Transcript from Epic of Evolution: Life, the Earth and the Cosmos (BEP 210A)

April 12, 2000 - Lecture by Michael Wysession

Okay, I want to pick up talking about where water is within the system, the system meaning the surface of the Earth and then I’m going to go through some of the different environments on the surface -- deserts, glaciers, shorelines and talk about how the water moves through them and how that shapes the land and carries rock through. So what I began talking about on Monday was essentially how water is distributed at the surface of the Earth and I’ll just finish up with that but then I want to talk about how the very appearance of the surface of the Earth is almost totally due to how that water moves -- and that water again can be air, water or ice, moves through the whole rock system at the surface. At the last part I will sort of show you a couple of pretty slides of national parks and sort of give you a sense of the general appearance that the surface of the Earth takes but let’s do a little bit more work first.

Okay, I had gotten through glacial systems. Let me talk for a moment about groundwater and I had said that groundwater was 20 percent of non-ocean water. That means because the oceans are most everything only 0.5 percent of the total water. And the residence time is on the order of about a month for soil and up to or greater than sometimes ten thousands of years for deep ground. We don’t think much about the water at the surface because we just don’t see it but there is a whole separate world of water motion deep within the Earth. Essentially underlying streams and lakes is a huge region of saturated water and that water is a tiny percentage of the volume, but it fills in every pore space in the rock. The rock in the ground is not solid. Certainly soil isn’t solid but even solid, when we think of solid rock is not solid. There are small pore spaces between the grains of the rock and water flows through it. And it’s just like the surface. Water flows downhill so we have this region that is defined by what’s called the water table which is the top part of the ground that’s fully saturated with water and if you dump water in let’s say in a mountain region it will sort of in general flow downhill. The water table fluctuates up and down with the season though so for instance there are some places where you get rivers only in certain times of the year and not in others. For instance, normally a river represents the intersection of the water table with the ground and so does a lake. However, if we have a dry period this water table may sink below the surface and then that river will be dry because the river is actually fed not only from runoff at the surface but from water flowing into it from the ground table underneath. So we don’t think about that but rivers are largely fed from underground and if the water table drops the level of the river drops as well.

Okay, the next thing I want to talk about just briefly is that of lakes, and lakes are really a tiny percentage of the total water. This is only 0.017 percent and lakes are half freshwater lakes -- the volume is split between half saline or salty lakes and half freshwater lakes. The freshwater lakes like Lake Michigan are lakes that will eventually drain into the ocean so they have a fairly fast circulation time. Water flows into them but then flows out eventually to the sea. There are however certain very large salty lakes like the Caspian Sea for example which is bordered by lots of countries whose names I can’t always pronounce like Turkmenistan and Azerbaijan and Armenia and all these other places in there. The Caspian Sea does not drain to the ocean. It’s like the Great Salt Lake in Utah in that sense. Rivers drain into it but there’s no way out. Where does the river go? These tend to be in very arid areas and the water just evaporates off. So the water evaporates at the same rate that the water flows into it and so you end up with a stable lake level. And that’s as I said about half the total of lakes and the residence time for lakes is on the order for big lakes of about 200 years. And it’s really the same for freshwater and saltwater lakes. You know water will sit in Lake Superior for about 200 years before it eventually flows out the St. Lawrence Seaway into the Atlantic. Water will sit in the Great Salt Lake or the Caspian Sea for about 200 years before it evaporates off.

Okay, let me next talk about one of the most significant in terms of shaping the surface of the Earth and that’s rivers. Very little water actually exists in rivers, 0.001 percent of total water. However, its residence time is days to up to 20 days for the very longest rivers. So take the Nile or the Mississippi or something, a very long river will take about 20 days for that river to flow all the way up but most rivers take much less. So what happens is even though there’s a tiny bit of water in the river system it moves through so fast that it ends up having a lot of water total pass through the system and that’s why rivers end up doing so much shaping of the Earth’s surface. Rivers are a continuous process but it’s important to remember that they do not carry most of their water continuously. They actually carry most of their water catastrophically and essentially the 100-year flood or 1,000-year flood that comes will end up moving as much water through the system sometimes than the continuous process.

[What was the definition of residence time?]

Residence time is the average time that a molecule of water will spend in that system so for groundwater you’ve got maybe months before that water usually is absorbed by plants and then transpired back up out. For lakes, a couple hundred years. For rivers, a matter of days.

