Ancient Tides Recorded in Indiana Rocks

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 equatorial plane (as much as 28°). As the moon orbits the earth its position relative to the earth's equator changes. Since approximately 75% of the tide is attributed to 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.

Select the link below to download a lesson plan for use with middle school or high school students: