Geology: Rock Formations and Units

Hi guys and welcome to Field Notes. Now
when i first started learning geology one of the things that confused me the
most was how you even begin to look at a hunk of rock and determine what happened
to get it there how would you know that those cracks are
not man made and how can you determine the line one rock starts and another
rock ends? and what do you do when you have found it some of this comes from stratigraphic
hierarchy or how we divide rocks into units it's much like how we divide
plants and animals into King Philip came over for good spaghetti kingdom phylum
genus what is it to kingdom phylum class order family genus species? By doing this
this gives us a starting point as to how we divide the massive amounts of rocks
on earth so to start this is what the hierarchy looks like. This hierarchy is
dependent on being able to tell when rock composition or rock properties have
changed.

A lot of the time you will have an actual list of all the different
rocks that you are going to be expected to see, like say if you were out mapping,
but you're still going to spend a lot of time hunched over one rock determining
if it's a limestone or a dolomite sometimes it is really obvious, and other
times it is way more blurry. here is a list. Here's one such lists you see it it's a
list of all of the different rock types that you will find near Red Lodge
Montana this is the only example i have of it look at it so dirty so to get started
learning about this hierarchy we need to know that the base term is formation
like we've seen many times before geology can be kinda wishy-washy so
there's no hard and fast rule for what qualifies a new formation and is
dependent on how complex the geology of the area is in other words what
qualifies as a new formation in Illinois may not be the same as what qualifies
for a new formation in Montana.

So past the formation ranking we can get larger and we can get smaller
let's get larger first. If there are two or more formations that seem to share
some sort of characteristic, either they are the same kind of rock or they have
the same distinctive fossil within them we can group them together in a group
and if there are two or more groups that seem to go together we can group them
into the less common but still a thing super group. Super groups are not really
common but they do exist so they're a good thing to keep in mind now going the
opposite way getting smaller from formations we have members there's no
real standard for separation for a member basically if you think this hunk
of rock is different enough in the formation you can have a member and then
finally the last two that I want to mention are bed and flow.

Bed is the
smallest formal unit. Only distinctive beds are given formal names and really all a
bed does is help you determine where you are within a member or a formation and
finally a flow is kind of what it sounds like it is a volcanic event that you can
see in a member or in a formation and much like members these are only given names
if it is distinctive and widespread. So while these generic terms are all well
and good sometimes things just get too blurry and
too complicated to divide anything past a formation and honestly sometimes you
just look at it and go well there's a lot of different things going on here
but it's generally the same kind of rock so you put it into a group a great
example of something that is just too complex to divide any further is the
Madison limestone group now i have heard this called the Madison limestone
formation i have heard this called the medicine limestone supergroup i have
personally been taught that it is the Madison limestone group.

The Madison
limestone is a rock that is found out west it is found all over the place out west if
you see a humongous hunk of limestone were probably looking at the Madison
limestone. So the Madison LS is not only limestone it has
interbedded shale sections it has sections where it is more fossiliferous
but then it will switch back to this original limestone so instead of
dividing each of these tiny sections of shales and tiny sections of different
types of limestone geologists have kind of grouped this whole thing together into
one big limestone group ok so now that we've got these basic
terms down how do we use it in active geology, active geology as opposed to
that sedentary geology Oh that all reeks of a bad geology pun doesn't it? The two main ways are going to be in stratigraphic columns and in
mapping stratigraphic columns are basically vertical column representations of
layers of rocks and really they deserve their own videos so that's all we're
going to say about that but you'll notice that they're labeled with the
names of the rocks and that each of those rocks sections are of varying
thicknesses which corresponds to how thick the unit actually is when you saw
it again depending on whether you're doing a snapshot of a huge depth or
simply 10 meters is going to determine how far you break the units down.

If you're
doing a column all the way down to bedrock then you might just stick to
labeling groups and formations but if you're looking at one group in
particular you'll probably label members beds and flows. Next we use these names as well as the
strat columns for the age-old geology pastime of mapping now mapping with a
strat column and determining where the original breaks are in a rock are pretty
similar with typical mapping that you will do for classes and research you
probably are already going to have a strat column with all the names of rocks
that you should expect to see but like I said you're probably going to spend a
decent amount of your time just bending over a rock determining what it is. Just
like somebody would have when they were first making the list so that is all
we've got for an introduction of stratigraphic hierarchy.

Something else
that is really helpful when you're dealing with these is to remember Steno's
Laws i have a whole video whole separate video on Steno's Laws, it's really quick be sure to check it out remember that field notes is now on patreon so pop over to my patreon
page to check out some of the perks that you could get or suggest some if you
have something in particular that you would like from me maybe google hangouts
been playing with that idea for a while be sure to leave any questions that you
might have in the comments i try to get to just about everybody and remember to
like the video if you liked it subscribe if you would like to see more and i will
see you guys next time.

Mr. bug i'm going to save you Drew does not believe me that you're in here.
I found a bug. now supergroups are not super common.

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