Some Curious Facts about the Origin of the Lakes

I have written again and again about the lakes, and I return to them because I keep discovering new things to enjoy and to learn about them. Every summer I find new paths, new hills, new places to enjoy, even new villages which I bring within my range by starting earlier and riding harder.

And then there are the eternal spots on the banks, where the water is at its best, high points on the hills, from which the views are at their most spectacular, the people-less woods filled with croaking frogs, or squirrels jumping from tree to tree, the quiet places that seem to breathe slowly and calmly as though the world did yoga there.

There is always more to discover, and more to learn. And recently I’ve been trying to find out a little more about the geology of the area, because people ask, and I like to have answers.

The whole region is formed from limestone rocks. An awful lot of geology seems to be about calcium, water and carbon dioxide*, in fact:

The geological formations of Ruidera are, chemically speaking, based on the process of precipitation and dissolving of the carbonates (essentialloy calcite). The conditions for precipitating and dissolving of CaCO3 depend on the relatively high level of Calcium Ca2+ and (H2CO3) ions, or of the carbonate (HCO3-) respectively in the water. The dissolution of a calcareous sediment or of limestone in water with a given level of CO2 can be described by the following reactions:
 

·        Ca2+ + HCO3-  --> CaCO3 + H+.                

·        H2O + CO2 --> H2CO3  y CaCO3 + H2CO3 --> Ca2+ + 2HCO3- 

The chemical process of dissolution or precipitation of CaCO3 depends, among other things, on the following factors:    

·       The carbon dioxide (CO2) content. 

·       The pH of the water: A low pH value favours the dissolving of  CaCO3, and vice versa.  

·       Temperature: The dissolution of  CaCO3 in pure water decreases with an increase of temperature. 

It turns out- I finally got round to looking up the history of the area- that the current lakes are only about 10,000 years old. Our ancestors knew only the river, and earlier there were similar formations at higher levels, the remains of which can still be seen rising above the banks in some places. The Cave of Montesinos, which Cervantes riffed on brilliantly in the Quijote, and the Grieta del Toro, were once part of that system, but are now 100m above it.

The most curious thing, it seems to me, is that, while the banks of the lakes are being eroded away by the water**, they are also being created by it. The karstic formations which form the banks and the divisions between the lakes in much of the system are deposited directly by precipitation of the calcium compounds from the water, which is very rich in them, presumably from having come down from the mountains to the east which are made of them. So while the gentle flow is widening and deepening its own channels and basins, it is also building them up around it. Which will win, over time, will, I imagine, depend on the rate of flow and the exact concentrations of minerals and carbonates in the water. History suggests erosion will win.

*My previous experience of geology, derived from walking in the hills with a friend at University who was studying the subject, suggested it was mostly about drinking beer and whacking things with a hammer before laughing maniacally and jotting strange figures on maps. I now realize there is more to it than that.

**Although they are described as lakes, they are part of the river system, and there is a continuous, though usually very gentle, flow throughout the system, enough to coause erosion over time. And karstic rocks are very soft.