[Do those percentages vary year to year?]

Oh, they vary tremendously. These are just total rough numbers right here. They vary by season, they vary by the size of the river, the size of the lake. These are really just sort of average back of the envelope type calculations but they’re important because they give you a feel for how the process works. Rivers are broken up into drainage systems and if I were to turn this around it might look like a tree or something. And they have certain fractal dimensions and what I mean by a fractal is if you were to take one little segment here and zoom into it, it would look exactly like this picture and that is you have a system of branches with smaller branches and those smaller branches have smaller branches and if you look at any given scale even a matter of meters sometimes you get little streams of water that join up and become larger streams. The basic idea is that water always flows from lots of smaller areas into the large major stream which in this case is this branch of the river right here but essentially a drainage system is a way for any drop of rain to fall anywhere in this area to eventually make it into the main stream, make it into a very tiny little rivulet first and then larger and larger streams. These drainage systems can be very large which I will show you and they can also have very significant effects shaping the land. This is a Landsat photograph of the Grand Canyon and it kind of looks like one of these little stream systems that I showed in the previous one, but it’s a mile from the bottom of this stream here up to the top of the Colorado Plateau here and several miles across here. But you can see the way it’s just like a stream and the little branches are beginning to work their way up into the Colorado Plateau and this is a very young feature. The Grand Canyon is no more than about 3 million years old. It’s due to a very recent uplift of the Colorado Plateau and the river has essentially stayed where it is and as the Colorado Plateau has come up the river has just kept carving its way right through it, but given enough time this is going to look exactly like this. It’s going to look like a large set of fractal branches covering the whole Colorado Plateau. This is what the river has done in 3 million years. Give it another 20 million years and it’ll all be little streams and valleys. There are certain very large drainage basins for the very largest rivers and we happen to sit in the middle of one of them, and that is the Mississippi Drainage Basin. Almost any drop of rain that falls in the middle of the continent that makes it to the ocean eventually all comes out the Mississippi and it covers from the western edge of the Appalachians to the eastern side of the Rockies and as far north as Canada, and it all drains out through the Mississippi. That’s one of the largest basins along with the Zaire Basin, the Nile Basin, the Amazon Basin and the Ganges. And the Indus is large as well. There are a couple -- the Yenisey and the Ob in Siberia and the Huang He or Yangtze in China. The basins are separated by divides, and you may have heard the term continental divide. It essentially is a line that separates if rain is to fall what ocean it will end up so if rain falls just west of this edge of the Mississippi line here it will usually make it into some rivers that will eventually work their way out into the Pacific. Divides aren’t always impressive features. When I was in Evanston for graduate school in Chicago, any of you who have been to Chicago most of the city is in a nice rectangular grid pattern but there’s one little road that cuts across parallel to the shoreline called Ridge Avenue and the area in Chicago’s remarkably flat. Ridge Avenue, if you walk to it goes up about 5 feet or so and then you cross the street and it goes down about 5 feet on the other side. That’s about as much topography as you get in Chicago. It’s an ancient shoreline of Lake Michigan but it is actually a major drainage divide because rain that falls on the sewer on the east side of the street flows into Lake Michigan and then on out the St. Lawrence Seaway into the North Atlantic, and rain that falls on the west side of the street flows into eventually the Mississippi River and out and down into the Gulf of Mexico. So the divides are sometimes very subtle. Seventy-five percent of rainwater goes right back to the atmosphere. And that’s an important thing to remember. I talk about the rain falling and then all this water going other places, most of the rain goes right back, it evaporates or it gets absorbed by plants and transpired and so most of the water that falls on the land -- I should say most rainwater on land doesn’t participate in the water cycle on the surface. However, of the remaining 25 percent, 90 percent goes into the ground. Okay, so that means only 10 percent goes into runoff of 25 percent. That’s 2.5 percent, 2.5 percent of rain on land goes into streams and rivers. That’s fairly insignificant of the total amount of water that falls on land. Most of it gets evaporated up, the rest goes into the ground, a little bit runs off into streams. To give you some idea, I mean you’re obviously aware of the fact that we impact our land in many different ways. This changes dramatically in urban areas though so in areas where you have streets essentially 90 percent of the rain goes into streams and only 10 percent into the ground. So in the city you’ve got all these efficient sewers that essentially funnel all that water into sewers and head them off down into a river someplace and in urban areas and suburban areas we are actually dramatically cutting down the amount of water that’s going into the ground and that is having some significance in places because we often get a lot of our drinking water out of the ground. And so if we whisk it all away we don’t get a chance to recharge that. Claude?

