Complex sentence: A critical unsolved problem in the early history is the relationship of these cultures, overlapping in time, the Frankish material culture, perhaps, according to H.R. Loyn Loyn, "Anglo-Saxon England and the Norman Conquest", 2nd ed.1991:39-43.
Split into simpler sentences: A critical unsolved problem in the early history is the relationship of these cultures, overlapping in time. One explanation, espoused by H.R. Loyn Loyn, "Anglo-Saxon England and the Norman Conquest", 2nd ed.1991:39-43.