Dating еду

Stratigraphy and the Principles of Relative Dating

Relative dating falls under the sub-discipline of geology known as stratigraphy. Stratigraphy is the science of rock strata, or layers. Layering occurs in sedimentary rocks as they accumulate through time, so rock layers hold the key to deciphering the succession of historical events in Earth’s past.

The fundamental principles of stratigraphy are deceptively simple and easy to understand, but applying them to real rocks and fossils can be quite challenging. Here are the four fundamental principles of stratigraphy that form the foundation of our understanding of Earth’s history:

The Principle of Original Horizontality: When sediments are laid down on Earth’s surface, they form horizontal or nearly horizontal layers. This means that non-horizontal rock layers were tilted or folded after they were originally deposited.

The Principle of Lateral Continuity: Rock layers extend for some distance over Earth’s surface—from a few meters to hundreds of kilometers, depending on the conditions of deposition. The point is that scientists can relate layers at one location to layers at another. This is critical for stratigraphic correlation (see below).

The Principle of Superposition: As layers accumulate through time, older layers are buried beneath younger layers. If geologists can determine which way was originally “up” in a stack of layers, they can put those strata in the correct historical order. (Rarely, after a sequence of layers has been deposited and compressed to form rock, it may be literally overturned by thrusting of the Earth’s crust as continental plates collide. In these rare places the youngest rocks in a sequence are on the bottom, but such overturned sequences can be identified by the extensive faulting and breaking of rocks, and because the same original sequence of rocks is frequently present elsewhere in undisturbed order.)

The Principle of Faunal Succession: This principle is attributed to William Smith, an English engineer in the late 1700s. Smith noticed that the kinds of fossils he found changed through a vertical succession of rock layers, and furthermore, that the same vertical changes in fossils occurred in different places. Using the fossils collected from rocks in one part of England, Smith could predict the succession of rocks and fossils in other parts of England. The observation that fossils change in a consistent manner through stratigraphic successions can be extended to the entire world. Smith’s discovery formed a key line of evidence for evolution (it predates the birth of Charles Darwin in 1809), but it is an observed property of the rock record and is independent of natural selection, Darwin’s proposed mechanism of evolution.

Relative dating of rocks and fossils from an area is based on the Principle of Superposition, which enables scientists to put historical events in order. Relating the succession of events in one region to those in another requires that the two areas be stratigraphically correlated. Correlations can be made by tracing rock strata from one area to another by using the Principle of Lateral Continuity or by relating the fossils of the two areas using the Principle of Faunal Succession.

The Grand Canyon as an Example of the Principles of Stratigraphy

The Grand Canyon spectacularly exposes rocks spanning hundreds of millions of years of Earth’s history.

Many of the rock layers exposed in the walls of the Grand Canyon have not been disturbed by mountain building or other forms of deformation since they were originally laid down on Earth’s surface. This is an example of original horizontality. Some older layers, however, have been tilted; the surface where these tilted layers are overlain by undeformed strata is called an angular unconformity.

Many of the undisturbed formations can be traced from one end of the Grand Canyon to the other, a distance greater than 435 kilometers (270 miles). This is an example of lateral continuity. Some of the same formations are also exposed hundreds of miles farther away in other parts of the Southwest.

The oldest rocks in the Grand Canyon are exposed at the base of the gorge and are late Proterozoic. These rocks are overlain by younger Paleozoic-age rocks. This is an example of superposition: In a pile of sediment, the oldest deposits are at the bottom of the pile, underneath younger deposits.

Each major layer of sedimentary rock in the Grand Canyon contains different types of fossils. The succession of fossils in the Grand Canyon is consistent everywhere in the canyon and is also similar to the succession of fossils in other parts of North America and on other continents. This is an example of how the principle of faunal succession has been used to recognize that the Grand Canyon includes rocks from the Cambrian, Devonian, Mississippian, Pennsylvanian, Permian, and other geologic periods (each characterized by different fossils).

Correlation based on fossils is the focus of biostratigraphy. Most species live only for a few million or tens of millions of years before they become extinct or evolve into new species. This fact implies that rocks containing fossils of the same species were probably formed within a few million years of one another. The more species that can be matched in this way, the more precise the estimate of the relative age of a rock layer. Organisms with high turnover rates (meaning that new species appear very rapidly and last for a short period of time) and a high likelihood of being preserved as fossils are ideal for biostratigraphy. Graptolites and conodonts are good examples from the Paleozoic, and mammals, foraminifera, and pollen are often used for biostratigraphy in Cenozoic rocks.

