Source: Hell Unleashed, pg(s). 38ff.
The Hanging Marches in Hell's first layer, Avernus, are the realm of the infernal duke Furcas. They are a tangled morass of writhing vegetation so vast that a flying observer could mistake it for a green sea, where the plant life covers the ground to such depths that only the highest peaks emerge above the foliage. Most of these are volcanoes, whose continuous eruptions barely keep them free of the growth and burn large swathes of vegetation as the fires they kindle snake out into the Marches. Great stretches of swamps and jungles lie hidden in the green darkness beneath the canopy, alongside small settlements. At the center of Marches lies the volcano Barrowsiege, on whose peak is Furcas' fortress, the Forked Pyre.
The Hanging Marches are highly fecund and home to a great variety of plant life, and plants from numerous worlds are known to grow within them; assassin vines, in particular, make up much of the canopy. A druidic order sworn to Furcas, known as the Leeching Vein, is also active in the Hanging Marches.
The Marches have recently—in planar terms—expanded to overtake those sections of Typhon's realm of Arima, which was left abandoned after the archdevil's death; Avernus' new ruler Barbatos did not claim Arima for constructing his own realm, so Furcas did. The ruins were swiftly overtaken by the Marches' vegetation, and the temples and serpent-lairs of Arima became a new province known as Viperpool.