About science; About sand dunes
This is about science produced by the California Institute of Technology and originally broadcast by station KPCC in Pasadena California. The programs are made available to this station by national educational radio. This program is about sand dunes meeting to discuss this subject. Dr. Robert McGregor and his guest Dr. Robert sharp chairman of Cal Tech's division of Geological Sciences. Here now is Dr. McGraw the scientific discipline of geology has many compartments and elements as do most other scientific disciplines. In his field geology professor Sharp is known as a geomorphologist. Tell me what is encompassed in the term geomorphology. You know if simplest concept Robert geomorphology deals with the land forms of the earth's surface I like to think of it as today's geology because it is very current. We are dealing with
dynamic activities going on on the surface and what they do to the land surface. And that's one reason of course why we are involved in things such as running water and being ice vulcanism or even with him as it moves across the land surface. These create land forms and it's geomorphologist we like to know how they were created and what they mean. You include such structures as mountains and valleys beaches everything Bob from a very small feature along the beach where we all like to go on a weekend to the highest mountain you can think of on this earth all of these would be in volved in geomorphology and though we often speak of the dry land surface we shouldn't overlook the features on the floor of the sea which after all constitute about three quarters of the globe. They to an offense are of interest to us as your morphology. It's the form what they are and how they were
made. When you refer to the dynamic forms this means temporal change. I should imagine that something like a modern structure would be many many years in the changing and how do we examine or study such a phenomenon is that. Well that's today's course is the frustrating aspect of some parts of geology and that's one reason why lots of times I like to focus on the more dynamic things but let's talk about your mountain range for a moment from the standpoint of geology. The vast realms of time we have to deal with a mountain range is not here very long. It doesn't last for a very long time. Though we as people can hardly see any change in it. Geologically it got here just a little while ago and won't be here very long but as people we like to look at things that change more rapidly the mountain ranges and though we can study mountain ranges in terms of their historical evolution and by certain age dating methods we could
say that something happened in this mountain range one million two million three million or ten million years ago and we can actually do this pretty well. The top of the Sierra Nevada for example in California can be da dated as having been a surface a very gentle relief back perhaps 10 12 15 million years ago. Now some fairly recent No. Your logic leaves fairly recent for you and me that's all the time. But basically you are interested in things that happen on a short a time scale. Personally I find this more and more satisfying to to be able to go out and see something as it as it happens to all the ologists a pretty satisfying thing. As a matter of fact what might be some farms not change in a more rapid scale. Volcanoes are a good example. Volcanoes are built in periods frequently of just a few years. Changes occur very quickly. Running water will modify of course the channels that it flows in in a very short period of
time. Glaciers work more slowly than running water but nonetheless they are moving all the time and you can record this movement and as a matter of fact in some ways they're a little easier to work with than some aspects of running water because it is moving more slowly you can work with them more carefully. And then of course wind. I like personally very much. Cause when things happened just within a matter of hours or days or weeks or do something for instance you're talking about there in terms of wind the most doing any important influence the thing that we see the most of course is the movement of sand and dust. We've all experienced sand and dust storms usually sometime or other in our life and the storm itself can be a rather unpleasant experience or if you can take a detached look at it. A very interesting nature experiment and the result of the transport of material by
wind that is Rees That is the things that are caused on the surface of the land by this transport in terms of where on fixed objects the movement of vast amounts of material for rather long distances and most interesting of all I think is the ultimate accumulation of the material and that accumulation that we see most commonly is in the form of piles of sand that we call sand dunes. Your interest in sand on stone stems from basic background and the discipline and geomorphology and elements of our land forms that change rapidly in time. Have you been able to extend your research of our work in this area on a local matter. I should think there be plenty of opportunities in California. That's really one reason I got into it Robert had been living in the east and I moved out to California. Lots of sand lots of sand in the desert I've always been extremely fond of the desert I like to work out there anybody who spent much
time in the desert know that the wind blows a great deal out there. There's not a lot of vegetation to hamper the movement of material by the wind and the result is that right in our own back door within a matter of Dish's Very few hours you can drive to several areas where we have unusual accumulations of wind blown sand. And these are not fossil accumulations there are active accumulations things going on there today at a very rapid rate. So as I say when I first came out California first but when I came back to California in the late 40s why I was looking for things that I could work on easily in my backyard and the dunes of the Mojave Desert appealed to me very much so that's how I got to know it. You're suggesting then that the activity of sand dunes and the variations in their farming their movement and something that's measurable easily within a man's lifetime. You know a shorter period than I have measured notable changes in dunes
