The Precambrian Shield, also known as the Canadian Shield, formed over a very long period of time, spanning from approximately 4.5 billion years ago (bya) to 540 million years ago (mya).

Here's a breakdown:

* Archean Eon (4.5 - 2.5 bya): This is the earliest period of the Earth's history. The majority of the Precambrian Shield formed during this time, with the oldest rocks dating back to almost 4 billion years.

* Proterozoic Eon (2.5 bya - 540 mya): This eon saw further rock formation and development of the Precambrian Shield. While not as intense as the Archean, it still added significant volume and complexity to the shield.

Important Note: The Precambrian Shield is not a single, monolithic structure. It's a vast, interconnected network of ancient rocks that have been affected by various geological events over billions of years. Its formation was a complex process with different stages of rock formation, deformation, and erosion.

Therefore, it's more accurate to say that the Precambrian Shield formed over a vast span of time rather than a specific period.