A Hole In The Ground

While on a recent hike I came across a karst feature on our property. We've seen it before, but this time I decided to photograph it and see how deep it was. I hiked out today and found it again.

I've posted about karst features on the Onion Creek Water Quality Protection Land, but it's neat to find one on our property. Karst is a geological feature, basically a hole or cave, that allows rain water to enter directly into an aquifer. They are formed by water percolating through limestone and are widespread throughout our part of Texas. Mary found this one years ago but we never paid much attention to it and haven't always able to find it.

It used to be surrounded and almost hidden by a very large prickly pear cactus, but that has died and the hole is now easily visible. The entrance hole is about 8 inches long and 3 inches wide.

Here's a better view of the inside. The rock at the top of the  hole is about 8 inches thick.

I lowered a weighted string into the hole to see how deep it was. The weight went down 5 feet! I think the  hole actually veers to the side, so the actual depth may be greater.

I wonder how long it took rain to form the karst? Is it relatively quick, by geologic standards? Maybe a few hundred or a few thousand years? Or did it take hundreds of thousands or millions of years?

And why there? Is the rock especially soft right there? The spot doesn't look like it's in the middle of a wet-weather creek, so I don't think water runs through there regularly.

I also checked the wildlife water supplies today. The recent rains have them operating well. It's been over a month since I last trucked water out to them. Both of them are full. Here's a photo of the newer, smaller one. The re-bar spikes I put in to keep the dishes from being moved around by animals also seem to be working.