By using the four principles of relative dating, geologists can compile a detailed sequence of events based on relative time. The process is not always straightforward, however, because the geologic record is often discontinuous. Rock layers representing a particular time may be missing from an area because no sediment was deposited there at that time, or because sediments that were deposited were eroded. Furthermore, fossils may be absent or poorly preserved, and interpretations of evolutionary relationships within fossil groups may be incorrect. If sediments were deposited in different environments (such as land and oceans) comparison is difficult because most organisms are adapted to a relatively narrow range of environmental conditions.

Until the advent of radiometric dating, there was no independent way to test the accuracy of relative dating of sedimentary sequences. In stratigraphic sections around the world radiometric dating techniques have verified the relative ages of sedimentary rocks that had been determined long before from the fossils they contained.

Rocks whose ages have been determined by absolute dating can be incorporated into a succession of strata determined by relative dating. Then geologists can use correlation to infer the ages of rocks and fossils that cannot be directly dated.

Tree-Ring Dating

A tree was felled in 1982 giving rise to Stump A which dates back to 1906. The 1906 ring pattern in wood Sample A (which was cut from Stump A) correlates with a 1906 ring pattern in Sample B which was cut from an older, undated Stump B. Wood Sample B dates back to the year 1840. By matching up similar spaced rings in Samples B, C and D, the ages of ancient timbers can be determined. As long as the wood samples being compared have some ring patterns that coincide, time may be extended back through an unbroken succession of growth rings. In this example, wood Sample D dates back to the year 1691. This represents an unbroken succession of 291 annual rings, almost three centuries of time recorded in four small pieces of wood. Today, scientists have painstakingly established an unbroken succession of rings extending back in time over 80 centuries (8,000 years). Cross-dating is a valuable tool in dendrochronology and archaeology.

Lack Of Visible Annual Rings In Tropical Trees?

In the tropical rain forest, relatively few species of trees, such as teak, have visible annual rings. The difference between wet and dry seasons for most trees is too subtle to make noticeable differences in the cell size and density between wet and dry seasonal growth. According to Pascale Poussart, geochemist at Princeton University, tropical hardwoods have "invisible rings." She and her colleagues studied the apparently ringless tree ( Miliusa velutina ) of Thailand. Their team used X-ray beams at the Brookhaven National Synchrotron Light Source to look at calcium taken up by cells during the growing season. There is clearly a difference between the calcium content of wood during the wet and dry seasons that compares favorably with carbon isotope measurements. The calcium record can be determined in one afternoon at the synchrotron lab compared with four months in an isotope lab.

Advice about Teens Dating

Jan 2008

My 15 year old daughter informed me yesterday that she's been texting an 18 year old boy she met at the bus stop. She had told me about him a month ago, telling me that this really cute guy kept approaching her to talk with her. Friendly-like. Clearly, she was/is flattered. At the time, I talked with her about ''the dangers'' while also validating how good it felt to have someone notice you etc. I told her she was not to give him her number. So, now they're texting and she's grinning and blushing and feeling all special. and I'm not sure about how to approach it. My mother-bear self wants to lay down the law and tell her no way in hell is she going to be dating an 18 year old who we know nothing about. (There's a chance that he's a friend of a friend of a friend!) On the other hand, my saying that will only encourage her as she is seriously rebelling these days. I tried talking with her about it but she was angry that I was ruining her fantasy and finally said ''Fine! I won't write him anymore!'' which I trust about as much as I trust George W. I want to support her to start thinking through some of this for herself, but whereas she used to have very good judgment, these days she's running on low self esteem and hormones and I believe would follow anybody home who told her she had beautiful eyes. ( And yes, we've been addressing the low esteem for years.)She has no experience with dating. She says they could just be friends. I say I could get behind that better if he would stop commenting on her physical beauty and if she would stop gushing. I find myself in the unsettling position of wanting to say very conventional things like ''An 18 year old boy only has one thing in mind.'' And then I feel bad. Could they just be friends? Right. Any feedback from parents who have been through this? Thanks!

