Complex sentence: Mill Creek lies in the Stockton Formation (from the Triassic period), which is composed of coarse-grained arkosic and other sandstone, shale, siltstone, and mudstone.
Split into simpler sentences: Mill Creek lies in the Stockton Formation, a sedimentary laayer of rock laid down during the Triassic. Mineralogy includes coarse-grained arkosic and other sandstone, shale, siltstone, and mudstone.