By Bill Ward
Published in The Boerne Star on January 20, 2006
It goes without saying that the soil type and, consequently, the vegetation in this part of Texas are intimately related to the limestone country rock. But what is limestone?
Limestone is a special kind of sedimentary rock composed almost entirely of one mineral called calcite (calcium carbonate). The Cretaceous limestones in this area were derived from bits and pieces of calcium carbonate shells and hard parts of marine organisms, such as bivalves, snails, sea urchins, calcareous algae, coral and other organisms.
During the early Cretaceous period, shallow seas covered most of what is now Central Texas. At that time, over 100 million years ago, the mainland was low-lying, and thus the rivers brought very little sediment to the sea.
Unlike the muddy Gulf today, the Cretaceous sea was sparkling clear. It is in clear, warm shallow seas that calcium-carbonate-secreting organisms proliferate.
For tens of millions of years, calcium carbonate sediments accumulated on the sea floor across Central Texas. As these carbonate sediments were progressively buried beneath subsequent deposits, they were trans our Hill Country bed rock.
To tell the strict truth, not all of the country rock around Boerne is limestone. At some time during the history of becoming rocks, certain of the calcium carbonate sediments were subjected to pore waters rich in magnesium, and thus they were transformed to calcium magnesium carbonate, a mineral called dolomite.
Some other layers of rock around here contain clay and sand, because from time to time some sediment did flush into the Cretaceous sea from the ancient mainland.
Also, there are gypsum (calcium sulfate) layers, that were precipitated from hot, high-salinity waters of isolated bays in near shore areas of the marine shelf. formed into the thick sequences of limestones we now know as
For the most part, it is safe to say, our bedrock is carbonate rock, limestone or dolostone. What makes carbonate rocks different from most other sedimentary rocks (sandstone, shale, conglomerate) is that carbonate rocks easily dissolve in fresh water. Limestone terrains are full of caves, sink holes, and other dissolution features.
Soils in limestone country are sure to be alkaline. Kendall County soil maps show several different kinds of soils. I don’t pretend to understand classification of soil types, because there are too many variables that are inconsistently chosen to designate a soil type. But one thing is for sure, virtually all the soils around here are alkaline or basic, the opposite of acidic. Plants that grow in our region have to tolerate alkaline soils.
Most people in this area live either on the thin soils of the Glen Rose and Edwards Formations or on floodplain deposits derived from those formations. Soils on the Glen Rose outcrop are more variable than on the Edwards outcrop.
For example, I’ve heard some friends who live on the Glen Rose outcrop say they can’t get a certain native plant to grow because of poor drainage, but other friends who also live on the Glen Rose can grow this same plant easily.
This is because some units of the Glen Rose Formation have high percentages of clay. These clayey members are weathering into soils which have lower permeability and poorer drainage. Of course, the clayey soils may contain a few more elements that nourish some plants better than soils derived from pure limestone.
By the time the younger Edwards Formation was being deposited, the shoreline had migrated far inland and little material eroded from the land reached this region. In other words, the Edwards rock typically is pure limestone, with soils and vegetation reflecting that.
Wouldn’t it be interesting to know just how closely the distribution of different native plants in the Boerne area is controlled by the soil type and underlying rock? Probably many details of this subject are yet to be documented, but let’s explore it some more in future columns.
