Tide Spring
In the 1920s, families would pack a picnic basket on Sunday afternoons and head for a scenic overlook or an idyllic millpond. Around Harrisonburg, one of the favorite destinations was a stretch of rolling farm pastureland near the community of Cherry Grove in Rockingham County.The attraction was neither overlook nor millpond, but a hole in the ground. At one moment, water would pour out of the hole, sometimes at a rate of 1,000 gallons a minute. Then the water would shut off abruptly, like a faucet. No one knew why.
The place was called Tide Spring for its resemblance to ocean tides. In 1936 the ebbing and flowing water caught the attention of the state geologist, who apparently had more humor than explanations. He wrote the phenomenon off as "uncanny," calling it perhaps the work of "mischievous elves" residing in underground caverns.
According to current thinking, Thomas Jefferson, who wrote about the spring in 1787, may have been much closer to the scientific explanation. Jefferson speculated that Tide Spring was a "syphon fountain."
Scientists and geologists today say that Tide Spring is a reservoir in the porous limestone and dolomite rock typical of the region. It belongs to a type of spring with a variety of names—ebb and flow spring, periodic stream, temperamental spring, and breathing spring. Only a few exist in the world, and Tide Spring is one of the most unusual.
The complex dynamics involve a syphoning action with a self-cycling timer. As the reservoir beneath the surface fills with groundwater, water is forced to the surface where it overflows and runs to a nearby stream.
When the rising column becomes higher than the source end, pressure changes cause the flow not only to shut off but also to flow backward to regain equilibrium. The action is turbulent and noisy and so interesting to watch that the landowner allows school groups to visit his farm.
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The "self-cycling timer" noted in the description above runs at variable times depending on the amount of water in the ground. When we arrived we noticed that it seemed as if the spring had recently run, so we weren't sure how long we would have to wait. In wet weather it can run as often as every 20 minutes; in dry weather as infrequently as once a day. Here are the photos of our adventure:
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| Standing on a rock in the cattle field on the way down to the spring |
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| The black hole to the right of Grandma is where the spring bubbles up. You can see the water standing there from where it had run not long before we arrived. |
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| Watching and waiting for the water to reappear. We brought entertainment. |
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| Bubbles while we wait. |
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| Grandpa on his phone, still waiting. |
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| Finally, about 40 minutes after we got there, we start to see the water bubbling up through the rocks. |
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| It rises quickly and steadily ... |
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| ... until it's gushing down over the rocks. |
The girls had fun playing in the crystal clear water during the almost 10 minutes that it ran before beginning to slow down and then recede and reverse its flow back into the earth. A truly special activity for Easter Sunday!
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