Red Hill EarthCache EarthCache

Like much of South Central Texas, what is now Palmetto State Park was underwater during much of geologic time.  During the Jurassic, thick layers of salt were laid down; during the Cretaceous, thick layers of chalk and limestone (such as the Austin Chalk) were deposited in the area.  This was followed by marine clays of the Midway Group, and the Paleocene littoral sands and clays of the Wilcox Group.  During the Eocene, mud and sand continued to be washed in from the rising Rockies far to the west, while at times sea levels rose to inundate the Texas coastal plain; the mixed group of formations formed during this time is known as the Claiborne Group.  

 

At the bottom of the Claiborne Group is the Carrizo Sand, which formed as a thick layer of coastal plain and shoreline sand at the edge of what was then part of the Gulf of Mexico, with fairly high resistance to weathering.  As sea levels rose in the Eocene, marine mud and near-shore sand was deposited and altered to form the Reklaw Formation, a much less resistant clay-like formation largely made of glauconite and characterized as either shale or greensand.  

 

Other alternating layers of sand and shale followed, including the Queen City Sand, Weches Formation, Sparta Sand, and the Cook Mountain Formation; the highest member of the Claiborne Group, the Yegua Formation, outcrops as characteristic "knobbs" in Lee and Bastrop Counties, to the north.  After the late Eocene, South Central Texas was above water for good; many of the subsequent formations such as the Whitsett and Catahoula Formations are of volcanic origin - ash and tuff blown onto the south Texas plains from volcanic areas in West Texas.  

 

The stresses of these added layers of sediments washed down from the Rockies strained and effectively liquefied the underlying salt layers from the Jurassic, causing fault systems such as the Balcones fault system far to the northwest and the Luling fault zone which passes through this location; the accumulated sediments slid toward the center of the Gulf of Mexico, kind of like a sled on snow.  During the Pliocene and Pleistocene, with sea levels up to 400 feet lower than today, rivers cut deep into the Texas plains; the subsequent deposition of late Pleistocene and Holocene alluvial deposits in now-shallow river valleys such as those of the Brazos, Colorado, Guadalupe, and their tributaries now form the basis of the Texas blackland, responsible for much of the state's agriculture.  Meanwhile, the sandstone formations, being porous but resistant, retain many of the state's aquifers.

 

 

Q1: Based on what you have learned above, and the map shown immediately below this question, what rock formation comprises the harder outcroppings here, and in what geologic epoch was it formed? Keep in mind that you are located just north of Ottine, but also note the resistance, makeup, and appearance of the rock here before sending in your answer. The map key is linked here and both the map and key are also available on the image gallery of this geocache page.

Q2: Here's a basic geology test: given the nature of this rock formation, would you classify this as igneous (formed from magma or lava), sedimentary (formed by deposition of eroded sediments), or metamorphic (changed from the other two by extreme pressure or chemical change)?

Q3: How has the relative density of this particular rock outcropping altered the course of the nearby river, if at all?  Do you see any clues as to where the course of the river may have been over time, such as meander scars?

Q4: Name at least one geographic or geologic feature mentioned in the text that might account for the discontinuity between these rocks and similar rock formations many miles to the northwest.