summation - Geometrical interpretation of $(sum_{k=1}^n k)^2=sum_{k=1}^n k^3$

Using induction it is straight forward to show
$$\left(\sum_{k=1}^n k\right)^2=\sum_{k=1}^n k^3.$$
But is there also a geometrical interpretation that "proves" this fact? By just looking at those formulas I don't see why they should be equal.