[Is that 90 percent of the 25 percent?]

Right, 90 percent of the 25 percent, right, of the rain, whereas here 90 percent of the 25 percent went into the ground. In an urban area 90 percent of the 25 percent goes into streams into what we call runoff so we flip that in an urban area. Thanks.

Okay, last couple ones -- the atmosphere has almost no water (.0001 percent). If all of the rain were suddenly to fall down onto the surface of the Earth it would make a layer 2 millimeters thick. Okay, you can barely see 2 millimeters. Alright, so all of this is a layer over the Earth 2 millimeters thick. This seems like almost nothing, however, the residence time is less than 1 day. And in fact, in 1 day we have enough water that passes through the atmosphere for a global layer 2½ millimeters thick. The point is more water passes through the atmosphere in a day than the total amount of water that’s in the atmosphere. This has to happen because this is the only way that water on the continents and in the ground and rain and ice and snow gets there from the oceans. So this is the evaporation process. We take the water up off the oceans and most of it rains right back down very quickly but a huge amount of water will end up moving through the atmosphere even though there’s not much there at any given time. The other things are living organisms and I will say also the tectonic system. We and other creatures like us contain water within our systems and we’re even much less than the atmosphere but we often move water through even quicker so we (we meaning life, everything from bacteria to algae, trees, plants, animals, bugs, beetles) actually move a very significant amount of water through us on a daily basis and play a very important role in shaping the surface of the Earth in a variety of ways. And the tectonic system is what I’ve talked about in the first part of the class with subduction and mid-ocean ridges. There is water that does go down into subduction zones and it comes back up as the volcanoes. How much water is involved, how much time it takes, the residence time is probably on the order of tens of millions of years. Water is not really known. So I’ll just say this is a big question mark here. We don’t really know how much water. There’s actually a fairly active debate within my field as to how deep water exists in the Earth. Is it just within the top few tens of kilometers or is there a little bit of water distributed throughout the rock of the Earth? That’s a big unknown.

Okay, the magnitude of this system is a little bit hard to imagine so I’ll continue this. Each year we get the equivalent of 400,000 cubic kilometers of water evaporates into the atmosphere. This would be equivalent to a giant cube of water about 50 miles on each side. Okay, each year that’s how much water is evaporated up into the atmosphere and comes back down. So what goes up must come down. If this is how much goes into the atmosphere, this is how much comes out of the atmosphere every single year. Each day that works out to be 1,100 cubic kilometers and this is a cube that is about 10 kilometers on a side. So imagine a cube of water where one side is from here to the Arch. That cube of those dimensions goes up each day and comes down. Over 1 year you get 400,000 cubic kilometers and the amazing thing is that when that comes down it carries with it a tremendous amount of potential energy. And remember when I talked about how the Earth formed and I dropped things on the table and said you start out with kinetic energy or gravitational potential energy, as it falls down it has kinetic energy and when it stops that goes into heat energy. Well, think of all of this water everyday coming down on the Earth and falling back into the Earth. This is the equivalent, the work done by this water is the equivalent to a mile-high waterfall with about 90 Mississippi Rivers flowing across it. Okay, that’s the volume of work and the amount of work that we’re dealing with. Now, of course it’s spread out over the whole Earth and so that work goes into essentially tearing away the Earth and washing it back into the ocean but that’s significant. That’s about twice our total world’s energy use is what the rivers have within them. I mean if we could tap all the energy in the rivers which of course we can’t tap efficiently we would have way more energy than we would ever need. Now, in some places this is actually the case. Canada for instance has a lot of streams, a lot of rivers but not a lot of people and they are a net exporter of energy. They make more hydraulic power from their rivers than they can use and they sell it to New England. Much of the New England power companies’ business is involved with buying electricity from Canada which gets essentially shipped down through the wires into New England and sold in Massachusetts and New Hampshire and places like that. So the rivers carry a tremendous amount of energy and most of that goes into erosion and transportation of rock (and I’ll talk about that a little bit more). Let me go back to my rock cycle picture. Okay, 3 weeks ago and then 6 weeks ago I dealt with this part of the rock cycle. Remember I talked about how if you have magma (or molten rock) it will cool to form an igneous rock but you can also take that igneous rock or sedimentary rock and increase the temperature and pressure and that can metamorphose that rock and alter it through temperature and pressure.

