Tidal Time - Ancient Tides Recorded in Indiana Rocks; Page 2

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Geologic Time & Ancient Environments > Tidal Time

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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 upper diagram animates the semidiurnal tide (click the right-arrow button to play the animation). The continents that rotate by may seem a bit odd to you, but this is the configuration of the continents 300 million years ago. The approximate position of Indiana is marked by the red X.

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 contains actual measurements of the core. Note the thick and thin layers representing the dominant and subordinant tides.

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