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A geologic formation is a rock unit with a distinctive set of characteristics that can be easily identified and mapped. They are formed through various geological processes that result in the accumulation and deposition of sediment. Over time, these sediments solidify into rock layers, or strata, which make up the geologic formation.

The boundaries of a geologic formation separate it from other formations. They are defined by either a change in rock type, a gap in the sequence of rock layers (known as an unconformity), or an age relationship between different rock layers.

Several factors can influence the age of a geologic formation's upper and lower boundaries: 1. Tectonic activity: Movements of the Earth's crust due to plate tectonics can cause a variation in the age of a formation's boundaries as rocks may be pushed together or separated over time. 2. Erosion: The removal of rock material by natural processes such as wind, water, and gravity can change the surface of a formation, making the age of the boundary difficult to determine accurately. 3. Depositional environment: Sediment deposition occurs at different rates depending on the environment. For example, a slow, calm environment may result in a more uniform age for the boundaries, while a rapidly changing environment could result in more variability in the age of boundaries. 4. Time-transgressive deposits: A deposit may start to form at one location and then gradually spread across a larger area over time. This could lead to the same rock layer having different ages in different areas.

Due to these factors, especially the varying depositional environments and time-transgressive nature of some deposits, a geologic formation will not necessarily have the same age for its upper and lower boundaries everywhere. Different parts of the formation may have experienced different rates of erosion, deposition, or tectonic activity, leading to variability in the age of boundaries within the geologic formation.