Initiation points or origins of a plumose structure are commonly marked by geometric or material inhomogeneities, like a cavity (Figure 1), a fossil, an opaque mineral inclusion, or a depositional feature, such as a cusp (Figure 2). These are often referred to as 'flaws' in a mechanical sense. Initiation points are also called the origins which sometimes are captured by naked eye for joints.

Figure 1. Photo showing joint initiation point as well as the radial pattern of the associated plumose structure. The initiation point is a void in the rock due to an air bubble, circled at the center of the photo. From Pollard and Aydin (1988).

Figure 2. Photo showing joint initiation at a sole mark and the associated plumose structure. Watkins Glen, NY. From Pollard and Aydin (1988).