Understanding the Different Elements of a Hillslope

If you observe a hill, it is a layered staircase rather than a simple slanting surface. Each part of this hillslope profile plays a distinct role in shaping the landscape. Let’s study its four major elements—each having unique characteristics and processes at work.

1️⃣ Summital Convexity: The Crest of the Hill

Think of the summital convexity as the rounded peak of a hill. This is the uppermost portion where:
✔ The surface curves outward, forming a convex shape.
✔ It is covered with weathered material (broken-down rock and soil).
✔ Soil creep (slow, downhill movement of soil) dominates transportation here, shifting material gradually over time.

This zone acts as the source region for material movement, feeding debris to the slopes below.

2️⃣ Free Face: The Steepest Drop

If the summital convexity is a gentle dome, the free face is like a vertical cliff. This element:
✔ Resembles a wall-like, precipitous slope made of bare rock.
✔ Is devoid of loose debris—any material falling from it quickly moves downward.
✔ Undergoes backwasting, meaning it erodes in a way that causes it to retreat parallel to itself over time.

Picture a fortress wall—over centuries, erosion chips away at it, but instead of crumbling from the top, the whole wall shifts backward.

3️⃣ Rectilinear Element: The Regular Slope

Moving downward, we reach the rectilinear element, the straight, uniform segment of the hillslope. It:
✔ Lies between the free face (above) and the concave slope (below).
✔ Has a constant slope angle, making it appear regular or uniform.
✔ Is often covered in rock debris, earning it the nickname debris slope.

This part of the slope serves as a transition zone, where material from above starts accumulating before moving further down.

4️⃣ Concave Element: The Gentle Base

At the lowest part of the hillslope, we find the concave element, which:
✔ Forms the basal segment of the slope profile.
✔ Has a gradually decreasing angle, meaning the slope becomes gentler as it extends downward.
✔ Is also called a waning slope, as the steepness diminishes progressively.

Imagine a river valley at the base of a hill—here, material finally settles, forming a stable depositional zone.

Conclusion

Each of these four slope elements plays a role in shaping hills over time. From the slow movement of soil at the summital convexity, the sheer drop of the free face, the steady debris flow of the rectilinear element, to the gradual flattening of the concave slope—hillslopes are dynamic landforms, constantly evolving under the forces of erosion and deposition.