What is the famous 4000 year old Ziggurat at Ur was constructed of a solid core of mud bricks and covered by a thick skin of?

The famous 4000-year-old Ziggurat at Ur was constructed with a solid core of mud bricks and covered by a thick skin of fired bricks. This architectural marvel served as a temple complex dedicated to the moon god Nanna and exemplifies the grandeur of ancient Mesopotamian civilization. The ziggurat's tiered structure and impressive height were designed to elevate the temple closer to the heavens, highlighting the city's religious significance. Its remains continue to be a pivotal site for understanding early urban architecture and society.