weathering and that suggestion together with other lines of evidence may prove that there was a river there or that the wind blew or the mountains were glaciated. First rule you should learn in this class: Keep description separate from interpretation.

Exercise 7: Look at each of the surface feature samples provided in the lab. Their background data is summarized in Appendix A.  Make sure you are familiar with each so if it should ever pop up again you could identify it.

Sphericity (Y)

Sphericity (Y) "The relation to each other of the various diameters (length, width, thickness) of a particle; specifically the degree to which the shape of a sedimentary particle approaches that of a sphere" (Bates and Jackson, 1980). Sphericity could be thought of as the degree of equality of the three axes of a grain where in a perfect sphere the length, width and thickness (Long, Intermediate and Short) are all equal.

The following terms may be used in the field as descriptive names made without any serious measurements. 

Bladed: L>I>S Think of a knife blade.

Roller: L>(I=S) Think of a rolling pin.

Discoidal: (L=I)>S Think of a CD. 

Spherical: L=I=S Think of a ball, but a cube is also spherical!!!

Quantitatively sphericity may be expressed as the Wadell Sphericity:

  where Vp is the volume of a particle determined by immersion of the grain in a fluid and Vcs is the volume of a circumscribing sphere which may be taken as the volume of a sphere with a diameter equal to the long axis of the particle. To find the volume of the grain find a graduated cylinder large enough for the grain to slide down into. Fill the cylinder partially with water or other fluid and record the height V1 of that fluid. Then slide the grain gently down the inside of the cylinder and observe the new height V2. Subtract the initial height  from the second and you have the volume Vp of the grain. The length of the long axis, L,  can be had by using a micrometer for granules, vernier caliper for pebbles, a ruler for cobbles, and a meter stick for larger grains.

Sphericity may also be approximated by: where L, I, and S are the long, intermediate and short axes of the grain.

If one is trying to relate shape to settling rate then a Maximum Projection Sphericity: would work better.

Some descriptive terms for sphericity that involves serious measurements:

Very elongate

Compact

Very platy

For thin section work (two-dimensional) the Riley Sphericity has been used where:

  where Di is the diameter of the largest inscribed circle and Dc is the diameter of the smallest circumscribing circle. See diagram below.

This would probably be best done with a photomicrograph of the grain or while observing the grain under a projecting microscope. A similar measurement called the Least Projection Elongation would be easier to make. This is simply the ratio of the least projected width to the least projected length. See diagram below.

Using these ratios then you can apply a name according to Folk (1974):

Under 0.60 very elongate

0.60 to 0.63 elongate

0.63 to 0.66 sub-elonate

0.66 to 0.69 intermediate shape

0.69 to 0.72 sub-equant

0.72 to 0.75 equant

Over 0.75 very equant

Exercise 8: Determine the various sphericities,       for each of the stones labeled A through J. To make the measurements of the lengths of the axes use a vernier caliper. Just figure the lengths to the nearest 0.1cm. The vernier calipers can be read much more accurately than that but is it really necessary?  Watch your digits!!!!! Try not to spill to much water with the graduated cylinders and dry each sample when you have removed it from the cylinder. Record your data in your lab note book.

 

Vernier Caliper. To make a measurement unlock the locking thumb screw on the top with a slight rotation clockwise, open the 'jaws' slightly wider than the object, place the object to be measured within the 'jaws' and slowly use the slide roller on the bottom to slide the jaws loosely but snug against the object. Loosely lock the instrument then read the measurement using the centimeter scale (bottom scale on most calipers), see detail below.

Close up of the vernier scale on the vernier caliper. This one current reads 1.19 cm (on bottom scale). The '1' and '2' and '3' refer to one, two, and three centimeters respectively. The divisions between are in tenths of a centimeter. On the outside oval area at the bottom there are an additional 10 marks. The left hand most mark (A) is to the right of the '1' hence 1 centimeter and just shy of or to the left of the second tenth mark, hence 1.1 centimeter. The 9th mark of the oval set (B) is directly opposite a mark on the main scale, hence 1.19 cm. (actually position now opposite the mark of the 2 cm position). All you need to do in this lab is read to the nearest tenth of a centimeter hence you should read this instrument now as 1.2 cm.

References Cited

Bates, R. L. and Jackson, J. A., 1980. Glossary of Geology, 2nd Edition. Falls Church, Virginia, American Geological Institute, 751 p.

Folk, R. L., 1974. Petrology of Sedimentary Rocks. Austin, Texas, Hemphill Publishing Co., 182 p.

Pettijohn, F. J., 1957 Sedimentary Rocks 2nd ed. New York, Harper, 718 p.

Powers, M. C., 1953 A new roundness scale for sedimentary particles. Journal of Sedimentary Petrology, v. 23, p. 117-119.

