What type of rock stops the flow of water?

Igneous rocks, particularly those with a high density and low permeability, can effectively stop the flow of water. Examples include granite and basalt, which have tightly interlocking mineral grains that prevent water from easily passing through. Additionally, sedimentary rocks like claystone can also act as barriers to water flow due to their fine particle size and low permeability. These types of rocks are often found in geological formations that serve as aquitards or confining layers in aquifers.