Lazily meandering its way through the southern portion of Texas, the Frio River is lined by cypress trees and provides no shortage of recreational opportunities to Texas residents and visitors alike in Garner State Park, just 75 miles west of San Antonio. The region is home to geological formations dating from the Cretaceous period, which are reflected in the colorful rocks of the Frio. Does this Spark an idea?
Limestone
Dating back 145 million years, the area surrounding the Frio River, along with much of the rest of Texas, was covered by a vast Cretaceous sea. As this sea evaporated, it left behind limestone deposits that remain to this day. Limestone is mainly composed of calcite and mineral fragments from the bones of ancient crustaceans, including coral. These are shallow beds of limestone in some regions along the river, but form more impressive craggy cliffs in other sections.
Shale
Shale's muddy origins are apparent in its appearance.
The secondary layer of rock that is located around the Frio River is primarily composed of shale. Known as the Glen Rose formation, it combines the shale with siltstone and marl beds which were then covered by the limestone that visitors to the region can see upon arrival. Shale, composed of mineral-rich mud which is then finely compacted through time and erosion, is common along riverbeds in this region.
Siltstone
Siststone is a third classification of rock that makes up the bed of the Frio River and can be seen in its surrounding regions. This is quite similar to shale but possesses finer grains and pores because it is composed of silt rather than clay. However, the two types of rock are found in close proximity to one another in this area because they were formed around the same time period, before the Cretaceous Sea covered the Texas plains.
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