Glaciers don't have a distinct middle layer like a cake. They are made up of ice, which is constantly flowing and changing. However, there are different zones within a glacier, based on their properties:

* Surface Zone: This is the uppermost layer, exposed to the air. It's often covered in snow and can be very porous.

* Firn Zone: Below the surface zone, the snow becomes denser and more compact, turning into firn. Firn is granular ice that is still relatively porous.

* Glacier Ice Zone: This is the deepest and densest part of the glacier, where the ice is solid and compact. This zone can be further divided into zones depending on the depth and pressure:

* Upper Zone: This is the zone where ice is still relatively brittle and can fracture easily.

* Lower Zone: This is the zone where ice is highly compressed and can flow more easily.

So, while there is no "middle layer" in a strict sense, the firn zone can be considered the intermediate zone between the surface snow and the dense glacier ice.