Ancient Tides Recorded in Indiana Rocks
by:
Erik P. Kvale
Introduction
Ever hold a month of time in your hand? Of course it is impossible, but you can hold a time recording instrument in your hands (like a stopwatch or an hourglass). Did you
know that rocks are like stopwatches in that they record time? A block of sandstone records the amount of time it took to deposit and then cement the little sand grains into
a sandstone. The problem with "rock stopwatches" is that the face is difficult to read. Typically, it takes much longer to cement the sands than it does to deposit them but
exactly how long is not always clear. However, for certain rocks in Indiana we know precisely how long it took to deposit them (down to almost the exact hour). These
rocks were deposited by ancient tides when Indiana had a lot of beachfront property and an ocean covering its southwestern corner (no humans, however, just big insects
like dragon flies with two-foot wing spans and amphibians large enough to eat them). To understand and fully appreciate these rocks one needs to first understand tides.
An understanding of oceanic tides has been important to human kind ever since people decided that there are good things to eat in the oceans and that oceans make
good highways to go from one place to another. But what are ocean tides and how do they form? From a geological point of view and a coastal engineering perspective,
tides are also important since they are capable of generating currents that erode, transport, and deposit sediments (or houses or wharfs or ships). This article will try to
answer "what are ocean tides and how do they form" and show you that, even in Indiana, tides were once important.
What are tides?
Tides are the rise and fall each day (daily) and sometime twice-a-day (semidaily) of the ocean.
For instance, if you were to walk along the shores of the Bay of Fundy in eastern Canada you
would not want to go far out on the tidal flat for long. If you walked way out on the tidal flat at low
tide in the morning, by late morning or early afternoon you could be under as much as 50 feet of
water at high tide if you stayed in one spot. If you could hold your breath for about 6 hours you
would once again be on dry (well...damp anyway) ground and able to exhale. Not all coastlines
experience that much daily or semidaily rise and fall of tides, but all ocean-facing coastlines have
tides.
Did you know that without the sun and moon, the earth would not have noticeable tides? Tides are
caused by the gravitational pull of the moon and sun on the earth's oceans. The amount of tidal
influence (how high the tide rises each day) along any coastline varies with the positions of those
bodies relative to the earth.
In some cases, the high tides will actually deposit a thin layer of sediment on coastal tidal flats.
This phenomenom is illustrated in the top animation to the right (click the right-arrow button to
play the animation). If the daily or semidaily tidal rise and fall is large enough (the amount of rise
and fall is termed "tidal range") thin layers of silt or sand will continue to stack up on each other
(think of a stack of poker chips) and actually leave a record of the tidal activity (bottom
photograph to the right). Geologists call such deposits tidal rhythmites. If you understand this you
are beginning to understand how we can read certain types of "rock stopwatches."
"Tidal Rock Stopwatch"
The thickness of each layer of a tidal rhythmite deposit is determined in a general way by how
high the tide rises that day. Thicker layers reflect higher tides and thin layers reflect lower tides. In
some cases, tidal rhythmites consist of stacked successions of layers in which successive layers
gradually thicken and then thin. This progressive thickening and thinning is in response to the
moon and sun changing their positions in the sky relative to our coastal tidal flat.
To understand these changes it is often useful to think in terms of purely astronomical tides and equilibrium tidal theory. By definition, equilibrium tides are ideal and
defined by the gravitational forces of the moon, and to a lesser extent the sun, on an idealized earth completely covered by deep water of uniform depth that is capable of
instantly responding to changes in tractive forces. Of course our world is not covered by an ocean of uniform depth (otherwise we wouldn't have dry land to stand on), but
the model does help us to understand what is going on.
Tidal Cycles
First of all, in some areas of the world like the Bay of Fundy, the tides rise and fall twice a day. This is because on our equilibrium earth the tidal forces from the moon and
sun together produce two tidal bulges. The bulges are on opposite sides of the earth (see upper diagram to the right). The rotation of the earth through each of these
bulges once a day produces two tides a day (the semidiurnal tide). Typically, one of these tides will be higher (dominant) than the following tide (subordinate) (see lower
diagram).