What I want to talk about now is this part of the cycle and that is bring the rock all the way to the surface and weather it and erode it. And we have essentially three stages to the top part of this rock cycle. We have a process of erosion, of transportation and then deposition so that’s what I’m going to talk about now in terms of these systems that I’ve just dealt with. The first part of it is erosion and erosion has essentially two parts to it. Erosion involves the weathering and removal of rock. You’ve got this big mountain there, solid granite and by the end of this discussion it’s going to be sitting as sand at the bottom of the sea floor and water is our main mechanism for both eroding that rock and transporting it and depositing it. And again that water is driven by the heat from the sun so this whole process is often referred as the solar engine of the Earth because it’s the sun that drives evaporation and that comes down as rain and causes this whole process. The first part (weathering) is essentially a physical or chemical decomposition of rock. You have a rock and you break it into little pieces. This occurs in two ways -- mechanical and chemical. All of you are familiar with this process in various ways. Why are the streets of Chicago in such rotten shape every spring? Okay, or any sort of, you know New York, Boston, Los Angeles doesn’t count but someplace in a more northerly latitude.

[Water has gone through …]

Right, it’s a process called ice wedging. What she said was water gets into the streets and when water turns into ice its volume increases by 9 percent and if you don’t have any room for that 9 percent you make room. And the streets are filled with frost heaves at the end of the winter season because of the repeated process by which water gets into a crack, freezes, expands the crack, thaws, more water gets in, freezes again, expands the crack some more, and you shatter your roads with this process over time. It shatters rock and roads are no different. Why is it that the streets of the Chicago are much worse than the paved streets in a town in the northernmost part of Canada?

[The temperature changes more]

Right, up there it’s frozen all winter long so you don’t have the freezing and thawing occurring. It’s the warm during the day, cold during the night process that does most of the damage. So this mechanical process works the most in cold areas but actually works a little bit less in the very coldest areas because you need that freezing and thawing. And I should say there are other ways of mechanical weathering as well. Anyone who has helped with gardening in the house or keeping a sidewalk tidy knows that the grass seems to grow much better in the cracks in the sidewalk than on the lawn. Plant roots are also remarkably efficient powerful tools at finding any little crack and essentially opening it up over time, and whole mountain sides can be torn away by tree roots slowly working their way in and cracking open the rock. And there are other aspects. There are other mechanical methods of weathering -- dust storms, forest fires that cause the rock to suddenly expand and contract, can all do this as well. The chemical process is also largely a result of water because all the chemical reactions that dissolve rock involve water. It turns out that chemical weathering actually is more important than mechanical weathering in terms of total volumes. Mechanical weathering causes about 250 million tons of sediment per year and chemical weathering has about 300 million tons. I made an allusion to this about a month ago. This would be the equivalent of eight freight trains of rock that are as long as Los Angeles to Boston, so that’s how much rock gets weathered and then carried away by rivers every year. The chemical process involves three major chemical processes which I won’t go into. They’re called dissolution and hydrolysis and oxidation. And the names aren’t important but what is is that different rocks will essentially break down in different ways. Rain has naturally has a slight acidity of about a pH of 5. Now, a pH of 7 as you know is neutral in terms of acidity. It turns out with all the filtering that goes on in St. Louis the water you drink actually has a pH of 10 so it’s the equivalent of having I don’t know a Tums a day or something. It’s actually fairly rough on some plants. But most rain is a pH of about 5, between 5 and 6. Now, of course we cause some trouble by putting sulfur and nitrous and carbon dioxide into the atmosphere and in some places we bring that acidity down to 4 or 3. But the natural acidity of rain dissolves rock and certain rocks like limestone end up being dissolved, just washed away. And I’ll talk about caves in a little bit which are primarily holes in limestone where the rock has been dissolved. Question?

[Like in communities that do it like my hometown that use a lot of the salt on the road during the winter, does that have much to do with the breaking down of roads?]

Salt is fairly corrosive so it probably does, salt is more corrosive on metals. I don’t know what the effect would be on tar, tarmac or concrete. In some places actually salt ends up being a significant agent of mechanical weathering where you get salty water and then all the water evaporates and you get salt crystals that will form and will grow and break apart the rock. But I would imagine the salt probably does play a role but I don’t know what it would be. It probably doesn’t help.

