What happened to the strips of clay as they were pushed from opposite ends?

When the clay strips are pushed from opposite sides, the tectonic plates move, causing a collision and elevation of the earth's surface.

Explanation:

The pushing of the clay strips on opposite sides causes the tectonic plates to move, causing them to collide.

When the tectonic plates meet and collide, the most impacted or the lightest ones, it sinks below the denser plate, creating an elevation of the surface, while folding and forming a double layer and higher, forming what we call mountains.