limestones and dolomites dominate the madison. because the rock is highly soluble, it often develops caves and karst topography. lewis and clark caverns, montana, is an example of a cave developed in the madison. the rocks were deposited in a generally shallow marine setting, indicated by the richly fossiliferous rocks of the madison. in the williston basin, water was shallow enough for oolite shoals to develop; they later became reservoirs for oil.