* Igneous Rocks: These are formed from the cooling and solidification of magma or lava. Common types found in shields include granite, gabbro, and basalt. These rocks are often found as large batholiths, which are massive intrusions of igneous rock.

* Metamorphic Rocks: These are formed when existing rocks are transformed by heat, pressure, and/or chemically active fluids. Common metamorphic rocks found in shields include gneiss, schist, and quartzite.

These rocks are generally very old, often dating back to the Precambrian era (more than 540 million years ago). They are also very resistant to weathering and erosion, which is why they have been preserved over such long periods.

It's important to note that while igneous and metamorphic rocks are the primary building blocks of continental shields, they are often associated with sedimentary rocks as well. These sedimentary rocks can form within basins within the shield, or they can be deposited on top of the shield during periods of geological activity.

Here's a simplified explanation:

1. Early Earth: Molten rock (magma) erupted and solidified, forming the first igneous rocks.

2. Further Heat and Pressure: These rocks were further transformed by heat and pressure, becoming metamorphic rocks.

3. Continents Form: These ancient igneous and metamorphic rocks formed the cores of the continents, known as continental shields.

4. Sedimentary Rocks: Over time, sediments eroded from these shields were deposited in basins, forming sedimentary rocks.

Continental shields are the oldest and most stable parts of Earth's continents, offering valuable clues about the planet's early history.