* Grain size: Fine-grained rocks, like shale or mudstone, have tiny particles. This can lead to a smooth surface if the rock is well-compacted and the grains are evenly distributed.
* Weathering and erosion: Over time, weathering and erosion can smooth the surface of any rock, including fine-grained ones.
* Surface features: Some fine-grained rocks can have features like fossils, ripple marks, or bedding planes that make the surface less smooth.
* Rock type: Even within fine-grained rocks, there are variations. For example, a smooth, polished shale might have a very different surface texture than a rough, crumbly mudstone.
In short: While fine-grained sedimentary rocks *can* be smooth, their surface texture depends on a variety of factors.
