This is from Mike Smith Enterprises Blog Tragedy at Table Rock Lake
The dark cloud being formed on the right side of the photo is a shelf cloud (?), warm moist air is being forced upwards by an inflow of cool air (the gust front). The cool air from the thunderstorm is moving from left to right in the photo (thunderstorm to the left).
It has a flat bottom typical of cloud formed by vertical development, like this one;
An illustration of a shelf from here (good explanation) Shelf Clouds: One of Nature’s Most Alarming and Awesome Phenomena
Here is another good diagram:
