From the hydrogeology point of view, rock formations are categorized conveniently as unconsolidated (loose), consolidated (hard) and semi-consolidated. Recent and older alluviums are unconsolidated sedimentary formations, usually as alternate beds of sand and clay (or shale) with varying thickness and proportion. The sand formation is a natural carrier of water; the coarser the grain size and lesser the compaction of the sand, the better the water content and its flow. On the other hand, clay and shale are impervious and are natural barriers to groundwater flow.

Water is also unable to pass through compact rocks like granite, basalt, quartzite, etc., which are usually devoid of any primary porosity. However, over time, the top portions of these hard rocks can develop numerous fractures and get weathered into loose formations with granular consistency when exposed to extensive weathering. Weathered and fractured portions can transmit groundwater commensurate to the degree of secondary porosity developed due to weathering. 

Although formations like sandstone are compact, they are prone to quick weathering and develop extensive joints and fractures and turn out to be suitable aquifers. Similarly, limestone is prone to develop cavities and solution channels. Both sandstone and limestone yield a moderate to a good quantity of groundwater and may be considered semi-consolidated formations.