Many parts of Texas are experiencing severe drought (though not quite as severe as the b*tch that was the 2011 drought). As a result of these exceptionally dry conditions, a certain riverbed in Dinosaur Valley State Park has run dry, exposing previously undiscovered dinosaur tracks that are roughly 113 million years old. How is this possible—like, how have they not been eroded by the river? Water is pretty excellent at eroding rock, especially over the course of 113 million years. On the one hand, I do understand that most likely the river has not been there as long as the dinosaur tracks have been, but on the other hand (1) we actually don’t know this for sure, and (2) even if it has been there for less time than the tracks have been, there certainly has still been ample time to erode them away. What am I missing here?
[https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjWsazb4t_5AhXRk2oFHVUIBawQvOMEKAB6BAgGEAE&url=https%3A%2F%2Fwww.cnn.com%2F2022%2F08%2F23%2Fus%2Fdinosaur-tracks-discovered-texas-park%2Findex.html&usg=AOvVaw2co9awHFuiD-qAeMOP5DNk](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjWsazb4t_5AhXRk2oFHVUIBawQvOMEKAB6BAgGEAE&url=https%3A%2F%2Fwww.cnn.com%2F2022%2F08%2F23%2Fus%2Fdinosaur-tracks-discovered-texas-park%2Findex.html&usg=AOvVaw2co9awHFuiD-qAeMOP5DNk)
*EDIT/UPDATE: Aha! It now makes sense. It turns out that these particular dinosaur tracks were first discovered in 1908, when a layer of rock that had been covering the tracks broke up and washed away due to a flood. In the time since then, there have been multiple paleontological excavations that have revealed even more of the tracks. And the most important thing to know here—the thing that ties it all together and makes everything “click”—the River* ***has*** *been eroding the prints ever since the first tracks got exposed in 1908. It’s just that obviously on a geological timescale, the 114 years between 1908 and now are just a tiny, minuscule blip in the grand scheme of things… nowhere near long enough for the tracks to have been eroded. So it all makes sense, knowing that the river has only been eroding the tracks for 114 years instead of 113 million years.*
*Source:* [*https://www.nps.gov/…/nature/making-dino-prints.htm…*](https://www.nps.gov/dena/learn/nature/making-dino-prints.htm?fbclid=IwAR3MZ-SQHiJg8uZ7LEjeSZTNgE705yDE4x54VFmrHZgGdySHhI5kLdXgcO4#:~:text=When%20dinosaurs%20walked%20through%20the,where%20people%20can%20see%20them)
In: 1541
Go to [https://txpub.usgs.gov/txgeology/](https://txpub.usgs.gov/txgeology/) and search for “Dinosaur Valley”, it will take you to the state park. You can see the Kgr units around the riverbed. If you click on it you will get a popup saying:
“Limestone, alternating with units composed of variable amounts of clay, marl, and sand…..soft to hard….thickness 40-200 feet”
So the rock beneath the river is layered, with different materials, and some of them can be soft vs hard, and pretty thick.
The river, as it erodes, will go through the soft stuff quickly, but slows down on the hard stuff, leaving that exposed much longer.
In some scenarios at least, the same water has eroded something further above and deposited it on top of the footprint. Being of different make it wouldn’t adhere so well to the already solidified riverbed while still protecting it, allowing researches to remove it like a kinder surprise (and a lot of care)
I have some geology background but not a paleontologist by any means, so I may not be totally correct. But basically, in order for those prints to be preserved in the rock at all, they had to be buried by layers of sediment and allowed to eventually turned to stone. The water is in fact constantly eroding the stone its flowing on, eroding away layers of sediment and eventually uncovering the footprints. The footprints are definitely eroding over time, they just haven’t been exposed long enough to erode away completely.
In the pictures, along the side of the riverbed, you can see the layers of rock the river had to erode through before uncovering the footprints.
1) Step in soft mud to leave a footprint
2) Footprint in soft mud dries into kinda hard clay
3) Kinda hard clay gets covered by other soft soil. Lots of soft soil.
4) Soil compacts for a few million years and and the kinda hard clay turns into very hard rock
5) A river shifts and begins to erode an area.
6) Erosion takes away any soft and kinda hard stuff, exposing the footprints that are now in very hard rocks.
The short answer is that most of them ARE eroded away.
Fossilized footprints form when a footprint gets filled in and covered up, which gives the soil that the footprint is in time to harden. After a while, something changes, and that protective layer gets eroded away without damaging the fossil too badly.
But most fossils are destroyed in exactly the way you describe. The fossils we find are the lucky ones that survived.
Timing is everything. We need to find ’em at the time where they’re exposed by weathering et al and before they’re destroyed by weathering et al.
What others have said about the sediment being on top. Also, rivers don’t stay in one place, they constantly move. These prints may have been in the river for only a few hundred to a few thousand years.
I think the bigger question to ask here is how do they exist through 110 million years of tectonic plate activity. Those footprints could have been made all the way in South America and even covered with ocean at some point.
The flat terrain and winding path of the river means that the river is eroding its bed very slowly. During normal flows it’s probably depositing sediment from up river and only erodes the bed during floods.
water is a powerful erosional force but what really gives it the power are things like suspended material (sediment, tiny pieces of other rocks/minerals (think silica sand), debris, etc) within the water, the overall discharge (how MUCH water is moving through the space, is it narrow and a lot of water or wide and shallow) and the gradient of the overall watershed (meandering stream over 200 miles in Nebraska vs rapid loss of elevation from high point to low point over 20 miles). If you have crystal clear water w/ no suspended load (either in the water column or kinda dragging/bouncing along the bottom of the channel) then your erosional power is muted. A moving water source with lots of suspended load (like the Colorado River, pre dams) then your erosional power is greater (Grand Canyon). Like I stated before, combine suspended load with gradient, discharge & the general temporal history in the space (did a fault move and the stream was redirected and therefore hasn’t been flowing over that space “all that long”) impact the overall erosional effectiveness of moving water.
So that is a very simple approach, in application a lot of other factors could play a part, like others have said, a clay-rich rock without much interstitial cement (between the grains) is gonna be easy to erode while a sandstone with abundant silica cement holding the grains together is going to be much tougher… I mean even down to the grain shape (surface area) will effect how quickly it erodes.
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