The walls of the Grand Canyon are made up of layers of different types of rock, including sandstone, limestone, and shale. Dikes, which are intrusions of igneous rock, cut across these layers. The dikes formed *after* the sedimentary layers they cut through.

Here's why:

* Sedimentary Layers: The layers of sandstone, limestone, and shale in the Grand Canyon were formed over millions of years by the accumulation and hardening of sediments.

* Igneous Intrusion: The magma that formed the dikes pushed its way through the already existing sedimentary layers. This intrusion happened much later than the formation of the sedimentary layers.

* Cooling and Hardening: The magma cooled and solidified within the layers, forming the igneous rock that we now see as dikes.

Therefore, the dikes formed after the sedimentary layers of the Grand Canyon were in place.