Google maps terrain image of Ronas Voe.
The fjord head is separated only by a low col from Yell Sound to the east
Definition: an overdeepened glacial valley drowned by the sea through postglacial sea level rise
Shetland's voes and firths are valleys that have been drowned by the sea. Many of these valleys show undoubted evidence of glacial erosion, with ice-moulded bedrock on the flanks of the valleys, steep sides, a straight or gently-curving course and the presence of enclosed rock basins. These features indicate that the valleys should be regarded as fjords, although the depth of glacial erosion has often been modest and nowhere exceeds 100m.
Many of the fjords have dendritic valleys systems at their head. Examples include Sullom Voe and the Toft-Ulsta channel that drains into the Sound of Yell, the valleys draining into Swarbacks Minn and Laxa Firth and its neighbours. Valley incision has been encouraged by the steep gradients for ice flow provided by the drop between the current landmass and the sea bed offshore which falls quickly towards -100 m OD. The fjords generally lack a steep trough head at the landward end, reflecting the limited relative relief available on the islands.
Rock basins occur offshore and may lie hidden beneath sediment on valley floors onshore. Yell Sound has deeps down to -90 m on either side of a central ridge that includes the islands of Little Holm and Muckle Holm. The inner basin of Sullom Voe is 30 m deep near the oil terminal and the deep water along the voe is of course a major factor in its accessibility to supertankers. The Deeps off Skelda Ness reach -90m and rockhead in the deepest part of St Magnus Bay lies at -200m, with 80m of Late Quaternary fill.