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BSB Geology Texas Hill CountryMore knowledge = better decisions
Geological forcesdetermine the
look of our Earth.
There was a time when continents were together. When bumping into each other, mountain ranges were pushed up and folded. The area of today's southern United States had a huge mountain range, the Ouachita Mountains. They were pushed up by the African plate pushing against the North American plate. During the Triassic Period about 240 million year ago, the continental plates of North and South America and Africa were together and started drifting apart. |
Texas and the southern part of the American plate was pushed up and folded by the African plate, the Ouachita Mountain range was built.
The name Cretaceous period comes from the Latin language for "chalky" and is usually abbreviated K for its German translation Kreide = chalk. At the beginning of the Cretaceous period, about 145 million years ago, the ancestral Gulf lapped upon the edge of the Texas continent.
The Bear Springs Blossom Nature preserve was covered by a warm shallow ocean about 120-125 million years ago.
Much later nearly all of Texas was covered by the sea. Multitudes of sea life deposited their calcium carbonate sediment – the origin of limestone or "lime" shells and skeletons.
The Cow Creek, Glen Rose, and Edwards were derived largely from skeletal calcium carbonate sediment.
The Glen Rose is the oldest of these limestone layers on our Nature preserve. In the subsurface under our preserve lies the Cow Creek limestone. The Glen Rose in Central Texas was deposited from about 113-108 million years ago and consists of layers of yellowish limestone, compacted clay, compacted silt, and soft lime mud.
The "soft lime mud" usually is a mixture of lime mud and clay, which is called marl.
You can see the Glen Rose formation in stream valleys and road cuts throughout Bandera County.
The sea that deposited the Glen Rose progressively advanced onto the continent.
After the sea level was driven about to the middle of Texas, pure carbonate sediment was deposited in this area.
This deposition is now called the Edwards Group of formations, which overly the Glen Rose formation. The Edwards was deposited from about 108 to 100 million years ago. Remnants of the Edwards are on the high hills in Bandera County, TX.
You all heard about the Balcones fault zone.
The Balcones faulting occurred around 20 million years ago.
Balcones Fault Zones
Graphic by Professor Bill Ward, member of BSB
Bear Springs Blossom Nature Conservation
Far below the limestone layers, a thick salt layer was deposited by evaporating sea water.
The northwestern margin of the Gulf of Mexico subsiding caused enormous tension and movements far beneath the layer-cake surface. A huge part of Texas surface bulged, and a huge section was sliding on the thick salt layer downward toward the Gulf Basin.
Because of all the movement of this huge weight, the region was lifted up, building the Edwards Plateau.
The "hinge" of that uplift or downwarp runs parallel to Interstate 35 between Austin and San Antonio along what we call the Balcone's Escarpment. The Balcone's Fault Zone forms the eastern edge of the Edwards Plateau and the southern edge from San Antonio nearly to Del Rio.
The increased elevation of the Edwards Plateau area increased the gradient of streams, thereby increasing the ability of the streams to downcut and erode. Since that time natural processes of weathering and erosion have carved the limestone into hills and canyons that you see today in Bandera County. You can also say that the streams cut the canyons and what is left between the canyons are our Texas Hill Country hills.
Stone is hard and water is soft, but water is powerful and persistent and wears away stone. Water penetrates pores and cracks - when it freezes it expands, sometimes breaking the stones. Rainwater picks up carbon dioxide from the air and soil and forms a dilute solution of carbonic acid, which can easily dissolve limestone.
Caverns, sinkholes, and fantastically shaped pieces of limestone result.
Vegetation and mulch slows down the flow of water across slopes, holding water in the soil and allowing it to settle into aquifers.
By slowing the flow of water, vegetation protects slopes from excessive erosion and increases infiltration.
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