Can a geologist here explain this to a dumbass?

>Sedimentary dikes or clastic dikes are vertical bodies of sedimentary rock that cut off other rock layers.

>They can form when shallow unconsolidated sediment is composed of alternating coarse-grained and impermeable clay layers the fluid pressure inside the coarser layers may reach a critical value due to lithostatic overburden. Driven by the fluid pressure the sediment breaks through overlying layers and forms a dike.

What causes the coarse clay layer to shoot up exactly? Is it lighter than the impermeable clay? I know nothing about geology, but I'm wondering how this works.