Steps to repro:

1. Open attached project and 't1' scene on Mac with integrated Intel Iris GPU;
2. Observe the Game view window.

or

1. Create new project and scene;
2. Add Terrain gameobject (Create> 3D Object> Terrain);
3. Zoom in/zoom out in the Scene view and observe terrain's surface in the Scene view.

Expected result:
Terrain is rendered smoothly without artifacts in the Scene and Game view. See attached expected.png screenshot.

Actual result:
Shadowy artifacts are shown on terrain's surface. See attached actual.png screenshot.
Regression introduced in Unity 2018.1.0.

Notes:
- Reproducible in Version 2018.2.0a1, 2018.1.0b2;
- Not reproducible in Unity 2017.3.p2, 2017.1.2p4;
- Reproducible only on OSX Metal with integrated Intel Iris GPU; not reproducible on MacBook with the discrete NVidia or AMD GPU; not reproducible on Windows running DX11;
- Also reproducible in Standalone Player;
- Turning off shadows in the project solves the issue.