Welcome to Phil Hardberger Park! The Geology Trail is a short, easy hike under shady trees. It is called the Geology Trail because of the different rock outcrops along it. This earthcache takes you to the Borrow Pit and explains the geology you see there. The trail is level and paved all the way from the listed parking area to the cache coordinates.
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1 The name of this earthcache: GCAXQ49 Phil Hardberger Geology Trail: The Borrow Pit (if you're using the app or message center, this is done for you).
2 Describe the wall in front of you: colors, consistency, layers, etc.
3 Compare: Describe at least one difference, if any (other than the tall hackberry trees) in the living things you see outside the pit from those you see inside. The reading below may give you some ideas what to look for.
4 Analyze: Based on your observations for 3 and the reading below, how are these differences the result of the borrow pit's microclimate? Alternatively, if you didn't notice any difference, are you convinced there is really a microclimate here?
GOOD GRACIOUS, IT'S THE CRETACEOUS!
According to accepted scientific theory, the Cretaceous period, between 65.5 to 145.5 million years ago, featured a warm, tropical climate. This part of Texas was under the ocean at the time.
Illustration: Phil Hardberger Park in the Cretaceous Age (Carter Keairns/Ron Blakely).
All ocean water contains some calcium carbonate (CaCO3), which coral, shellfish and other marine life use to harden parts of their bodies. This calcium carbonate can precipitate, or crystallize out of the water, to form a layer of mud. Remnants of sea creatures can also settle into this layer, becoming fossils. The mud eventualy gets compressed by the weight of layers above and cemented together with calcium carbonate to form limestone.
Limestone, of varying ages, is the dominant rock type in the park. Edwards limestone gets a lot of press in local earthcaches, and with good reason: we depend on the water stored in cracks in the Edwards limestone for our water. But here along the Geology Trail, we get traces of Buda Limestone, named for its outcrops near Buda (just south of Austin). There is a layer of Buda limestone, about 95 million years old, right under your feet, but it doesn't contain many fossils. (You can learn more about Buda limestone at this earthcache.)
COBBLING THINGS TOGETHER
Fast forward about 93 million years. A crack formed in the land, stretching from around Del Rio to San Antonio, then curving up to Dallas started to lift up from the surrounding area, called the Balcones Fault. When it rose up, it formed a ridge called the Balcones Escarpment. You can learn more about the escarpment by visiting this earthcache in Stone Oak Park.
Due to gravity, what comes up must come down. Stones, made of limestone and a similar rock called marl, washed downhill from the escarpment, rounding off as they moved. They eventually formed a layer of gravel and cobblestones here, called conglomerate, which is loosely stuck together by dirt and calcium carbonate.
You can see the live oak roots pushing through and even into the rocks in the walls - this process is called biological weathering. You can learn more about that process by visiting this earthcache in Cibolo Canyons.
The farmers who used to work here found this layer of conglomerate useful for different projects, so they dug this pit to "borrow" fill for use elsewhere.
A GAP IN TIME
Normally, you'd expect there to be more layers of stuff between the relatively young conglomerate in the walls and the 95 million year old Buda limestone just under your feet, but that's not the case. About 93 million years of geology has worn away, due to wave action in the ancient seas that were in the area and other effects. Geologists call areas like this that are missing such layers an unconformity.
AND NOW, FOR YOUR VERY, VERY LOCAL WEATHER
Studies have shown that the walls of the borrow pit can affect the weather conditions in this small area differently than the surrounding area. This is called a microclimate. Here are some of the ways the pit's geology and formation can affect things differently than the surrounding area. The list below does not provide all examples, so if you can think of one that isn't, feel free to share it in your email or message to us!
- There are hackberry trees here, which have pretty shallow roots. They normally only grow about 20 feet tall before they blow over. But because the walls of the borrow pit shelter the trees from the wind, they've managed to grow 50 feet high without getting uprooted.
- When rain falls on the layers above the pit, it would normally seep down through the conglomerate layers until it hit a layer of rock it couldn't go through. But because the layers are exposed, some water can make its way out of the walls of the borrow pit here. This is called a seep. Plants can grow here to take advantage of seeps that wouldn't otherwise do well here.
- The walls of the pit also create more shade, which keeps things cooler. This means the borrow pit doesn't dry out as quickly, which can create conditions more favorable for plant and animal life than the surrounding area.
SOURCES
"The Borrow Pit." Phil Hardberger Park Conservancy
Ethan Bucholz, "Walk Through Time on the Geology Trail." Phil Hardberger Park Conservancy (2013)
Tom Dukes, "Phil Hardberger Park Geology." Texas Master Naturalist.
Carter Keairns, "Geology of Phil Hardberger Park." Texas Master Naturalist.