Interlocking spurs occur mainly in the upper course section of streams and rivers as rivers have little energy to erode.
Steep sided valleys
A narrow steep-sided valley.
Since water flows in small amounts and in predominantly steep areas in the upper course section vertical erosion is more dominant than lateral erosion.
This also facilitated by the load which the river carries because it cannot reach the upper levels of the valley walls once they have been formed so much of corrasion processes are limited to the lower sections of the river and this tends to deepen the channel.
The result are V-shaped steep sided valleys.
To learn more about landforms resulting from river action go to this page.