Sign me as: conflicted mother My daughter started seeing a 15 year old boy (on the water polo team)when she was 15 and I had her keep her door open when he came over and requested that his parents do the same. I talked to her about birth control and safe sex and she clearly was not ready. When she saw him I kept my cell phone on and when they were alone for the first time and he was pressuring her for sex, she called me and I picked her up immediately. Older guys pick on younger, more vulnerable girls because it's harder for them to say no. Talk to your daughter about different things that can happen when she is with him and let her know she can make a choice about what she wants to do. Make sure she has condoms if she will be alone with him. The new shot that prevents cervical cancer would be a good idea too. anon

March 2006

My daughter is 15. She's in a kinda bookish group at school, and few of her friends have ''gone out'' with guys yet. While she doesn't appear to have a specific boy in mind right now (though I may be wrong about that), she's asking questions about what her dating rules will be. Being clueless myself, I'm looking for suggestions--how have you handled early dating experiences? What kinds of dates have you allowed? At what age? With what curfew? Did you or another adult chauffeur, or was it at one of the kids' homes? Anything you would suggest or do different? All comments and suggestions are welcome.

Single mom of teenager Well, I don't have that much experience with this as my daughter is 15 also. She has had two (!) ''boyfriends'' already though. Both were fairly short-lived. They mostly spent time together at school, at lunch, or met before school for coffee. I met him on several occassions. I told her that I wanted to meet him before they went ''out'' anywhere. He came over to pick her up several times. They went to the movies, like on a real date! He came over to do homework a couple times. She went to his house once or twice, all when parents were home. That's the rule, parents must be home. As far as a curfew. I always ask her to be home by dark, no matter what she's doing, out of not wanting her to walk around alone at night because it's not safe. After the movies though he walked her home. I offered to give him a ride home but he didn't want one. When she has been out later with others I often pick her up (the joys of cell phones!). I guess when there is another boyfriend I will want to meet him also. I think it's a matter of what you are comfortable with and what she wants too. Just make sure she knows that she can call you anytime and you will pick her up. That you are on her side and supporting her. That way she might talk to you about any things she doesn't know how to handle. I also tell her if he wants to ''go farther'' than she does, she absolutely has the right to say ''no''. This totally embarasses her and she doesn't want to talk about it, but I think she's listening. I give my daughter possible things to say to boys in different situations because sometimes we just can't think of the right words at the right time! I am trying to teach her to stick up for herself, to not deny her inner feelings about someone, to use good judgement, and to be tactful also. It's a work in-progress. Good luck! anon Group dating seems to be the best idea I heard until. maybe 18? Kids can get together in a group in homes where parents cruise through at times (From experience, I know some parents at home may mean zero supervision.) Our daughter is 16 and figured out early that a lot of so-called dating is really an excuse for sexual experimentation. Some kids pretend they are drunk, and hence not responsible for their sexual behavior. When my daughter asked me when I thought a girl was ready to have sex, I replied ''when she's ready to handle a baby.'' We all know that every form of birth control has a failure rate, so I don't think it's too far out to talk about pregnancy. A lot of oral sex goes on in dating, with definite health risks, loss of reputation, etc. I'm generally very liberal but not when it comes to early dating. Our daughter has a nice social life and is not suffering by not dating solo. (Drugs also come into so - called dating.) If our kids hang with a crowd that don't date, it's easier.

We're all in this together

Dec 2001

I would like to hear from parents about what kind of parameters they set for dating for 16 year old girls --particularly sexually active ones who ( in addition to being great kids) can be defiant, unreliable, do not tell the truth and have various emotional problems for which they are in treatment/ on medication.

Past efforts to monitor a dating situation have failed, either because of being lied to or because of being let down by a past boyfriend's dad who promised to "supervise" if the two spent time at his house when he WAS there and did not.

My daughter is beginning a new relationship. Knowing I cannot control what she does, I want to set age - appropriate parameters and attempt to hold her to them. If they are too tight it will backfire. Also, these parameters cannot be based on expecting truthfulness or abstinence. They have to be based on something else, giving her reasonable guidelines within which to learn to take responsibility for herself. Its a complicated situation ( late, international, cross racial adoption, early trauma, etc. ) So please, no preaching. I would very much appreciate just seasoned experience and practical advice . Quite agree no preaching - if you believe your kid to be sexually active - and if it agrees with religious beliefs - I advise putting her on the pill. I am only speaking from experience my daughter, found herself pregnant at 14 and although it brought us closer together emotionally, it was not a pleasant experience. She is now 18, more mature and a fabulous person, looking back she says "I was just a kid - what did I know that something like that would happen to me." Good luck with your daughter. I have a 16 year old daughter, though she is pretty "easy", but I was a "troubled" 16 year old once (and I have also transracially adopted a child with challenges). I will speak from my experience as a troubled teen. The most important thing you can do (and obviously already are) is to be involved, and concerned. She needs to prove to you and herself that what you think, is of no concern to her - but it is a ruse - it matters more than she can accept or let on. If you are not condemning (and sounds like you are not) but instead acknowledge the fact that this is ultimately a choice she must make about her own body, she will be more able to hear the things that you want to say about the situation. That does not mean, that if you think she is too young that you need to hide that opinion, but present it in a way that acknowledges her ability to make different decisions, and make sure it conveys your concerns about the effects her choices may have on her rather than an issue of right or wrong. When adults approached me in this way when I was a teen, they made the most impact.

