Use two structuring elements shown below

Since we have two different structuring elements, we shall use the formula:

$A⨂B=(A ⊝B_1 )∩(A^c⊝B_2)$

The structuring elements for erosion contain boyh 1’s as well as 0’s

The pixels in the structuring element containing 1’s define the neighbourhood of the structuring element. Only this neighborhood is considered for erosion.

Now we have to place the structuring element, this is done by looking at the neighboring 1’s, choosing the minimum value and placing it at the center.

Now ,

Hence only the top edge of the original square matrix is detected.