There isn't a single type of rock that specifically stops the flow of water. It's more about the rock's properties and the context of where it is. Here's why:

* Permeability: Rocks with low permeability (meaning water can't pass through easily) will act as a barrier.

* Examples: Granite, Shale, Quartzite.

* Size and Shape: A large, solid rock can block water flow in a stream or river.

* Location: A rock placed in a riverbed will naturally disrupt the flow, even if it's somewhat permeable.

Here are some specific examples:

* Aquicludes: These are layers of rock, like clay or shale, that are very impermeable and act as barriers to groundwater flow.

* Dams: Humans build dams using concrete (which is made from crushed rock) and other materials to stop water flow for various purposes.

* Glacial Till: This is a mixture of rock debris deposited by glaciers, often creating barriers to water flow.

Let me know if you have more questions!