likely due to the collision of the Greater Antilles arc with the Bahama Platform of the North American Plate. Deformation increases northward across Saint John. The Narrows pluton and Virgin Gorda batholith, which are dominated by tonalite followed the episode of compression, forming around 39 million years ago in the Eocene. East-west compression brought brittle fractures filled with veins of calcite and limonite. Spreading at the Cayman Trough may explain north-plunging late folds, due to eastward movement of the Caribbean plate. The formation of the Cayman Trough produced the last significant tectonic feature on the islands after 39 million years ago, with sinistral strike-slip faults. A structural block produced as a result creates a 5.7 kilometer sinistrally offset contact between the Water Island and Louisenhoj formations. Hydrogeology One of the most in-depth studies of groundwater in the U.S. Virgin Islands was conducted by the U.S. Geological Survey in 1995, focused on Saint Croix. The south-central part of the island is underlain by alluvium and carbonate rocks, including lenses of silt and clay. The Oligocene to Miocene Jealousy formation, the Miocene and Pliocene Kingshill Limestone and Pliocene Post-Kingshill Limestone are all a single hydrological unit. The water-table aquifer ranges between five and 68 feet below the grounds surface and overall groundwater in the area contained comparatively high sodium chloride concentrations. References +U.S. Virgin Islands Geography of the United States Virgin Islands