The contact between the shale, which dominates the bottom part of this sedimentary sequence, and the sandstone, which dominates the upper part, is best described as

Gradational.

Explanation:

Gradational means stage wise successive changes. The contact between shale and sandstone is gradational. It has different layers of rocks and soil which are lying below the surface. The sandstones dominates all the parts as it is largest part and occupies more space.