As for the rules themselves, I think that the rules we set for teenagers are a safety net, not a protective coating, the kids can get around them if they are determined. Set the rules that you think are appropriate for her, if they are not 100% enforceable acknowledge this to her, and be clear about the consequences if you do find out that they have been broken. You are obviously aware that a teen who is acting out like this will rebel against very restrictive rules, but that still leaves you room to maneuver. The task of a teenager is to become an adult, when a teen is troubled it is imperative that they learn that the responsibility for their actions is theirs alone. It is a hard concept for teens (anyone?) to unravel that while the problems may stem from some terrible situations and horrible stuff that other people did to her, that she is the one who must now move forward with her life and find ways to make choices that support her. I guess that is what I would most emphasize: share with her your genuine concern that she make choices that support her and tell her that to that end you will set certain rules with certain consequences, but then acknowledge her responsibility for being the one who ultimately will make these decisions. Good luck. PS - Alanon might be supportive for you as the parent, even if she is not using any substances, the issues are close enough. So I must ask first: What is it that you hope to accomplish with your rules? It's unclear to me what your goals are, and this is the first step in setting up guidelines or procedures of any sort. It sounds from your message as if you are conflicted with regard to your goal. If you could, you might want to have a goal of "get teen to stop having sex." But you seem equally convinced that this is not achievable. Certainly if you set a goal (and therefore rules) that *you* do not feel are achievable, then they will not be followed, nor will they be enforcable. One possible starting place to think about your goals might be: "Have teen and parents be both happy and safe." Then you can identify the sub-parts of this goal that will contribute to this. For instance, as you mentioned, garnering cooperation from another parent might be one route (though it hasn't worked so far). Getting appropriate health care for the teen might be another--taking her to the Planned Parenthood, or your physician, and getting her a complete checkup including STD screening, and good solid information on both pregnancy prevention and STDs, with access to appropriate condoms (required in this day and age) and possibly also hormonal protection as well (pill, Depo-Provera, whatever). If it were *me* (and it has been!), what I would hope to accomplish with this sub-goal would be safety for both parent and child--knowing the child is safe from the unwanted effects of accidental pregnancy, and also from the very serious possible consequences of STDs. By bringing this out into the open, and *partnering* with your teen about it, you will begin to set up an atmosphere of trust on both sides, but also send the message that you are serious about caring about her safety. Dragging her to this would not help, but having her understand that you are in partnership with her to ensure her safety as well as your peace of mind probably will.

Now I'm going to suggest a really STRANGE book to read that might help you learn to negotiate this process. It's a business book called "The Goal", by Eliyahu (?) Goldratt. It is a novel about manufacturing processes. No, really! And more strangely, it's very readable--even enjoyable! The reason I think that it's useful in this case is that it talks about identifying what your goal is, and how to figure out where your bottlenecks are in the process. Even better is the second book, called "It's Not Luck." In that one, they set out some really powerful "thinking processes" that can help you identify a conflict, and see where seemingly irreconcilable differences can be shifted, if you can identify incorrect assumptions. The two together are actually pretty amazing, and there are several occasions when the examples used are from the protagonist's family life, so it's even clear how to apply it outside of the business world. The process is very powerful, and my husband and I are planning to incorporate it into our personal and relationship coaching tools.

Feel free to e-mail me individually, if you wish. I have much more specific advice to offer, if you want it. My now 17-yo daughter sounds similar to yours in many ways, and it has been quite an adventure to get to our current family life, which is mostly downright WONDERFUL. And I assure you, it was not wonderful a few years ago, so it's not like we just have some miraculous kid, or are some unachievably enlightened parents ourselves! ;^) But we did have to throw out a lot of assumptions (like, that we were at all in control of her!), and set up a lot of new models (like us being in partnership with her to create the situation in which she could achieve the best life for herself possible). Challenging, but highly worth it.

