all right so how does cliff retreat happen um meaning the cliff slowly but surely gets eroded away so you know uh one of the big factors we talk about here in encinitas and san diego in general is just um there's homes that are beach front right on the cliff and those homes are slowly but surely going to fall into the water one way or another because the ocean is eroding away at the cliffs and then this collapses and eventually their home will go in we'll talk a lot about this in the next lecture but for right now we're just gonna leave it here okay so how does this work right what you got going on is if you look here you can actually see this notch that's cut out of the bottom of the cliff that's from wave activity okay and then you can see just next to it there's this collapse so what happens is it's a little bit sure the waves cut at the bottom of the cliff eroding away the bottom the bottom the bottom and then it creates a big enough notch to where it collapses this is why you don't set up right here and hang out right because then you'd be under this and i don't want that to happen to you so what you got going on this is called the notch this is a collapse the way this works can be seen in this figure here the waves come in and erode away at the bottom of the cliff and then this slowly but surely collapses so here's your old cliff line the cliff used to be out this far and the waves were eroding away at this notch eroding away roading away eroding away until it collapsed out and now you don't have any more cliff this is where homeowners live now out here hoping that this doesn't happen to them right um here's a great example of uh what i'm talking about right so this is a huge area of cliff recession you can see this large zone um this is occurring this is a low tide picture right so what would the moons be that's right full or new right very low tide because it's you can see all of this um at high tide all this would be underwater and it is the waves have been cutting away at the base of this cliff and eroding away for years for hundreds of years and what it is left is this nice wave cut platform right wave cut bench that's the bench that you would sit on if you were a giant if you wanted to be cool and put your legs out into the ocean but if you were cold and you wanted to be warmer you could sit up on this one and this would be that marine terrace that we were talking about so this used to be down here this one day will be up here and let me show you more about what i'm talking about so here's another example you've got your wave cut uh bench here okay there it is your wave cut platform or bench and then up here you got your marine terrace so this marine terrace used to be your wave cut platform this wave cut platform will be your marine terrace later on if we take another look at it what you're seeing here i'm telling you this is a marine terrace okay one of the pieces of evidence that i would use to explain that to you is what what are what are these right what's what's going on here as a geologist or an oceanographer right you would look at these and you're trying to answer questions about why things look the way they do because these processes are occurring over hundreds to thousands to tens of thousands to hundreds of thousands of years so you're just getting this quick shot how can you create the story right that would make the most sense well i'm going to give you some insight into something called occam's razor check out occam's razor what it really means is that the simplest explanation is usually the most true right or just the truth so what i'm looking for is a very simple explanation as to why this would be a marine terrace and i'm using these pieces of evidence what i would tell you is well if i look out into the ocean i find well these look just like those therefore these must have been this at one point meaning this whole area used to be out here and under water and look just like this but with uplift of our coastline we have actually created a nice marine terrace here so just like this and this is our new wave cut platform just like this so that is a simple explanation it's not convoluted right um it's it's easy to understand um another explanation would be maybe that these rocks somehow all came down like bowling balls from some cliff nearby and all made it out and sprinkled out just the right way that's a lot harder to explain than what i just did with the fact that this could just be uplifting right it's hard to think about these large processes but that's what's occurring okay so that's kind of the realm of if you will geopoetry right you're telling your story um in the simplest fashion and that's how you're gonna get the most people um believing in what you say right occam's razor and and it makes the most sense so take that uh with you okay in other things as well so let's take another look at these marine terraces this is san clemente island and believe it or not each one of these flat spots is an old marine terrace so here's our current wave cut bench out here and now i like to think of this as a staircase for a giant right so walked out across the ocean basin and now i'm going to use my stairs to get on top of san clemente island i got one two three 4 5 6 7 8 9 10 11 12. it's like 13 14 steps all the way to the top for a giant so there you go this is because san clemente island has slowly been uplifted and caused this stair-stepped terrace looking thing on san clemente island those are all old wave cut benches this is kind of the idea of how it looks right this is slowly what's really uplifting and these are all those nice flat spots that we're looking at along here now you might wonder how this happens um i know we've been talking about it but here's the idea of the constant uplift of the land