Welcome to Phil Hardberger Park and the Salado Creek Greenway Trail! This earthcache takes you to view limestone rocks in Salado Creek and explains the geology you see there. The trail is paved until the last 20 feet or so of dirt trail, which is still flat and stroller- and wheelchair-friendly. Optional stage 2 is a little rougher and not so stoller- and wheelchair-friendly.
TO LOG THIS EARTHCACHE
Earthcaches have no container to find or log to sign. In order to log a find, please message (preferred) or email us answers to the following questions (preferably, copy the questions and paste them into your email or message with your answers - this helps us keep track of which earthcache you’re logging). You don't need to score 100%, just do your best.
Please do not post the answers in your log, even if encrypted. There’s no need to wait for confirmation before you log a find, but we will respond if you send a message or include your email address in the email. Group answers are fine, please just let us know who was with you.
1 The name of this earthcache: GCAXQF5 Salado Creek: Bio-tur-what now? (if you're using the app or message center, this is done for you).
2 Describe the rocks you see in the carved-out creek bed: shapes, colors, and so on. Other than the holes, can you see other evidence of ancient life, like tracks or fossilized remains?
3 Analyze: Based on your observations and the reading below, can you determine the size of the creatures that made these holes based on measurements made today?
(Optional) 4 Compare: examine the holes in the loose stones in the creek bed at stage 2 and compare to the carved-out stream bottom under the bridge. Do you think the loose stones are all Buda limestone? Explain your reasoning.
HOW THESE ROCKS FORMED
According to accepted scientific theory, the Earth's climate was warm and tropical between 65.5 to 145.5 million years ago. During this period, called the Cretaceous, if you were here, you'd be more than a hundred miles out to sea.
Illustration: Phil Hardberger Park in the Cretaceous Age (Carter Keairns/Ron Blakely).
The sea water would contain a dissolved mineral calcium carbonate (CaCO3). Sea creatures such as shellfish and coral don't have bones, but they can pull calcium carbonate out of the water and make shells and other hard body parts.
Calcium carbonate slowly precipitated (crystallized) out of the sea water here to form a thick layer of mud. Over time, other layers of rock and sediment formed above the muck, pressing it together. It got stuck together with more calcium carbonate to form limestone.
Most of the rock you see in this area is limestone. The rocks along the stream bed of Salado Creek are about 95 million years old and are called Buda Limestone, because it was first studied near the town of Buda (just south of Austin).
IT AIN'T EASY BEING CHEESY
What's with all the little holes? When the limestone you see here was still just mud, different invertebrates, or creatures without a skeleton inside, burrowed through it, searching for food, much like earthworms still do today in the dirt. They swallowed sediment, digested nutrients from it, and then excreted (pooped) the rest out behind them.
When evidence of ancient life is preserved in rock, that's called a fossil. Often, we think of fossils as actual plant or animal remains preserved in rock, like dinosaur bones or petrified trees. Buda limestone does not contain many of those types of fossils, but the burrows, tracks, nests, and coprolites (preserved excrement, or yes, poop again) that it contains are technically fossils as well.
But hang on, mud doesn't just leave holes in it. It's soft enough that any burrow holes would fill back up or collapse. So how did these rocks get all these holes?
Limestone can be easily dissolved by acid. When rain falls through the air, it can pick up carbon dioxide gas, making it slightly acidic. Thisis called carbonic acid, which eats through limestone rock. This is a form of chemical weathering, and when it happens, the resulting landscape is called karst.
When the Buda limestone became rock, the trails of sediment and such left by the invertebrates were still part of that rock. But the acidic water ate through the sediment that the invertebrates excreted (or...you know) more easily. That doesn't mean that these worms and such were the exact same size as all the holes you see today, however. Some of the bigger holes could be from nests. Also, the rock that wasn't burrowed-though mud also has been eaten away some by carbonic acid. But the holes can be a rough guide as to the size of the creatures that made them.
SOURCES
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.