Good luck! Dawn I want to refer you and any other parents to a wonderful resource for any kind of parenting issues: Parents Leadership Institute (PLI). I speak from my own experience as a teen who was sexually active at 14 and avoided getting pregnant but did NOT avoid sexually traumatic and exploitive situations (invariably by older adults and not my peers). First to the extent possible make sure that your daughter uses the pill or another highly effective form of birth control. Encourage her and her regular boyfriend (if she has one) to go as a couple to Planned Parenthood for an information session; in any case, make sure that someone other than you ensures that she is very well informed about birth control and STDs and gets some coaching on the latest ways to persuade partners to use a condom.

Second, help her to understand deeply that she alone can decide if she is "consenting" in "consensual safe sex. Being pressured into having sex when she doesn't want to have sex attacks a girl's core self esteem and can lead to other problems with alcohol, drugs, self-cutting, etc. Each pressured/forced situation undermines self esteem. Let her know how very important it is to ask herself how SHE feels and if SHE is ready to have sex with this particular person at this particular moment.

She needs to know that although having genuinely consensual safe sex can be a joyful experience, she may need help to handle the feelings that come up because sex is a radical form of intimacy. Sex is powerful stuff. Who can she talk to about how it really feels?

To avoid trauma and to keep her safe, TRY to instill ONE RULE for both of you that she can call you and you will come to pick her up whenever wherever she is with NO QUESTIONS ASKED. If she is driving her own car, encourage her to come home at a reasonable hour and to routinely tell her friends that she "will be grounded forever" if she doesn't make the curfew. This will make it a little easier for her to get out of situations where she is in over her head.

Most important is to encourage her to LOVE herself, to exercise her power to take care of her core self, which is indistinguishable from her body, with confidence and joy. Just want to say thanks to the people who responded to my question about teen dating. Your responses were a help and a support.

7th Grader son wants to date

Sept 1998

Your thoughts and ideas on the subject of dating would be appreciated. I have a 12 yr old 7th grade boy who has been getting calls from different girls to go out with him (often 8th graders). Usually it's a girl calling for a friend who wants to go out with him. Years ago (when it was all just theory) my husband and I talked about the issue of dating and decided that we would allow our kids to group date in 9th & 10th grades, and then go out on actual dates in 11th & 12th grade. But now reality hits: my son has had a crush on a girl for about 9 months. Evidently she likes him, too and they've shared their feelings with one another. They see each other only at school. She has let her friends know that she wants to go out with him and they in turn have told him. Question: would that mean go out on a date, or go steady with her. When kids are in seventh/eighth grade what do kids do at the school - do they kiss, hug, just hold hands or grope? Parents with older kids: how did you handle all this? I want my kids to be open and honest with me and not sneak around if I'm too strict. When I was growing up I snuck out on dates by saying I was going to a friends house. I don't want that with my own kids. If your kids did go steady, how did they handle breaking up? I know a lot depends more on maturity level than on age, but have any of you come up with any rules of thumb methods? Here I am supposedly guiding my son and I'm just as confused as he is. He has definitely been feeling pressure with these phone calls. This much I've learned: girls have gotten very aggressive with phoning boys as I've spoken to others parents of boys who are getting the same thing. I wish the phone calls would stop! How does a kid say No and not sound like a nerd. If a kid says yes, just what is he getting into? Any insight would be greatly appreciated. My daughter graduated from 8th grade in June. There was lots of talk among the girls about "going out with" so and so. This term means "going steady", not actually going on a date, however they may want to go on dates while "going out". I have told my daughter that she can go on dates when she is 16 or 17. Meanwhile, they have gone on group dates; this does not imply that physical closeness is out the window. Lots can happen on a group date. Because of the media hype of sexuality (especially on TV) these kids are under a great deal of peer pressure to be grown up and cool. Not only are kids' hormones running wild at this age, but TV programming implies that sex is constantly on every adult's mind, and is the primary component of humor. I find this portrayal skewed, to put it mildly.

Times are different now. Girls do call boys and I can appreciate your dislike of this practice. I think kids are being pressured to take on all this boy/girl stuff much too early. As an alternative to the overemphasis on this, I suggest supporting involvement in sports for girls and boys and helping them to develop and get involved in things they have a strong interest in. a Mom I've just read a chapter in "Your ten to fourteen year old" by Louise Bates Ames, et al (the revised addition) on this subject that was very interesting. For instance, the terms, like "going steady" have a different meaning than they used to. That book might be useful to the parent. Asking other parents of kids in this age group in this region is a great idea! I'm eagerly awaiting the relpys that are generated, being the mother of an eleven year old who is clearly gearing up for the complexities of the middle school social world. It's fascinating to watch as the posters of "The backstreat boys" and others begin to cover her bedroom walls, a swirl of peoples faces wherever you look. And the time she spends looking at her own face, refining all its different expressions. Hmmm. Katherine My 8th grade son, who has no personal experience dating or going steady, tells me that "going together" at Willard Jr. High in Berkeley means walking together in between classes. He says "they hug" at school, but kissing is not allowed on the school grounds. As far as he knows, no dating goes on outside school. My son's friend agreed, and suggested that you invite the girlfriend over for dinner sometime.

