Here's how it works:

* Subduction: Oceanic plates (thinner and denser) slide beneath continental plates (thicker and less dense) at convergent plate boundaries.

* Scraping Off: As the oceanic plate subducts, sediments and fragments of the oceanic crust get scraped off the top and accumulate along the edge of the continental plate.

* Accretion: Over time, these accumulated fragments, called terranes, become welded onto the continental plate, adding to its size and complexity.

Examples:

* The Coast Mountains in British Columbia, Canada, are a prime example of accreted terranes.

* The Sierra Nevada Mountains in California were also formed by accretion.

So, the large slabs of crust that were stuck on to the North American plate were accreted terranes.