The nature of the Maclay Coast (Rai Coast or Bereg Maklaya on modern maps) is interesting in many ways. Located at 5° S, this area has features typical to the equatorial landscapes: constantly hot and humid climate without pronounced seasonal temperature fluctuations, lush forest vegetation, which is formed by hundreds of species of evergreen trees, rivers of high-water discharge, and original fauna. Gorendu and Bongu villages are located in the system of fold mountains of the Finisterre Range and Adelbert Range, stretching along the northeastern coast of New Guinea. Geologically, this is a fairly young area, which was the ocean floor at the beginning of the Quaternary Period. The formation of the mountain relief continues at the present time, accompanied by earthquakes and volcanic eruptions, which play an important role in the life of the inhabitants of this area. In the recent geological past (in the Pleistocene), the space between the Finisterre and Adelbert Ranges was occupied by a wide sea strait. The subsequent uplift of the landmass led to the formation of low terraced plains stretching along the Gogol River, as well as narrow strips along the edges of mountains to the north and south of the river mouth. The strait itself has turned into a vast bay, reflecting the curve of the coastline between the Ranges. This is Astrolabe Bay. The panorama of the Maclay Coast in the vicinity of Gorendu and Bongu is very picturesque, and it hasn’t changed much since N.N. Miklouho-Maclay’s first visit. Below is a precise and vivid description of the area’s nature in general and its unique features (landscape, vegetation and weather) made by the great traveller: “On the top of the mountains were thick masses of clouds, which did not permit one to distinguish the outline of their summits. Under the white layer of cloud, the steep slopes were covered with dark thick forest, darkness contrasted with the light green strip along the coast. The coastal area was raised in a series of terraces or ledges (of approximately 300 meters in height) which presented a very characteristic appearance. The uniformity of these terraces was more apparent from below, at a lower level. Numerous clefts and ravines, thick with vegetation, cut through these terraces and connected the jungle of the higher level with the narrow belt of coastal jungle. At two places on the shore smoke could be seen, witnessing to the presence of man. In some places the coastal belt became wider. The mountains retreated into the depths of the land and the narrow terraces, approaching the sea, were transformed into broad fields fringed with dark vegetation”.
Soviet scientists who visited these places in 1971 saw with their own eyes how precisely and figuratively N.N. Miklouho-Maclay described not only the general nature of the landscape, but also its individual features — relief, vegetation, weather.
Studies conducted during the Soviet expedition in 1971 showed that the coastal line near the village of Bongu is formed by six terraces that gradually rise moving from the ocean inland and reach a height of about 200m 7-8 km from the shore. The lowest terrace, about 5 m above the ocean level, is quite flat, but as soon as 400-500m from the shore it is followed by higher terraces. These high terraces, as N.N. Miklouho-Maclay noted, are dissected by a dense network of rivers and rivulets flowing from the Finisterre Range into the ocean and forming deep valleys at the intersection of the terraces. The largest valleys in the vicinity of Bongu are formed by the Mindjim and Kabenau rivers, and not far from the village there is the Sobol River.
At a distance of 6-8 km from the shore, the hills are followed by mountain ranges, the nearest of which is 700-800m high. And 15 km from the shore, the mountains reach 1500m in height. At a distance of 30-40 km from the shore, the mountains have a height of more than 3500m. The lower terrace is composed of young coral limestones, which, when destroyed, turn into a loose sandy material. Farther away from the shore, clay shales and limestones appear in the surface structure. The alternation of layers of different density and water permeability in the conditions of abundant moisturization lead to frequent landslides. Traveling through the hills and mountain ranges in the vicinity of Madang, the Soviet scientists often encountered steep, vegetation-free cliff walls — landslides. A lot of landslides occur during earthquakes. N.N. Miklouho-Maclay, during his second stay in the vicinity of the village of Bongu, was struck by the changes in the landscape of the mountains after the earthquake that occurred when he was absent. “At my departure (in December 1872) the vegetation covered even the peaks of the range, but now in many places the summits and steep slopes were completely bare”.
The Maclay Coast has a typically equatorial climate. In Madang, the average temperature of the warmest month (May) is +27.3 °C, and the coldest (July) is only 1°C lower (+26.1 °C). The lowest temperature ever observed here is +18.9 °C. The maximum recorded temperature is +33.7 °C. During the day (at any time of the year), the temperature varies from +23 °C at night to +29-30°C.
The precipitation regime is not as stable as the air temperature. On average, annual precipitation in Madang is 3,200mm. This is almost one and a half times more than the amount of precipitation measured by N.N. Miklouho-Maclay in 1871-1872 (however, the observations were conducted only for one year at that time). In the conditions of mountainous relief, the annual precipitation varies greatly from place to place. Comparing the data of weather stations located on the northern coast of New Guinea, Soviet scientists assumed that the annual precipitation in the Bongu area should be at least 2400mm.
Despite the relative uniformity of annual precipitation distribution on the Maclay Coast, it is still possible to distinguish some seasonality, which is explained by the fact that in summer (October—March), a powerful monsoon air current arrives here, bringing a large amount of atmospheric moisture from oceanic spaces. In winter, the Equatorial Current prevails. The northeastern winds blowing perpendicularly to the mountain ranges leave moisture on the mountain slopes. Low cumulus clouds coming from the Ocean often completely envelop the mountain slopes south of Bongu. Anyway, the trade winds carry less moisture than the monsoon, so winters in this area are drier than summers.
Seasonal humidity rhythms influence the Papuan farming greatly. At the end of the monsoon rains, the trees are cut down and the bark is removed on forest plots in preparation for gardening. In dry time, the trees are burned and garden plots are fertilized with ash. There is some seasonality in the cultivation of certain crops. The diurnal pattern of the weather, usually attributed to areas with an equatorial climate, is not typical for the Maclay Coast. Rain can fall at any time of the day, although N.N. Miklouho-Maclay noted that it mostly rains at night.