so these arrows right are representing constant uplift of the land and then the uh ocean the global sea level it changes through time so what you see here is this is um there's no ice okay there's no ice so sea level is very high you cut out a spot on the cliff you make a nice notch there's your wave cut bench right then what happens next is the land is slowly uplifting okay very slowly and we got a bunch of ice sheets formed so then the global sea level is down then the ice all melts again and we bring ourselves back up to the same level of ocean that we were before but because the land has slowly uplifted we're now hitting a part of the coast that we didn't hit before um the ocean is what i'm talking about when i say we so that old wave cut bench is now a marine terrace up here and now this new level um where we're hitting is creating a new wave cut bench then ice gets formed again sea level drops this whole time the land is uplifting and then ice melts again we've got our same sea level back what we had from original and we're cutting out a new wave cut bench and now we've got um two other terraces and if it drops again we'll have three terraces and guess what we do we build on these terraces nice flat land where would you want to build a house on the cliff side or on the flat spot usually on the flat spot right and then what we're going to talk about next lecture is this idea as to okay so now the homes are starting to fall back into the ocean yeah but that you know if you talk to a geologist you'd probably know um you probably wouldn't have bought that cliffside property now i know that might be a touchy subject for some of you um maybe you have a beachfront home i'm not sure right but we're just going to talk about i'm going to give you the information i'm not going to give you my opinion i just want you to have the information to be able to make decisions on how that goes but that's next lecture this all i want you to get is hey marine terraces how they form land is slowly uplifting all the time sea level is fluctuating depending on how much ice there is in the world creating these sea terraces okay where could we see them guess what locally look at this mount soledad i told you rose canyon inglewood fault right it comes right in here it makes landfall right there well that bending action causes compression which uplifts this whole mountain mount soledad and what you can see is because of that uplift we've got a current wave cut bench out here we have the first marine terrace where everybody has built their homes on we have the second marine terrace right here where people have built their homes on we have the third marine terrace right here we have the fourth marine terrace here and then fifth here and if you look at that there's that staircase i was telling you about so you can actually see this locally you go down to uh the beach pretty much anywhere north of here and you could look south and see mount soledad for yourself and get an idea of oh my gosh those are marine terraces and i know how they form now and then you could tell people hey now i know what happened right my professor sort of told me that this whole thing has been uplifting and changes in sea level therefore the waves cut out these flat spots and the people build on them you probably have a home on a marine terrace that's much further inland um but used to be a wave cut platform often times people build on those they're nice things to build on all right mount soledad la jolla all right so look at it right when i say uh the ice volume so here's earth today we've got a bunch of ice here in antarctica but up north we don't have very much right 20 000 years ago we had the last glacial maximum so lgm last glacial maximum similar amounts of ice down south a little bit more in chile but if you look up north look at how much more ice there was right i mean covering bits in north america all sorts of ice up north all that ice came from the ocean so what i'm telling you now is that sea level would be low here and high here and that's what we see in this graph here's the high sea level so this is sea level and you see it's negative right so going less than 100 feet than it is today or minus 200 feet from today and this is current sea level at zero if you look on the x-axis this is present day and as you move to the left this is a hundred thousand years ago two hundred thousand years ago three hundred thousand years ago all the way to half a million years ago so this is current sea level this is what you see globally twenty thousand years ago it was four hundred feet below what we have today so imagine what the coastline would look like with that and then if you look uh back up here you could see oh this is an interglacial moment meaning there's not much ice and it kind of hangs out in this zone for a while and then we really got no ice and then check it out a bunch of ice again lowering sea level it's a classic sawtooth pattern with this what you see is that these interglacial moments actually last a lot longer so um when when sea level is high it has a longer time that sea level is high it takes a long time to form the ice right but once it forms it really takes off and takes it down to about 400 feet below so that's how much sea level is fluctuating through the last half million years is about 400 feet and we're at the upper echelon of that um currently and we're going to just continue to have sea level rise because less and less ice will be around so we had about six or so um glacial maximums here if you will so yeah five one two three four five of the periods in which we have a lot of ice but one two three four five six periods when we didn't have much ice so there you go that's kind of what the earth looks like and that's why you get those marine terraces