* Sedimentary rocks are made from fragments of pre-existing rocks. These fragments, known as clasts, can be igneous, metamorphic, or even other sedimentary rocks. Radiometric dating would only give the age of the original source rock, not the time the sedimentary rock formed.

* Sedimentary rocks are formed through processes like erosion, deposition, and cementation. These processes often mix materials of different ages, making it difficult to pinpoint a specific formation time.

Instead of directly dating sedimentary rocks, geologists use radiometric dating to date the surrounding igneous or metamorphic rocks. This is known as "bracketing":

* Intrusive igneous rocks: If a layer of igneous rock intrudes into a sedimentary rock layer, the igneous rock can be dated to provide a maximum age for the sedimentary rock. The sedimentary rock must be older than the igneous rock that intruded it.

* Volcanic ash layers: Ash layers from volcanic eruptions can be found within sedimentary sequences. Dating these ash layers can provide a precise age for the sedimentary rock layers above and below it.

* Metamorphic rocks: If a sedimentary rock layer is metamorphosed, dating the metamorphic rock can provide an age range for the sedimentary rock. The sedimentary rock must be older than the metamorphic rock it was transformed into.

By combining these radiometric dates with other geological evidence, geologists can construct a timeline for sedimentary rock formations.