The exposed rock types in the vicinity of the thermal springs are sedimentary rocks of Mississippian to Ordovician age, with the exception of younger igneous rocks (Cretaceous age) exposed in two small areas about southeast of the thermal springs (Potash Sulphur Spring and Magnet Cove, respectively), and in many small dikes and sills. Most dikes are less than wide. There have been 80 dikes noted about southeast of Hot Springs, on and near the Ouachita River. There is no indication that igneous rock occurs where the thermal springs discharge.