By Bill Ward
Published in The Boerne Star on November 12, 2002
In some ways the wildflowers and other native vegetation we enjoy in the Boerne area today were predestined by events that took place more than 100 million years ago, even before flowering plants existed anywhere on Earth.
This is because the native flora over the Edwards Plateau comprises plants that thrive in alkaline soils. We have alkaline soils, because they are derived primarily from weathering of limestone. And our “country rock” is predominantly limestone because of conditions that existed in this area about 100 to 110 million years ago.
During the Early Cretaceous Period, the region that is now Central Texas was on the northwestern margin of the ancestral Gulf of Mexico. Shallow seas covered the Boerne area. The type of sediment that turns into limestone was accumulating from the breakdown of shells and other calcium carbonate hard parts of certain marine organisms.
About 110 million years ago, the shoreline lay 20 or so miles to the northwest of the Boerne area. Sand and mud that washed off the land was occasionally carried out to sea by marine currents. This was the time when the sediments of the Glen Rose Formation were being deposited in the shallow Gulf. (The Glen Rose is mostly limestone with some mudstone layers.)
At the same time that lower Glen Rose carbonate sediments were accumulating in the sea around here, the Fredericksburg region was a land area. Streams of that area carried sand and gravel derived from erosion of the granites and other old rocks of the Llano Uplift. (In and around Fredericksburg there is non-marine to shallow-marine Hensel sandstone and conglomerate that are time-equivalent to our lower Glen Rose marine limestone.)
As time went on, the sea rose to flood the Llano land area. Then shallow-marine calcium carbonate sediments accumulated over the older rocks. This is the time when the upper Glen Rose and Edwards sediments were deposited.
Around both Boerne and Fredericksburg the upper Glen Rose and the overlying Edwards Formation are mostly marine limestones.
For many tens of millions of years after the Edwards sediments accumulated, this part of Texas continued to be covered by seas.
Eventually thousands of feet of marine sediments were deposited in our area. For about the last 40 million years, however, this has remained a land area, where streams have continued to erode the landscape. By now, the millions of years of erosion have stripped off the younger layers to uncover Lower Cretaceous sedimentary rocks. The two rock units now exposed at the surface around Boerne are the Glen Rose and Edwards limestones.
Native-plant assemblages growing on soil derived from the Glen Rose outcrop may be different from those growing on Edwards soils.
Even different parts of each formation support different plants, because the variations in composition and texture of the rocks affect soil thickness, moisture, and drainage.
All the soils, however, are alkaline.
More acidic soils develop on the outcrop of the Hensel sandstone around Fredericksburg and on the granite exposed in the vicinity of Enchanted Rock. We can expect to see some native plants there, which do not grow here on our alkaline soil.
A great place to see alkaline-soil plant assemblages is along the trails at Cibolo Nature Center in Boerne. Acidic-soil plant assemblages are seen on the trails of the Fredericksburg Nature Center. Both of these places are open to the public free of charge.
The Fredericksburg Nature Center trails are adjacent to Lady Bird Johnson Municipal Park off of Highway 16 S.
Many of the native plants along two of the main trails through different ecosystems are identified by signs. Checklists are available for all the trees, wildflowers, and grasses on the site. Well over 200 species of native plants have been identified.
It’s worth a trip to Fredericksburg.