 

Lab Report to be completed before next week's lab.  Recorded the data from the above exercises in the EXCEL file grainmorph.xls (link is on the course SeaPort pages) then use the spreadsheet computational powers of EXCEL to do the math for you. I have partially set up the spreadsheet for you but you will have to do some of the set up yourselves. I will be looking at your use of these computational powers and expecting you to use engineering format, maintaining significant digits and designating the units. This last item you will have to do in the column head the other two you can set EXCEL to do for you and I will be looking to see that you did such. I want you to arrange your EXCEL spreadsheet so it will print up as a real nice neat looking report. When you save your work to your disk make sure you name the file by the protocol listed in the course syllabus. It should be in this case like ABCDE02 where the ABCDE are the first five letters of your last name; the 02 tells me that this is the second item that you have emailed to me. If your last name is shorter than 5 letters then just use whatever you have, I will figure it out. Remember the subject line of the email must be GLY 312 and nothing else. When you are finished email to me, ideally prior to next week’s lab, this EXCEL file as an attachment, print out a copy of your report, and give it to me at start of next week’s lab period. In grading your work I will be looking at how you actually use the calculation abilities of EXCEL (I check your equations and math), how well you formatted the data, and how well you designed your report. Don’t put this off until the last minute because you probably have a sizable learning curve with EXCEL. Make use of the campus online computer training facilities. Also I suggest that you use one of the computer clusters outside of Deloach Hall where you will find better equipment and people who can offer help, especially getting started using the online computer training facilities.

Appendix A: Description of Samples

S-1: The object is called a ventifact or dreikanter or wind-cut stone. The surface is frosted. Look along one of the edges and find the spot where someone has broken it and see the high luster of the unfrosted quartz. The flat surfaces of the pebble resulted from the erosion of wind hence its name. The sample came from a dune field on Long Island, New York.

S-2: This gastrolith has a polished surface. I collected it and a dozen others from the rib cage of a dinosaur. Dinosaurs and birds are thought to have swallowed stones and sand to aid in the grinding and digestion of their food. No one has ever seen a dinosaur do this, why? Are their other way this stone could have become polished; yep.

S-3: This cobble from a stream in the Colorado Rockies has some of the finest percussion marks that you will ever see. Certainly these formed by on cobble smacking another in a high velocity stream. I should know I crossed the stream, just barely and got bruised feet in the process.

S-4: This cobble from a North Carolina Native America site has real faint percussion marks, you really have to look for them. Their faintness is probably due to the Native Americans using it to grind grain. Now are the percussion marks due to cobbles basing against one another in a stream or did someone get made and bash something with it?

S-5: This boulder has one flat surface, which has a few percussion marks. The chunk of rock is known as a core. It was found on top of a lava flow near Prescott, Arizona. Native American first ground down the flat surface to prepare a platform from which they later struck flakes for the manufacture of projectile points, scrapers, etc. The percussion marks resulted from bashing the platform to knock loose a flake. (Real pretty interpretation don’t you think?)

S-6: This polished pebble was collected from some very fresh un-weathered glacial till of Pleistocene age, dinosaurs were long dead at this point and it is to big for a bird to have used. So lets interpret it as glacial polish.

S-7, S-8, S-9, & S-10: Cobbles with striations. These were all collected from Pleistocene diluvium (glacial tills) in Iowa. S-7 was picked up from a groove, which it was in the process of carving in the limestone bedrock just north of Iowa City, Iowa. A valid interpretation is that all of these striations are due to glacial action. But do not forget the Appalachian State Prof. who got laughed at when someone pointed out that a logging chain had made the striations, which he had used a proof that the Blue Ridge of North Carolina had been glaciated!

S-11: A cobble from the shore of Lake Superior near Calumet Minn. The protolith of this cobble is a conglomerate. Note how the differences in the weathering resistance of each of the grains of this cobble have resulted in a very ruff surface that illustrates quite well differential weathering.

S-12: Two more fine examples of differential weathering.

S-13: This pebble has a polished surface but some folks would not even classify this as a sedimentary pebble. It comes from a diatreme where deep seated gasses explosively erupted through the sedimentary pile ripping loose a chunk of Devonian shale and abrasively polishing it in a matter of a few seconds while it traveled upwards in the explosive pipe. (I like to write pretty geopoetry.)

S-14: The surface of this coke bottle is frosted, but not by the action of wind but by rolling around in the surf at Carolina Beach, NC. Litterbugs do serve a purpose.

S-15: This is a piece of black granite (yep it be granite) from Cold Spring, Minn. This is the same stone used in the Viet Nam Memorial. At the quarry they cut it and then use silicon carbide abrasives on it to give it a polished surface (I watched it happen therefore this is a correct interpretation). Some less than scrupulous people do the same thing to pebbles to produce dinosaur gastroliths that they then sell to unsuspecting tourist for exorbitant prices. Nice rocks for the coffee table; an incorrect, but for a tourist, a valid interpretation of their origin!!!

S-16: A cobble from the Lance Formation of the Big Horn Basin of Wyoming with very obvious impression marks. This was collected when I was at geology field camp as an undergraduate. It comes from the apex of a tight fold that was cropping out along the highway in one of the area, which we had to make a geologic map of.

Remember the above bold type terms are descriptive terms, they have multiple possible interpretations as to their mode of formation.