Morphogenesis of high mountains entirely covered by granitoid boulders and blocks in the northeastern part of Australia

Boulder fields are a typical feature of granitoid landscapes. Previously reported boulder fields are commonly an accumulation of boulders on gently undulating surfaces; however, sometimes boulders form high mountains. The Black and Melville Mountains, northeast Australia, have relative heights of 300 to 500 m and are entirely covered by granitoid boulders and blocks. They are underlain by Permian granitoid rocks intruded into the Silurian–Devonian strata. We performed field surveys, UAV observations and satellite image analysis of these mountains using Digital Elevation Models (DEMs) obtained from satellite data. Melville Mountain has low-relief top surfaces surrounded by steep slopes covered by boulders with (sub)angular blocks. The low-relief surfaces are assumed to be an exhumed Jurassic unconformity, below which the granitoid rocks became corestones (boulders) by subsurface spheroidal weathering. The limited exposure of the bedrock suggests that the granitoids were columnar jointed, which facilitated the spheroidal weathering. The steep slopes are covered by boulders originating from the low-relief top surfaces and rock blocks from the rock columns below the zone of spheroidal weathering. Black Mountain lacks low-relief top surfaces and comprises steep slopes covered with rock blocks. Boulders on Black Mountain exist at the feet of steep slopes and are mixed with rock blocks. The boulders are probably from previous low-relief top surfaces that are now lost. Columnar joints, subsurface spheroidal weathering, uplift and erosion are essential for the formation of the high mountains covered by boulders and blocks of granitoid.