Formerly the Northern Range of Trinidad was assumed to be the south flank of an anticline, with the north flank either out to sea or displaced by faulting. Mapping has shown that the structure is that of an anticline overturned to the north.
The steep southern limb begins with nearly vertical dips in the Chancellor formation and the Morvant beds along the Lady Young Road, and between Port of Spain and Maraval. In this upright limb are local folds 100 m to 500 m in wavelength, parallel with both the main structure and minor folds. Folds of this size have not been recognised in the overturned northern limb. West of Maraval step-like folds with alternating horizontal and vertical limbs occur on east–west axes 10 m to 100 m apart. Axial surfaces dip about 35°N, but all other minor folds in the area have axial surfaces dipping steeply southwards.
Along the axis of the main fold, which occurs in the outcrop of the Maraval limestones, dips are 40°–60°S and the flanks are isoclinal. In the whole of the over-turned northern limb dips are much flatter, commonly less than 25°, and appear to become even flatter to the north. The size of this recumbent limb suggests that it is a nappe. The underlying autochthonous beds are not seen.
A number of phases of folding are recognisable. The first folds on east-west axes were refolded on more or less parallel axes. The step-like folding west of Maraval may belong to this