The Basistoppen sheet is a thick sill-like gabbro body that intruded into the solid Skaergaard intrusion. The Bassistoppen sheet is quite coarse-grained and contains internal layering, but it weathers gray instead of brown, suggestive of its being a more primitive Mg-rich magma than the Skaergaard. The Basistoppen sheet was probably fed by the macrodike that extends to the northeast of the Skaergaard (see the geologic map). Thin section here.
Geologic map of the Skaergaard region with ice removed and all pre-Skaergaard age rocks (but not unconsolidated sediments) colored white. This shows my interpretation of the Vandfaldsdalen macrodike that extends to the northeast of the Skaergaard pluton, and its likely connection with the Basistoppen sheet. For the original geology see the geologic map. My apologies for foolish or crude interpretations.
Photo from the Zodiac looking east up Basis Glacier. Basistoppen is on the left and Brødretoppen is on the right. The upper part of Basistoppen is made of the Basistoppen sheet, which dips to the right (south) such that almost the whole dark gray wall on the right (south) side of Basis Glacier is made of the Basistoppen sheet. The contact above the Basistoppen sheet on Brødretoppen can be seen as a transition from dark gray Basistoppen sheet below to lighter brownish UBZγ above. The base and top of the Basistoppen sheet are indicated here.
Basistoppen summit (885 m), photographed from a minor summit at 504 m just west of Basistoppen. The Basistoppen sheet occupies the summit cone of the mountain.
View from the southwest flank of Basistoppen, looking east to the head of Basis Glacier and Basis Pass. The Basistoppen sheet is above the break in slope on the left, and occupies all of the wall on the right. A small bit dark of UZc can be seen just below Basis Pass.
View southeast down onto Basis Glacier from the southwest flank of Basistoppen. Almost the entire cliff in the distance made of the Basistoppen sheet. Internal layering in the sheet is visible, as is the Basistoppen floor contact with the darker underlying Skaergaard UZc rock.
Looking northwest from the Douglas Plateau, with crevasses visible where the Douglas Plateau ice cascades down between Nunatak 1 and Basistoppen to the Forbindelses Glacier far below. The slope on the left is Basistoppen, with the grayer, shattered rocks above probably being the Basistoppen sheet, and the rocks lower down are part of UBZγ. In the middle distance on the right is Pukugagryggen, and in the far distance are mountains on the other side of Watkins Fjord.