It is far more efficient from a body (heat) management point of view to wear several thin layers as opposed to one thick layer. Conditions change extremely quickly in the mountains and it is better to be over rather than under prepared. Extra layers can always be taken off.
The diagram below will explain the layering system.
Insulation The Protective Layers
Base Layer (1st Layer) this layer traps air next to the skin and draws perspiration away from the skin and transports it into the next layer thus helping you to keep warm and dry.
Ultimate Base Layer (Layer 2) Additional layer for those who really feel the cold.
Mid Layer (Layer 3) Microfleece provides additional warmth to work with either base or ultimate layers.
Outer layer (Layer 4) combines weather protection, durability and comfort.