GY 111 Lecture Notes D Haywick 2008 09 1 GY 111 Lecture Notes Bed Attitude AKA Strike and Dip Lecture Goals A Beds in 3D space the problem of orientation B Strike and Dip C Geological maps 1 horizontal and inclined bedding on map Reference Press et al 2004 Chapter 11 Grotzinger et al 2007 Chapter 7 p 152 154 GY 111 Lab manual Chapter 5 A Beds in 3D space the problem of orientation Up until now I have been illustrating just about every important geological concept in two dimensions For example in the last lecture I illustrated the Principle of Superposition with the adjacent cartoon The thing is that beds are planes not lines and therefore I really should have used a three dimensional or perspective cartoon to illustrate superposition This one is more realistic albeit a bit more annoying to draw on the fly during a lecture GY 111 Lecture Notes D Haywick 2008 09 2 If you think about it it is relatively easy to describe the orientation of horizontal bedding All you have to say is horizontal and everyone immediately visualizes a flat surface e g a table the floor etc But we have a serious problem when it comes to describing the orientation of an inclined bed Picture a tilted plane like the one drawn in the next cartoon Were you to just say that the bed is inclined every insightful person that you met e g your fellow GY 111 classmates would ask how much and in which direction After all each of the beds in the next cartoon is inclined but every one is inclined differently Some are tilted more steeply than others Some are tilted in different directions GY 111 Lecture Notes D Haywick 2008 09 3 Geologists as I have said many times before are clever people and we needed to come up with a way to describe bedding that everyone everywhere would understand Even Vladimir in Russia The method that we came up with to standardize bed attitude is consequently clever and despite first appearances relatively simple1 It does however require an understanding of simple trigonometry However as the Hitchhiker s Guide to the Galaxy2 states Don t Panic B Strike and Dip The system that geologists came up with to describe the orientation of inclined bedding is named after the two principle components required to describe 3D spatial orientation strike and dip Picture an inclined sedimentary bed Now picture a horizontal plane intersecting that bed see the cartoon below When two planes intersect they intersect in a line It stands to reason that if one of those planes is horizontal the resulting line must also be horizontal This is the strike component of attitude Measuring the orientation of a horizontal line is relatively simply You just have to use an magnetic compass You hold the compass parallel to the strike line and read the orientation relative to magnetic north In the first cartoon on the next page the strike line trends north I should mention at this point that a line by necessity trends in two directions If a line strikes north it must also strikes south If a line strikes east it also strikes west Very soon you run into a problem as far as the strike is concerned Consider the strike lines portrayed in the second cartoon on the next page Allowing that the first one trends northeast southwest how would you describe the strike of the other 1 I have also mentioned that geologists are uncomplicated We tend to use more intuitive means by which to describe the Earth and all its workings Had a biologist or worse a chemist or psychologist devised the system it would doubtless involve quantum mechanics applied calculus or something having to do with your mother 2 Coinsures of the Douglas Adams series of tales will argue convincingly that the recent Hollywood movie did not do justice to the 5 books in the trilogy which in turn did not do justice to the original BBC radio plays GY 111 Lecture Notes D Haywick 2008 09 two The second one trends a bit more east west than the first and the third trends a bit more eastwest than the second Life is a lot easier when you apply an absolute number to the orientation of the strike line and considering that all possible orientations describe a circle see adjacent cartoon it is reasonable to use compass degrees in reporting the strike direction So a strike line that trends north south also trends 0 180 A strike line that trends east west also trends 90 270 and a strike line that trends northeast 4 GY 111 Lecture Notes D Haywick 2008 09 5 southwest e g line 1 in a previous cartoon also trends 45 225 How would you deal with the other two lines in this cartoon Well line 2 trends at 55 235 and line 3 trends at 60 245 I m going to make your life easier still Since by definition a line trends in 2 directions exactly 180 apart you really only need to specify one direction By convention we go for the smaller of the 2 numbers Line 1 therefore strikes at 45 line 2 strikes at 55 and line 3 strikes at 60 Now we have to tackle the other component of bedding orientation the amount and direction of inclination Consider the two inclined beds below Both beds have the same strike 0 but the amount of inclination and the direction of inclination are both different The dip is defined as the amount of inclination measured from the horizontal plane downwards Bed A is inclined about 65 toward the west and Bed B is inclined about 45 toward the east The dip of the first bed is therefore define as 45 W and the dip of the second bed is defined as 60 E Now let s put it all together We use the following format to report strike and dip Strike Dip dip direction the symbol separates the two components For the two examples given in the previous figure the strike and dip would be given as follows Note that both of the beds strike at 0 The strike and dip of the bed A is therefore defined as 0 60 W GY 111 Lecture Notes D Haywick 2008 09 6 and the strike and dip of the bed B is 0 45 E In class and later in the labs you will be given several examples of strike and dip You will also be given the opportunity to plot the strike and dip symbols of variably orientated beds on maps This is actually fairly simple You use the following strike and dip symbol The long side of the symbol records the strike direction this time 45 and the smaller tick shows the direction of dip this time toward the southeast The number gives the amount of dip 30 So this symbol indicates that the bedding is orientated 45 30 SE I will provided with a few more examples of strike and dip during the lecture We will also discuss a few other