Final answer:
Grain size helps determine if a detrital sedimentary rock formed close or far from the source; larger, coarse grains suggest a proximal origin while fine, well-sorted grains indicate a distal origin.
Step-by-step explanation:
To tell if a detrital sedimentary rock formed close or far away from the source rock, one should consider the grain size of the sediment. The closer the sediments are to the source, the larger and less worn they tend to be.
As sediments are transported over distances, they become smaller and more rounded due to the process of weathering and erosion. A rock with coarse-grained sediments would suggest a proximal depositional environment, close to the source rock.
In contrast, fine-grained and well-sorted sediments would indicate a distal depositional environment, implying that the material was transported over a long distance before settling down and forming the rock.
The grade of sorting, where well-sorted sediments typically reflect a long transportation distance, and the roundness of sediments also provide insights into the transport history of the sedimentary particles.