Written by Tom Dukes, a past chair of the Alamo Group of the Sierra Club and long-time, local science teacher. EDITOR’S NOTE: Tom Duke’s Geology piece is meant to make more accessible to a wider audience some of the more detailed and technical concepts found in Carter Keairns’s piece on Geology. Readers wanting more in-depth information should follow the links to consult Carter’s piece: “Geology of Phil Hardberger Park”
Geology has helped shape Phil Hardberger Park, although much of the evidence lies hidden beneath vegetation and soil. The underlying rock explains why our creeks often go dry. It is the reason rainfall filters in or runs off quickly. It is why we have mostly thin, alkaline soils. Geology ultimately helps determine which plants and animals thrive here. As you walk, the Geology Trail, the “borrow pit” and nearby Salado Creek Overlook may serve as focal points, but subtle signs of geologic processes are all around you. The paragraphs below cover the basics, but a more advanced treatment can be found in Carter Keairnes’s piece referenced above.
Phil Hardberger Park, like most of central Texas, sits atop thick layers of limestone. Typically white, beige, or light gray, limestone forms in warm, shallow marine areas such as today’s Gulf of Mexico or around the islands of the Caribbean. All sea water contains trace amounts of dissolved calcium carbonate, CaCO3. It is this chemical that corals, clams and other shelled sea creatures extract to form their hard parts. When high evaporation causes some calcium carbonate to come out of solution, it precipitates, or settles to the bottom, as a limey mud. Tiny calcareous algae floating in the water also add to this sediment as their remains drift downward. Eventually this limey mud, compressed by further sediments above, hardens into limestone. If structures resembling the shells of clams and sea snails appear in this hardened sediment, we call them fossils. While fossils are fairly common in Texas limestone, not many can be found in Phil Hardberger Park. Elsewhere, if you are observant, you may discover many a curved clam or oyster imprint or the spiral coil of a sea snail within central Texas rocks.
The limestone beneath the soil of Phil Hardberger Park dates back to the Cretaceous Period, some 100 million years ago. This was a warm, tropical time when dinosaurs roamed the land. During the Cretaceous, our part of Texas was usually covered by shallow seas. The water gradually advanced and retreated as sea levels changed and continental masses rose and fell with immeasurable slowness. When this area was submerged, layers of limestone were deposited. As the seas retreated, erosion slowly ate away the exposed rock. This cycle repeated many times before the Cretaceous ended 66 million years ago. Each cycle left rock layers with distinct characteristics and unique sets of fossils, allowing geologists to study and map the layers.
Salado Creek skirts the eastern edge of Phil Hardberger Park. Depending on rainfall, it can vary from a dry creek bed to a raging torrent. When flooding, streams or rivers cause significant erosion as they carry away rocks and sediment. Once the water slows, it deposits those sediments downstream. Evidence of such deposition can be seen in the borrow pit on the eastern side of the Geology Trail near Blanco Road.
THE BORROW PIT
Here, the Geology Trail crosses the Salado Creek Greenway and enters a small borrow pit, a hole where people have dug out dirt or gravel to use elsewhere. Who do you think might have dug this pit? Notice how the Geology Trail drops slightly into the borrow pit and loops around it. Atop the walls of the borrow pit you can see how thin our layer of soil actually is. How would that affect the plants growing here? This is also a great spot to observe how plant roots thread themselves downward in search of water and nutrients.
The walls of the borrow pit expose loose materials deposited by flowing water during relatively recent Quaternary times, within the last two million years. Imagine a stack of books laid one atop another. Common sense says the bottom book was deposited first and the top one came last. So it is with rock layers. Here, older rock layers from Cretaceous times must lie beneath these relatively fresh Quaternary sediments near the surface. You might wonder, if the solid rock below is almost 100 million years old and these layers are less than 2 million, where is all the material in between? The answer is, it is simply missing. It eroded away long ago, leaving a gap in the rock record geologists call an unconformity. Notice how the small rocks and pebbles along the walls of the borrow pit are roughly sorted by size and arranged in poorly defined horizontal layers. This is because as rapidly running water slows down, it drops the biggest and heaviest gravel first. What rock type do you suppose all these little pebbles are? If you said limestone, you’re right.
