Fig. 1 a The dark layer is till. If a stream valley forms sub aerially
after the glacier has deposited the till (top image), the till layer will be cut
and not drape the valley (bottom image).
Fig. 2b. If the valley is formed subglacially (in top image, blue is ice,
dark blue is water in tunnel and in
bottom image white is air) after the tunnel valley forms , the advancing
ice of the glacier is likely to deposit till along the walls of the
glacier (bottom image).