73. Hillsborough River State Park 

Oligocene Tampa Member of the Arcadia Formation of the Hawthorn Group

Hillsborough River State Park

15402 N US Hwy 301 Thonotosassa, FL 33592

Oligocene Tampa Member of the Arcadia Formation of the Hawthorn Group

The unique hydrology and geology at Hillsborough River State Park support a lush ecosystem containing endemic plant and animal species enjoyed by thousands of park visitors each year. Some of the most visibly interesting geological features are the rapids on the upper stretch of the river. These rapids are caused by the flow of water over the erosion-resistant, partially silicified sections of the Oligocene Tampa Member of the Arcadia Formation of the Hawthorn Group exposed along the riverbed and the banks of the upper Hillsborough River.

The Tampa Member is a limestone ranging from soft and micritic to hard and crystalline, and it contains an abundant marine invertebrate fossil assemblage and small amounts of sand. More so than other rocks, limestone, which is composed of calcite (CaCO3), is subject to chemical weathering or erosion due to the chemical process of dissolution. However, when limestone has been partially or entirely silicified (a process by which silica replaces other minerals in a rock), it becomes much more resistant to this form of chemical weathering.

Silica is much harder and more erosion-resistant than the calcite in limestone, leaving a more durable layer of silicified rock.