in a matter of just a few hours. And certainly you can measure in a matter of a few days or a few weeks usually not in all instances but we're usually with these big permanent changes. These would be not permanent changes unless it so happens that the norm not permanent change because the dune is an active form that is continually changing all the time unless you fossilize it in some way by having vegetation grow on it or barrier or something like that. Noel at the present time any active doing any change that occurs and it cannot be permanent. Because by very definition exactly when it's going to change. But I found that depending upon the nature of the particular and its location its environment that these are capricious creatures and you have to spend quite a lot of time with them before you can understand them. You cannot look at them over a short period of time and draw a conclusion as to what they have done or are going to do in the future with any high
degree of confidence you've got to live with them for quite a while. You learn there. You'll get your secrecy just like people and then you can talk to them in a meaningful way. You ask them questions and if you ask the question in the right way back comes an answer you well yes if you're a friend. If you're a friend if you're not a friend I'll answer you. If you're a new friend they may teach you and they give you an answer that's a wrong answer and you find out three years later that they were teasing you. They do and what neighborhood neighborhoods have you made friends. My and my best friends among the dunes of the Mojave are a great mass of dunes we call the Kelso dunes. They are out in Kelso Valley which is a valley that is sort of halfway between the highways that go down to needles and up to Las Vegas. They lie between Amboy and Baker north of the granite mountains a little bit to the west of the Providence mountains that would be the eastern Mojave Desert.
Not over to the Nevada border but within about 40 miles of the border. They're big and they are over 500 feet high. We cover about 45 square miles of land and those are my best friends because I have been in effect living with those dunes off and on for about 15 years and how much one of the friends. Well I've got some friends down in the Imperial Valley a little group of dunes on the west side of salt sea down below Salt City. That's pretty wild on the west side of solvency I call it was the salt and do say I had no official name. They're beautiful little isolated crescent shaped booms that we call bark and dunes. Those I didn't live with so long but they're easier to understand because they're very consistent in their behavior. They move in one direction. They wouldn't be the way they are if they moved in other directions very well mannered done very well managed. There are what I would call the elite of the deuce their purest their
individual. They like to be alone although they don't mind travelling in a herd though I've heard of them but they won't mix. Because if they mix they lose their individuality. So there are they're easier to understand and easier to handle it's like a herd of animals all travelling in one direction instead of flying around willy nilly in all directions. Your interest has been of course for a number of obvious reasons in the local Dune structures. Of course we know that our major desert areas all around the world are there major differences between the thousand areas and the dune structures. Yes there are some differences. Robert in the Mojave Desert for example the departure fee is pretty rough. So that I just means a basic terrain beneath the dunes. Well not only beneath the dunes but surrounding the do have high high mountains and great broad desert basins. But it's a fairly rough terrain if you fly over an airplane and get a broad view of it for 30 or 40
thousand feet you see it's a rough terrain and that's quite different than the Sahara desert for example which has great broad extensions of rather flat line. Or the Arabian desert or the Australian desert just to name three desert areas that are famous for their sand dunes. And they have the same kinds of dunes that we have in the Mojave Desert. They also have some other kinds of booms that we don't have. Is this because of the difference in the size of the desert and nature of the terrain which surrounds and mostly in the nature of the terrain and perhaps consistency of wind patterns. The local strong topographic relief in the Mojave Desert influences wind patterns so that although we may have a prevailing westerly wind we have a lot of other winds that blow from other directions which are influenced by a certain mountain range in a certain valley and that sort of thing. They have in those other deserts that I mentioned. One very spectacular type of boom that we just don't
have called a lot of Dune is parallel to the general prevailing direction of the wind. Sometimes a single Dune Ridge will be one hundred miles long. This means running along the direction of the wind. Girls have a right to the direction of the wind and there will be a whole series of these parallel one to the other and they will be six seven eight hundred feet high. Some of these have been picked up on the Gemini photographed Yammer you know some of those those we don't have in the monitor and so we have some pretty good things but we don't have what most of what we have now coming that is characteristic far from the characteristic mostly we have what we call doom complexes where we have a irregularly shaped mass of blue and I heard you were talking about earlier. No not the heard so much the heard is pretty nice. It's a series of dunes a very beautiful shape all traveling in the same direction. I do complex occurs where there is a richer supply of sand where you just find the individual dunes in a herd.