Feb 2000

My 17-year-old son has never been on a date. Should I be worried? My worry is not about whether he likes girls vs. boys - he appears to like girls. What I'm worried about is that he has low self esteem and is afraid of being rejected and so won't ask any one out. Or that he he is so into peer approval (which he sort of is) that he thinks anyone who'd go out with him might not be acceptable to his friends. He does go to parties with his friends; about half his friends seem to have girlfriends who go along too. But to my knowledge he has never done anything socially with a girl and I can only remember one or two times when a girl has even called; he's never called a girl himself I don't think. He is cute, smart, sweet, and funny (I know, I'm his mom, but he IS.) I was dating when I was 14 or 15. So what's going on? Is it just a boy thing? Should I ask him about it or continue to keep my mouth shut? - anonymous My brother - and many of his friends - never dated until well into college. I guess he just wasn't ready. I always referred to him as a "late bloomer." He dated several young women in college and met his wife while he was in his masters program. They married when he was 23, had two wonderful children 6 years later, who are now 17 and 14, and stayed happily and solidly married. I was dating at 14 or 15 as well and couldn't understand what his problem was. In retrospect, I wouldn't worry at all. Anonymous Don't worry! I may be wrong, but I think that the high school kids who date are far outnumbered by the non-daters. I never dated in high school. I had a great group of friends (other girls) and we did things together. I enjoyed the boys in class and other gatherings but did not date them. I don't think my husband dated either.

I have an almost 17 year old son (also cute, smart, sweet and funny) who does not date either and I've been glorying (maybe I shouldn't admit that) in the notion that I don't have to worry about all the things it entails----sex, alcohol, increased peer pressure, etc. My son gets together with other guys for pizza, TV, sporting events, and the occasional Saturday night poker game. Friends are important but many people wait until they are out of high school to date. And, I bet that if he's cute, smart, sweet, and funny his self-esteem is probably well intact also. Enjoy him and don't worry. --Anon. Don't worry. Don't ask him about it. Since he seems to have friends and a social life, let him figure out when and how to ask girls out. These days, most teens seem to go around in groups. Teens who "go out" together are generally "going steady," as we used to say in my era, but with much more advanced social behavior at a much earlier age, if you catch my drift. If your son is generally happy, be glad he's a late bloomer.

My son also did not date as such when he was at Berkeley High although he partied a lot and had "just friends" relationships with many girls. As a UC student he still does not seem to have a single steady girlfriend, although he has many friends of both genders and definitely likes women. And, like your son, he is funny, smart, and handsome (if I do say so myself). I wish he had gone to his high school prom and look forward to seeing him in a committed relationship. But I am confident that he will take care of his relationships on his own without my help. Unless, of course, he asks me. anonymous I didn't date when I was 17 either--I just wasn't interested in developing an intimate relationship with a boy when I was 17--but when I went off to college. I wouldn't worry. L. I would like to respond to the person who is worried about her 17 year old son. As a parent of three sons, 22, 17, and 8 years old..I can tell you NOT TO WORRY. Every boy is different. My 17 year old son has never been out on a date himself and he is not worried about it. Why should I be worried about it? He will come to the decision himself when he feels he'd like to go out on a date. He goes out and socializes with both girls and boys and he's been to other girl's houses for homework type things as well as just pizza and videos (with the parents and other kids there). There is no hurry on my part for him to start dating. He just said the other day he didn't want to go to his Junior prom because, among other reasons, it was too expensive and he didn't want to spend his hard earned money on it.(go figure) I suggest you not say anything about it. He might think you think there's something wrong with him. Margo No, don't worry. And don't bug him about it. My son didn't date at Berkeley High till a girl asked him to the prom. He went to both proms. With friends who asked him. He had a great high school experience, with lots of friends and involvement in sports. He slowly got into dating in college. He is a very well-adjusted college graduate student now, and he is IN LOVE! Just wait. Anonymous I have a sixteen year old son who has lots of female friends both at school and work and obviously likes girls but admits that he is shy to get closer. I'm counting my blessings! I only warned him that, because he is shy, he is a target for aggressive girls who won't take no for an answer. He is going on an overnight with a coed group from school this week so maybe that will open up some doors. He seems comfortable so I think he should handle it in his own time. Not everyone is anxious to grow up and there are some pretty scary issues around getting physical these days that weren't there when I grew up. Barbara

WHO'S ON FIRST?

