Tidal Time - Ancient Tides Recorded in Indiana Rocks; Page 3

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Geologic Time & Ancient Environments > Tidal Time

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Tidal Cycles (continued)

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.

GEONOTE:
The Hindostan whetstone beds are siltstones that can be found in Orange County, Indiana. These rocks are 300-million-years old 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 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?

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.

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