Where the wind pattern has been complex not just from one direction but perhaps from three or four directions and these are perhaps like a herd of sheep they go in one direction for a little while and then turning on another direction for a little while and in a third direction they tend to keep bunched together and they're not individual Doom's really it's a great irregular mass of sand on the surface of which there are individual dew ridges. One of the most unusual dunes that we have and Southern California in the Mojave really down in the Colorado Desert are the algo Dani's dunes which are on the east side of the Imperial Valley between El Centro and Yuma. Many people have seen the Algodones dunes because their waggonette is in travels our highway crosses them. But you also see them very frequently in movies television shows and so forth they've been very widely used. Nearly always if you get a shot. A desert scene in a movie while you can be sure it was going to do
the thing that's unusual about the other good only these dunes is that they are a very long chain of doom so to speak about 40 miles long five or six miles wide and within them they have some very peculiar features that we call intra Dune flaps their flap area those that represent a part of the platform or floor on which the dunes rest which have been exposed right in the middle of the dunes they tend to be roughly elliptical shape and they're beautifully spaced about every half or three quarters of miles will be one of these introducing flats. And when you see them on a map or on an air photograph. You can hardly believe your eyes it looks like a gigantic ripple pattern it isn't quite that but there's Miranda lapses sort of an array of ellipses with the long axes and perpendicular to the long axis of the dune belt. Amazing thing which way is to do about that stuff which you will know the belt is headed
from the north west to the south east. Now it's not quite right is more north north west towards south southeast. How is that with relation to the wind and that would be parallel to one of the primary wind directions but almost right angles to the other primary wind direction inly in the aisle they don't need to do system which is the prevailing than Russia. I don't really know I think the prevailing one is the one from Lee. The right angle direction but these are new friends of mine. Bob I really haven't talked of these dunes long enough yet to feel that I know them well enough to say very much about them they're very unusual Doonesbury extremely interesting but we're just in the process of becoming friendly. And I would hope in another few years I'd be able to tell you a lot more than I can right now Matilda don't need to do how much your own friends in council. Well what would you like to know about my All-American Marchand's. No bargains at Kelso. Oh I think I got Barca fans on the Salton dunes on the west
side of the salt sea if you like talk about them. They're interesting because they they move so rapidly the barque and west of the Salton Sea move well on the average somewhere between 50 and 80 feet a year many of them move as much as one hundred twenty one hundred thirty feet or so for a two week you know I mean that seems like a fairly rapid movement. Yes and when you refer to the Amazon crescent shape which one is the convex portion of the crescent is that they will be up up wind that cried if you think of a brand new moon and the openside will be down when that is the priority of the brand new wind of the crescent moon point downwind and the convex side is up with what might be the dimensions of these beings from tip to tip if you think again of the new the brand from the tip of one harm to the tip of the other several hundred feet some will be four or five hundred feet across. He is hard to give you other dimension except the height of the dune above the
lamp surface and that runs anywhere from 20 30 up to about 40 feet down there up to the top of the highest portion of his advice and climb up the dune you'd have to climb about 40 feet to get to the top of it and then it slips off downwind and upwind upwind is a gentle slope and downwind is a very steep slope it's at the angle of repose a blue sandwich is about 33 degrees not 33 degrees is a steep slope that steeper than most mountain slopes for example. How do these parkand get started to begin with. Well probably first of all we have to have moving Sam Sam that is being carried on by the way and and there has to be sound thing that starts the sand to accumulate in one spot more than elsewhere. You know new cleaning tournament leaving Ross ass and that could be a Bush or two or three Bushes could even be the carpet out of your living room if you want to put it out there you'd find that your carpet would gather sand because the sand doesn't rebound.
You know I'm sort of trapped in a natural trap to begin with and once it starts to accumulate so that you get a layer of three or four Graeme's thickness then it traps more sand because it's a poor rebounding surface so that sand begets more sand. This continues until you build up a hay mound. Very interesting there's a crucial crucial stage in the evolution of that sand mound and that crucial stage involves the height of the mound and the wash of the of the wind either generally or at some particular time. The wind that is carrying the same across the surface has of what we call a sort of a boundary layer effect it tends to hug the surface but I think the small region next to the surface right the flow region next to the surface. So like skiing if you come down a hill and you go over a boat slow enough you don't separate from the surface but if you come down the hill fast enough and go oh that bumped you separate from the surface. Same thing happens to the boundary where and when it
separates from the surface then you really have things begin to happen. It's a little like Mickey Mouse remember the old cartoon runs off the edge of the building gets way out there and suddenly realizes he's off the edge of the building and then bingo down he goes. Same way the sand grains that are traveling in this boundary layer suddenly realized that they're not bouncing off the ground anymore they're out in the open and down they go and they accumulate on the east side of the dune they build it up to an angle of repose just about 33 degrees and then at that time the dune has. Really become I wouldn't say my tour but it's started to grow up. It's almost like a transition between a child that's been crawling and one that then starts to walk it suddenly can trap Sam very effectively. He mentioned that the sun would move down to the least side. This means then that the general movement of the doom is down when it has been blown down when they do not always move downwind. Anyone doom Ridge if it's subjected to wind always from one direction will
then move consistently in one direction and that's why these little mark and down on the saucer you're so cute they're moving in one direction they have this beautiful form and they can also doom as the wind blows from several different directions and the dunes there are confused again like a herd of sheep. The wind blows from the West so they move to the east. Two days later the wind blows powerfully from the south so they move back to the north and there will be a strong north east wind and they'll move back to the southwest. They have a mass of confusion there a mass of confusion in any one dune as I have said down there is busy tossing the sand from one side to the other side and back and forth. And it's amazing I watch some of those dunes for 15 years and the amount of ground they've covered has been tremendous and they end up within about five or ten feet of where they were 10 or 15 years ago. And you mentioned sand being tossed around by the wind. How large a grain are we talking about here when it was a range of sizes. Well they ate the finest material Robert is carried off in suspension the fine
dust. So we're not talking about real fine material we're talking about particles that are from something less. Let's say a tenth of a millimeter tenth of a millimeter would be the size of a very small dot made by a sharp lead pencil on a paper. Write things about that size. Surprisingly up to things there are 16 to almost an eighth of an inch in diameter practically pummels right Bickley pebbles and when one of them hits your windshield when you're driving across a highway you're well aware of it. So this material is is a conglomerate of which you talk about in your stand on. Yes Sam actually in dooms tends to average out at about three tenths of a millimeter three tenths of a millimeter would be 30 seconds when they enter something like that. Not not real big but it's beautifully short and they tend to be all the same size. Is there any effect on the distribution of these things over the
sand. Are they the small ones located in particular ways are the small ones for example that work up towards the crescent or what is it going to pop across frames. Yes this is a pretty complex problem. Generally speaking you'll find that on the surface of the dune the coarser material will be to the least side and particularly the base of the least slope and I think the reason for that is that the final material area is picked up and carried away in the the quarter is left. Also of course grain will roll down the least slope quite a bit. And although plenty of people have said that the sand at the crèche to the very crest of the dune will be finer than elsewhere. My studies would not tend to confirm that and I think I could find you a number of instances where they stand at the crest of the dune will be coarser and it will be down much of the windward side of the dune. You mentioned that the movement of a foot or so weak. How does one determine that.