Hydrolysis is a process by which essentially rocks like feldspars are turned into clays so the clay minerals are a byproduct of this chemical weathering. And oxidation you can think of as rust, and in the process of oxidation essentially we have the addition of oxygen to a mineral to form a new mineral and this breaks up the coherence. I mean rust has a lot less integrity than a nice piece of shiny iron and so you weaken the rock this way. Okay, so this is weathering. Removal of rock happens a lot of different ways and some of it is a process called local mass wasting. I don’t know why that term is used but I kind of like the term. And mass wasting refers to sort of the local motion, local movement of rock -- an avalanche, a landslide, a rock fall, a slump, even yearly soil creep where the ground essentially expands a little bit with the ice and then comes back down, the ground can slowly creep down. But this is largely aided by two factors -- water and steepness. You’re not going to get an avalanche in St. Louis but you will get an avalanche in the mountains where you have a steep slope. So you can begin to move rock downhill in a variety of different ways.

Okay, far and away the dominant way that you move rock and deposit it in that rock cycle of deposition and erosion is through rivers. So this is the main means of transporting and depositing rock. All of those 550 million tons of sediment a year or almost all of it comes down through the rivers. Rivers do something funny though and that is if they’re fast they erode. If they are slow they deposit. There’s nothing magical about this. Imagine that you have a very fast turbulent river and you’ve got a lot of mud and clay and sand. Well, that stuff is just going to be swept away by the river. Now, imagine that that river slows down tremendously. Well, even those little particles of sand can no longer be held up by the water and they will fall out, and that’s why essentially mountains erode because they are steep and that means that the rivers move fast. And that’s why we have deposition at oceans because the land there tends to be fairly flat and water moves slow and the water and the sediment drops out into the oceans, or also onto areas like St. Louis. The Mississippi River has a lot of sediment that it’s deposited about here because it’s a lot flatter and the Mississippi River and other streams move much more slowly than they do in steep areas. It’s important to remember though that as I mentioned before during a flood a stream can carry often much more than 100 times the normal sediment load. If you look at the base of the Grand Canyon, the Colorado River, there are all these gigantic boulders and the stream kind of trickles by and there’s no way that that stream is moving those boulders. However, in the once in a rare while, very large flood you get enough water and you move all those boulders down for awhile and those boulders will then sit there until the next large time of flood.

Okay, so that’s transportation and here’s a picture of one of these examples of a slump. Slumping is actually a real hazard in many urban areas. There was a great story some years back where a guy in the suburbs of Los Angeles up in the San Gabriel Hills went on vacation and left a sprinkler on in the backyard and the whole ground became saturated and his house washed away and it just washed right over a couple streets and down into a neighbor’s backyard because the whole land just kind of went down. And there was a period of heavy rain some years back and there’s a place where Big Bend crosses Route 40 right by the police station right there and on both sides the ground slumped right down, large pieces of it just scalloped right off and some of it went on to the edge of the road. And so this is a natural process when the ground often gets too saturated with water, however if you build houses there then that’s not something ideally you want to have happen.

Okay, when that water and sediment comes down from higher elevations -- again, as I said, erosion happens fastest in the mountains -- it dumps that material somewhere and that usually happens in one of three places. Where you have mountains that open out onto the plains like the front of the Rocky Mountains you end up with these large fans of sediment and essentially you go from steep streams to flat, the water flows slowly and the sediment just drops right out. In fact, here’s a picture of how that looks. This is in Death Valley, California. You can see this is a stream that’s actually fairly dry. Usually only runs during the rainy season and most of the time it’s not active at all, but this is all sediment that’s washed down out of the mountains and it’s formed this nice big fan at the edge of the mountain. The primary places that you deposit material is either at a delta like where the Mississippi goes out into the Gulf of Mexico or at the bottom of the actual edge of a continent. Remember I said we are at a time here where we’ve melted a lot of the polar ice caps and the water has actually flooded up onto our continents a little bit and the current coastline is actually not at the edge of the continent which is further out to sea. And you get to the edge of the continent and it drops off often about a kilometer or so and you get underwater avalanches of sediment down those continental slopes. Some of these can be absolutely enormous. In fact, there was one in 1929 off the coast of Newfoundland, Canada, it was actually triggered by a small earthquake and the underwater avalanche occurred at about 60 miles an hour and it flooded an area underwater of about 100,000 square miles. They actually know how fast it was flowing by where the transatlantic cable breaks occurred because it ripped through all the transatlantic cables and by which cables went out when they could actually determine the rate of flow. And apparently these happen all the time and the sediment builds up along the coastline and then will wash out one of these submarine canyons underwater.

