The coal-bearing Helvetiafjellet Formation represents a broad fluviodeltaic system (ca. 200 km wide), with high sand-influx and progradation potential, which gradually retreated landward due to rising sea-level. The subenvironments that hosted plant growth and peat (coal) deposition ranged from braidplain areas and mouth-bar lobes, to bay/inner-lagoon margins and back-barrier/outer-lagoonal zones. The climate (humid-warm) and ground-water conditions were highly favorable to vegetation growth, but the clastic sedimentary environment itself was unfavorable to longer-term establishment of vegetation and peat-forming mires. The depositional system was rapidly aggrading, varying frequently in its local physical conditions. However, within the longer-term pattern of these variations, local environmental “windows” for vegetation growth and peat deposition appeared. From the sedimentary record, it has been possible to recognize the nature of those “windows” and the mechanisms by which they were created and cancelled in particular subenvironments. With other major controls being favorable, the sedimentary environment itself became the critical, decisive control upon the vegetation and peat deposition; its effects could thus be studied, from the sedimentary record, essentially in isolation.