A sand dune is moving even over a long period which depends upon the situation it's easy enough if you put up a bunch of markers which use I do I usually take steel rods that I can drive into the dunes and into the ground surface you just ride these markers out there. Yes plan I'm up there and then you have to identify them so you know which one is which. Because various things will happen to them. But well there are various other ways if you happen to have a house in the path of one of these dunes you're well aware of that the ATC excuse me I shouldn't perhaps be so specific but a government agency build a base right in the path of the Salton dunes and they became well aware of the fact that these dunes moved moved in across some of the roads and highways airfields and buildings and so forth that they built on them. So it's pretty simple actually. And so my arranging the stakes across a variety of doom that you get some indication of how these things are travelling along.
Presumably with markers on the dunes and elevation markings. The sort of thing we can measure the change in doing elevation with respect of the top of any one stake it so it's a it's a it's it's a kind of a stupid procedure actually. There is a Consider amount of scientific curiosity motivates you in this work. I magine there might be some practical implications you mentioned ascendance crawling up on populated areas and these are very understandable. One of the basic underlying scientific interest other than the notion of the geomorphology this study of movement and formation. Well are there are two or three. Robert we have of course preserved in ancient rocks over sand dunes with deposits of wind blown sand and you know what do NOT look like you can then recognize these you know old deposits throughout the Navajo Indian reservation and all
deposits of sandstone all the part of the sandstone which are very clearly were very clearly formed as doomed and this right away tells us a lot about the environment that existed at that time. It tells us about the wind direction because we can a den of 5 at least side of those dunes just by looking at how the sand is layered and how the sand was required and the what we call the bedding that developed on the very steeply side of the dune and this is interesting because we like to know which way the wind was blowing. Millions hundreds of millions thousands of millions of years ago because this is related to the possibility that the Earth's north for and the magnetic pole related to it may not have always been where they are now. And this would change the wind patterns of the entire earth if you put the north pole down in Arizona you have a very different wind pattern and why should United States we have now. So the movement of the wind direction and wind patterns give you some
indication of the magnetic pole or a geographic pole. But of course one is related to the other and that's a long story we get over not a dissertation on but they're related in the change of wind pattern as indicated by these ancient dunes supports in general the idea that the North Pole of the earth has and be moved around over hundreds of million years of time. It's very interesting to find that you have so many dear old friends out in the desert and are rapidly making new acquaintances and from this knowledge which you're accumulating over the years we've got some indication of things that went on long ago. Thanks very much Professor Sharman. This was about science with host Dr. Robert McGregor and his guest Dr. Robert Sharpe chairman of Cal Tech's division of Geological Sciences. Join us again for our next program when two more prominent scientists will discuss a subject of interest about science is produced by the California Institute
- About science
- About sand dunes
- Producing Organization
- California Institute of Technology
- KPCC-FM (Radio station : Pasadena, Calif.)
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- University of Maryland (College Park, Maryland)
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- This program focuses on the science of sand dunes. The guest for this program is Dr. Robert P. Sharp, California Institute of Technology.
- Interview series on variety of science-related subjects, produced by the California Institute of Technology. Features three Cal Tech faculty members: Dr. Peter Lissaman, Dr. Albert R. Hibbs, and Dr. Robert Meghreblian.
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Guest: Sharp, Robert P. (Robert Phillip)
Host: Hibbs, Albert R.
Producing Organization: California Institute of Technology
Producing Organization: KPCC-FM (Radio station : Pasadena, Calif.)
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