A RELATIVE DATING ACTIVITY

MARSHA BARBER and DIANA SCHEIDLE BARTOS

INTRODUCTION

PALEONTOLOGY, AND in particular the study of dinosaurs, is an exciting topic to people of all ages. Although most attention in today's world focuses on dinosaurs and why they became extinct, the world of paleontology includes many other interesting organisms which tell us about Earth's past history. The study of fossils and the exploration of what they tell scientists about past climates and environments on Earth can be an interesting study for students of all ages.

Teaching about Earth's history is a challenge for all teachers. Time factors of millions and billions of years is difficult even for adults to comprehend. However, "relative" dating or time can be an easy concept for students to learn.

In this activity, students begin a sequencing activity with familiar items — letters written on cards. Once they are able to manipulate the cards into the correct sequence, they are asked to do a similar sequencing activity using fossil pictures printed on "rock layer" cards. Sequencing the rock layers will show students how paleontologists use fossils to give relative dates to rock strata.

Once students begin to grasp "relative" dating, they can extend their knowledge of geologic time by exploring radiometric dating and developing a timeline of Earth's history. These major concepts are part of the Denver Earth Science Project's "Paleontology and Dinosaurs" module written for students in grades 7-10. The module is an integrated unit which addresses the following National Science Education Standards:

*Science as Inquiry: Students develop the abilities necessary to do scientific inquiry — identify questions, design and conduct scientific investigations, use appropriate tools and technologies to gather, analyze and interpret data, think critically and logically to make the relationships between evidence and explanations, communicate results, and use mathematics in all aspects of scientific inquiry. *Life Science: Fossils indicate that many organisms that lived long ago are extinct. Extinction of species is common; most of the species that have lived on the earth no longer exist. *Earth and Space Science: Fossils provide important evidence of how life and environmental conditions have changed.

The complete "Paleontology and Dinosaurs" module takes approximately four weeks to teach. The "Who's On First?" activity is a 30-minute introduction to geologic time.

WHO'S ON FIRST? RELATIVE DATING (Student Activity)

INTRODUCTION

Scientists have good evidence that the earth is very old, approximately four and one-half billion years old. Scientific measurements such as radiometric dating use the natural radioactivity of certain elements found in rocks to help determine their age. Scientists also use direct evidence from observations of the rock layers themselves to help determine the relative age of rock layers. Specific rock formations are indicative of a particular type of environment existing when the rock was being formed. For example, most limestones represent marine environments, whereas, sandstones with ripple marks might indicate a shoreline habitat or a riverbed.

The study and comparison of exposed rock layers or strata in various parts of the earth led scientists in the early 19th century to propose that the rock layers could be correlated from place to place. Locally, physical characteristics of rocks can be compared and correlated. On a larger scale, even between continents, fossil evidence can help in correlating rock layers. The Law of Superposition, which states that in an undisturbed horizontal sequence of rocks, the oldest rock layers will be on the bottom, with successively younger rocks on top of these, helps geologists correlate rock layers around the world. This also means that fossils found in the lowest levels in a sequence of layered rocks represent the oldest record of life there. By matching partial sequences, the truly oldest layers with fossils can be worked out.

By correlating fossils from various parts of the world, scientists are able to give relative ages to particular strata. This is called relative dating. Relative dating tells scientists if a rock layer is "older" or "younger" than another. This would also mean that fossils found in the deepest layer of rocks in an area would represent the oldest forms of life in that particular rock formation. In reading earth history, these layers would be "read" from bottom to top or oldest to most recent. If certain fossils are typically found only in a particular rock unit and are found in many places worldwide, they may be useful as index or guide fossils in determining the age of undated strata. By using this information from rock formations in various parts of the world and correlating the studies, scientists have been able to establish the geologic time scale. This relative time scale divides the vast amount of earth history into various sections based on geological events (sea encroachments, mountain-building, and depositional events), and notable biological events (appearance, relative abundance, or extinction of certain life forms).

Objectives: When you complete this activity, you will be able to: (1) sequence information using items which overlap specific sets; (2) relate sequencing to the Law of Superposition; and (3) show how fossils can be used to give relative dates to rock layers.