The combined gravitational "pull" of the moon and sun on the earth, which
produces the tidal bulges, can vary in a number of ways. Perhaps you are familiar
with the neap-spring tides related to the phase changes of the moon (sometimes
referred to as the synodic month ). Daily tides are higher when the earth, moon,
and sun are nearly aligned (such as at full or new moon), and smaller when lines
to the sun and moon from the earth form a right angle (at 1st or 3rd quarter phases
of the moon). Watch the bar that appears in the left area of the upper diagram as it
plays (click the right-arrow button to play the animation). Note that the bar is at its
highest when the earth, moon, and sun are aligned, and it is at its lowest when
they are at right angles to each other.
The lower graph shows actual modern tidal data. Note the systematic rise and fall
of the semidiurnal tide over a period of about 30 days. If you move your mouse
pointer over this graph, you will see a core interpreted as showing neap-spring
cycles. Thinner neap tides and thicker spring tides are marked. A mouse click will
show a bar chart of measurements from this core.
If you understand the semi daily tidal cycles and the tidal cycles related to the
phase of the moon, you can see how tides have affected the thicknesses of the
layers in the delicately layered Hindostan whetstone beds. At this point, you are
beginning to understand how rocks can record time. You may also be beginning to
understand how geologists are able to determine how rocks were originally
deposited. We know the whetstone beds were deposited by tides because we know
of no other process that would produce such regular thickness changes in the
layers of the rock.
The orbital plane of the moon around the earth is inclined relative to the earth
The upper diagram animates the semidiurnal tide (click the right-arrow button to
equatorial plane (as much as 28°). As the moon orbits the earth its position relative
play the animation). The continents that rotate by may seem a bit odd to you,
to the earth's equator changes. Since approximately 75% of the tide is attributed to
but this is the configuration of the continents 300 million years ago. The
approximate position of Indiana is marked by the red X.
the moon, the tidal bulge follows the moon in its orbit around the earth. When the
moon (and tidal bulge) are over the equator, the height of the morning high tide is
equal to the height of the afternoon high tide (termed crossover in the lower
diagram to the right). The time it takes the moon to orbit the earth is called the
tropical month and it is 27.32 days.
GEONOTE:
The Hindostan whetstone beds are
siltstones that can be found in
Orange County, Indiana. These
rocks are 300-million-years old
The lower diagram is a graph of a semidiurnal tide. These tides can be
preserved as thick and thin layers in a rock. Position your mouse pointer over
the lower diagram to see a core interpreted as a semidiurnal tidal signal. If you
then click and hold the mouse button down on this diagram, the graph shown
Select the link below to
download a lesson plan for use
contains actual measurements of the core. Note the thick and thin layers
representing the dominant and subordinant tides.
with middle school or high
school students:
tidalites. From the early 1800s,
these deposits were mined as a
"whetstone" — stones used to
sharpen knives. In fact, by the end
of the 1800s Indiana led the world in
the production of whetstones.
Persons who used these
Modern and Ancient Tides lesson plan 622 K
Note: The lesson plan requires the Adobe® Acrobat® Reader®. If you don't have the Reader, select the button below
to go to the Adobe site where you can get it.
Persons who used these
whetstones really had "time on their
hands."
Prior to the Civil War, the Hindostan
whetstone was also used for
headstones in graveyards. These
whetstone tombstones can be found
in many old cemeteries in
southwestern Indiana (including
Bloomington) and along the
Wabash and Ohio Rivers in Illinois.
In all cases, the "tidal signature" can
be seen in the tombstones. In most
cases, the person whose grave is
marked with the whetstone lived a
longer period of time than it took for
the material in the headstone to be
deposited (but not nearly as long as
it took for the silt to turn into
siltstone). That's something to think
about isn't it?
The diagram animates the tropical month (click the right-arrow button to
play the animation). Because we are limited to the two dimensional
screen we cannot show the bulge actually following the moon. We do
show, however, where the apex of the bulge nearest the moon would
be. Note that there are two crossovers in each tropical month.
This diagram graphs modern tides. Follow the semidiurnal tides until
two are of nearly equal height. This is the crossover. Move your mouse
over this diagram to see the cross over points in a core. Can you see
the crossovers? Click on the graph to see the measurements again.
The crossover points are interpreted to occur where there is not a
pronounced dominant or subordinant tide.