Most of Uruguay is a Precambrian shield or craton, which consists of crystalline rocks, metamorphites and intrusive rocks. This craton is one half of an craton which was broken into two parts 145 Million years ago when Gonwana broke up and the South Atlantic opened. The other half is located in the Kalahari. Those old rocks were shaped by the Transamazonian orogeny 2 Billion years ago and the breakup of Rodinia (700–500 Ma). The Río de la Plata Craton underlies practically all of Uruguay, eastern Argentina and southern Brazil. In Uruguay, it only shows up in the south and east since in other parts of the country, it is covered by younger volcanic rocks or sediments.
The territory of Uruguay is covered by several sedimentary formations ranging from Devonian sandstones to Quaternary loess. They all form patches, as deposition have not been uniform and erosion has cleaned surfaces. The Lower Devonian sandstones cover the central part of Uruguay and form a narrow east-west to northeast oriented band.
Uruguay was affected by the Karoo Glaciation (360–260 Ma) and was subsequently covered by ice lobes of the great ice sheet that covered large parts of Gondwana. The result are numerous glacial striae on shales and varve-like sediments.
Quite impressive are the Paraná traps, the Paraná continental flood basalt province, forming a major lithological unit extending over northern Uruguay, Argentina and Brazil, and Namibia. This eruptions happened during the opening of the South Atlantic in the Cretaceous and was caused by the Tristan da Cunha plume. Basalts and rhyolites, but also syenite and other granitoid intrusions, are the result.
The geology is not supportive for the formation of caves, there are no limestones or other soluble rocks for karst caves, there is no young volcanism for lava tubes. Of course there are a few quite exceptional caves nevertheless, but only two caves are currently open for tourists.