Materials: two sets of sequence cards in random order (set A. nonsense syllables; set B. sketches of fossils), pencil, paper

Procedure Set A:

1) Spread the cards with the nonsense syllables on the table and determine the correct sequence of the eight cards by comparing letters that are common to individual cards and, therefore, overlap. The first card in the sequence has "Card 1, Set A" in the lower left-hand corner and represents the bottom of the sequence. If the letters "T" and "C" represent fossils in the oldest rock layer, they are the oldest fossils, or the first fossils formed in the past for this sequence of rock layers.

2. Now, look for a card that has either a "T" or "C" written on it. Since this card has a common letter with the first card, it must go on top of the "TC" card. The fossils represented by the letters on this card are "younger" than the "T" or "C" fossils on the "TC" card which represents fossils in the oldest rock layer. Sequence the remaining cards by using the same process. When you finish, you should have a vertical stack of cards with the top card representing the youngest fossils of this rock sequence and the "TC" card at the bottom of the stack representing the oldest fossils.

Interpretation Questions:

1) After you have arranged the cards in order, write your sequence of letters (using each letter only once) on a separate piece of paper. Starting with the top card, the letters should be in order from youngest to oldest.

2) How do you know that "X" is older than "M"?

3) Explain why "D" in the rock layer represented by DM is the same age as "M."

4) Explain why "D" in the rock layer represented by OXD is older than "D" in the rock layer represented by DM.

Procedure Set B:

1) Carefully examine the second set of cards which have sketches of fossils on them. Each card represents a particular rock layer with a collection of fossils that are found in that particular rock stratum. All of the fossils represented would be found in sedimentary rocks of marine origin. Figure 2-A gives some background information on the individual fossils.

2) The oldest rock layer is marked with the letter "M" in the lower left-hand corner. The letters on the other cards have no significance to the sequencing procedure and should be ignored at this time. Find a rock layer that has at least one of the fossils you found in the oldest rock layer. This rock layer would be younger as indicated by the appearance of new fossils in the rock stratum. Keep in mind that extinction is forever. Once an organism disappears from the sequence it cannot reappear later. Use this information to sequence the cards in a vertical stack of fossils in rock strata. Arrange them from oldest to youngest with the oldest layer on the bottom and the youngest on top.

Interpretation Questions:

1) Using the letters printed in the lower left-hand corner of each card, write the sequence of letters from the youngest layer to the oldest layer (i. e. from the top of the vertical stack to the bottom). This will enable your teacher to quickly check whether you have the correct sequence.

2) Which fossil organisms could possibly be used as index fossils?

3) Name three organisms represented that probably could not be used as index fossils and explain why.

4) In what kinds of rocks might you find the fossils from this activity?

5) State the Law of Superposition and explain how this activity illustrates this law.

Figure 2-A. Sketches of Marine Fossil Organisms (Not to Scale)

Dating

Dating

Here of some of the well-tested methods of dating used in the study of early humans:

Potassium-argon dating . Argon-argon dating . Carbon-14 (or Radiocarbon), and Uranium series . All of these methods measure the amount of radioactive decay of chemical elements; the decay occurs in a consistent manner, like a clock, over long periods of time.

Thermo-luminescence . Optically stimulated luminescence . and Electron spin resonance . All of these methods measure the amount of electrons that get absorbed and trapped inside a rock or tooth over time.

Paleomagnetism . This method compares the direction of the magnetic particles in layers of sediment to the known worldwide shifts in Earth’s magnetic field, which have well-established dates using other dating methods.

Biochronology . Since animal species change over time, the fauna can be arranged from younger to older. At some sites, animal fossils can be dated precisely by one of these other methods. For sites that cannot be readily dated, the animal species found there can be compared to well-dated species from other sites. In this way, sites that do not have radioactive or other materials for dating can be given a reliable age estimate.

Molecular clock . This method compares the amount of genetic difference between living organisms and computes an age based on well-tested rates of genetic mutation over time. Since genetic material (like DNA) decays rapidly, the molecular clock method can’t date very old fossils. It’s mainly useful for figuring out how long ago living species or populations shared a common ancestor, based on their DNA.

If you would like to learn more, we recommend visiting these two websites:

2. University of California, Berkeley Museum of Paleontology's Understanding Deep Time online resource. This is an informational tour in which students gain a basic understanding of geologic time, the evidence for events in Earth’s history, relative and absolute dating techniques, and the significance of the Geologic Time Scale.