Cold Shut is one type of surface defects and lines on the surface of the casting can be seen clearly with naked eyes. The flow marks indicate metals were solidified at different stage of time.
The main reason was caused as molten flow comes into contact with the cooler die surface then solidifies before completely filling of the cavity. If another molten flow and lie on top of the previous metal flow, it is likely to be a crack.
Proper maintaining mold/die temperature and gating design to avoid gas during filling processes are highly recommended to avoid this defects.
Here are some cause and root, why this is happening. There are air entrapment and rapid temperature drop in the last filled area. The cold shut possibility was even likely to happen to the parts.
Real defects of cold shut, with numerical simulation you can even predicted before the real parts manufactured.