PLANTS, MEET LIMESTONE
Our native plants have not only adapted to long droughts and little rain. They must also survive in the thin, poor, rocky soils caused by the limestone below. Limestone is chemically alkaline, another word for antacid. In fact, limestone and Tums, a popular stomach remedy, share the same formula: CaCO3, or calcium carbonate. As vegetation decays, it produces weak acids. Plant life depends on these acids to free up soil micro-nutrients essential for growth. But since limestone is alkaline, it neutralizes the acid. This leaves many soil nutrients locked away in chemical compounds useless to plants.
Every raindrop dissolves a bit of carbon dioxide as it falls, creating a tiny amount of very weak carbonic acid. This is the same acid found in fizzy soft drinks. When rain hits limestone, its carbonic acid is neutralized and a few molecules of limestone dissolve away. Given enough thousands and millions of years, this slight acidity will dissolve vast amounts of solid limestone. Other rock types slowly crumble and decompose, helping build thick, rich soil. Limestone does not. It gradually dissolves away, leaving thin, rocky, alkaline soil, often low in nutrients. Plants must be tough and resilient to thrive under such conditions.
RAINWATER, MEET LIMESTONE
The slight acidity of rainwater also slowly dissolves any underground limestone it contacts. This gradually expands cracks in the rock and eventually leaves it with tiny pores or holes capable of holding even more water. Continued long enough, the pores expand farther and the limestone may start to resemble Swiss cheese. Over immense time the pores may even grow so large that underground caves and caverns form. Rainfall and creek flows quickly drain away into these cracks and spaces below. In this way, an underground aquifer such as our own Edwards Aquifer is born.
San Antonio is very lucky to have the Edwards Aquifer, the underground rock layer from which we pump our water. The aquifer got its name because it occurs in the Edwards Limestone Formation. Its limestone is hard and dense, ranging from 300 to 700 feet thick. In many places underground it has been slowly dissolved away to resemble a hard rocky sponge full of water. The Edwards also has many larger water-filled passages and caverns. Long ago, all this pure, clear water reached the surface from numerous natural springs. Human use has caused the water level to drop, so today we must pump it out.
No rocks of the Edwards Limestone are exposed along the Geology Trail, but they are down there some 100 feet below us.
Certain features always seem to appear where limestone is abundant: caves and caverns, aquifers, disappearing streams, and almost no natural lakes. We call such areas karst regions. Our part of central Texas is a prime example of karst. Our streams are small and often dry. Our lakes are formed behind dams, and our nearby caverns inspire visitors with their beautiful dripstone features.
THE SALADO OVERLOOK
Geologists name rock layers for a location where they are exposed at the surface. The limestone of Phil Hardberger Park is part of the Buda Formation, found near Buda, Texas. It is around 94 million years old. Like the Edwards Formation, the Buda is Cretaceous in age, but it is younger, so its layers are above the Edwards. Here in the park, we can see the rocks of the Buda along the banks of Salado Creek at the Salado Overlook. During countless floods the creek has carved away much rock, creating a spectacular cliff below the overlook. Like the Edwards 100 feet below, much of the Buda exhibits the solution cavities and porous nature so common in limestone of karst areas.
Just a few yards downstream from the Salado Overlook is the lower entrance of a tiny cave, the Enchanted Forest Pit. It is one of only two known Buda Formation caves in Bexar County. DO NOT ENTER THE CAVE. IT IS ILLEGAL AND DANGEROUS. You can read more about Enchanted Forest Pit in Dr. Carter Keairns’s piece, “Geology of Phil Hardberger Park.”
As you walk the trails of Phil Hardberger Park, enjoy the sights and sounds that surround you, but consider how the geology beneath your feet has helped to form your experience. There are many fine resources available to continue your study of these topics. Visit the park’s Urban Ecology Center or ask a member of the Hardberger Park Conservancy for other suggestions.
For more information for children see, Rockin’ and Rollin’.