Okay, so that’s the major part of the transport and deposition of sediment. Oh, I should mention, let me say one more word about deltas. The Army Corps of Engineers essentially owes its existence to the Mississippi River delta and the process by which rivers work. Rivers normally move around all the time and the reason is think about the river from here all the way down to Louisiana. It’s fairly flat, the water’s flowing slowly and so sediment is constantly depositing on the bottom of the river. The sediment is from the Rocky Mountains carried down here. Well, over time you build up your river too much and the river jumps to a new place, essentially drops down someplace else. And we can actually see over the course of time, here’s where the Birdfoot Delta of the Mississippi River is currently but at first it was over here and then it was over here in what is now Lake Pontchartrain and then it was down south over here and then off to this side and then 5 and then 6 and now out here. And the river meanwhile moves all around. However, now we have a large portion of our country’s oil refineries dotting all along the current Mississippi River. We don’t want it to move because then you flood all these people’s houses and you have to go and move all your oil refineries to wherever the river happens to go again. Needless to say New Orleans is now about 15 feet below the level of the river and the only thing that’s keeping the river in place is a massive set of levees that are constantly rebuilt and maintained by the Army Corps of Engineers. Essentially we have a situation where here’s the land, here’s the levee, here’s the river flowing up here and here’s the land. There was a near catastrophe that happened about 20 years ago, actually north of this picture where during a time of real flood the Mississippi almost jumped onto the Atchafalaya River. And because it’s a shorter path to the sea it is actually much steeper and it’s a preferable route. In fact, the floodgates had almost totally been undermined by water flow and they were within like a day of losing the Mississippi River and having it -- this would have all dried up and it would have flowed out here. Of course they rebuilt those floodgates and have redoubled their efforts but they can’t keep it there forever. It’s just going to be a matter of time before there’s a giant flood and once it makes the jump there’s no way to get it back again essentially. And here’s a case where the rivers like to move around a lot, we don’t want them to move and so we’re taking some fairly severe efforts to prevent them from doing so.

Okay, there are some other systems that move sediment and one of the most dramatic is that of glacial systems. Ice is an incredibly powerful scouring agent and it literally just tears the rock right away. The reason this Matterhorn is shaped like a peak is because the mountains were all once up here but you’ve just carved them all out like an ice cream scoop literally just scooping the rock away and a glacier has removed rock on this side, another glacier on this side, another glacier on the back side, and all that’s left is this little bit of a peak which is itself going down. Where is this rock? All the black stuff here on top of the glacier is Alps, Alps rock and you can see for instance here’s one glacier that’s running down on one side of this rock and here’s another one that’s coming down here, another one coming out of this hill but notice there’s some brown rock right here and notice how this now gets carried out as a line right down through our glacier. Essentially it’s just this line of pulverized rock that’s been ground up by the glacier as it’s worn down. And the glacier moves slowly by our scale. It moves on the order of about 100 meters to a kilometer per year. So imagine something that moves from here to the field house in a year and that’s about the rate that the glaciers flow. It’s not significant but over hundreds of thousands or millions of years you’re just steadily carrying that rock away and it literally just tears it down in a very continuous process. We get some very characteristic features. If you’ve ever gone mountain climbing in the Rockies or the Appalachians you will often find these very characteristic U-shaped valleys and that’s how you know that the glacier has been there. Streams to tend carve out a V-shaped valley but the ice tends to scoop out the rock and you get these very large, very rounded U-shaped valleys. The rock goes somewhere and the glacial debris gets deposited in what’s called a moraine, and you probably know some of these moraines -- Cape Cod; Long Island, New York. Long Island is one just bit of glacial junk that was dumped there at the last ice age. It’s just a big long front of sand and in the last ice age that happened to be the front of the glacier and all the stuff that it tore off of mountains just kept getting dumped there and it built up into a nice long arm of sand sticking away from the shoreline. And I’ve run over so I’ll finish that up on Friday.

[end of lecture]