Hardberger Alluvial Plain EarthCache
Hardberger Alluvial Plain
-
Difficulty:
-
-
Terrain:
-
Size: 
(other)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the smaller area over which the rivers flood at a particular period of time, whereas the alluvial plain is the larger area representing the region over which the floodplains have shifted over geological time.
As the highlands erode due to weathering and water flow, the sediment from the hills is transported to the lower plain. Various creeks will carry the water further to a river, lake, bay, or ocean. As the sediments are deposited during flood conditions in the floodplain of a creek, the elevation of the floodplain will be raised. As this reduces the channel floodwater capacity, the creek will, over time, seek new, lower paths, forming a meander (a curving sinuous path).
The leftover higher locations, typically natural levees at the margins of the flood channel, will themselves be eroded by lateral stream erosion and from local rainfall and possibly wind transport if the climate is arid and does not support soil-holding grasses. These processes, over geologic time, will form the plain, a region with little relief (local changes in elevation), yet with a constant but small slope.
Hardberger Park is located in the transition zone of the Edwards Aquifer, where areas of groundwater recharge may occur. Surrounded by dense urban development, the park is a model for how landscape can function as a filter, cleansing stormwater before it infiltrates into the aquifer.
Three formations of the upper Cretaceous Age are exposed at Hardberger Park: Austin Chalk, Eagleford Shale, and Buda Limestone. A fault line runs through the park, displacing bedrock at the Salado Creek, where a thick layer of alluvial sediments called the Quaternary Terrace, has provided rich soils for plant communities along the creek corridor.
The erosion and weathering of bedrock along the Salado Creek corridor is an expression of thousands of years of water flowing through the landscape. Exposed limestone bluffs, deposits of fossils and karst features such as solutions cavities and vugs can be seen in the creek bed below.
A vug is a small to medium-sized cavity inside rock. It may be formed through a variety of processes. Most commonly, cracks and fissures opened by tectonic activity (folding and faulting) are partially filled by quartz, calcite, and other secondary minerals.
Identification of both active and inactive alluvial plains are important in city planning. Alluvial plains are helpful in identifying 100 year floodplains, which need to be identified in order for future building to occur. It is also important to understand if the alluvial plain is active or inactive.
In order to claim credit for this earthcache, you must send your answers to the CO via the Geocaching website. You may go ahead and log the cache as found. If there are any problems with your answers, we will contact you.
1. From this vantage point, you should be able to see at least 2 things that would identify this as an alluvial plain. What are they?
2. Wander along the side of the creek and see if you can determine if this is an active or inactive alluvial plain. Why do you think so?
Reference....Wikipedia
Additional Hints
(No hints available.)