Read the following context and answer the question.
Context: There was a bit of up and down riding , but it went through shady woods , mostly , and made for a very pleasant ride . After the woods we ran into a fairly steep hill , and the gravel road became more rough , so we decided to walk the bikes up the hill , hoping that the next little town was just on the other side . When we crested the hill , we saw that there was another , even steeper hill , so even though we may have been less than a mile from the town , we decided to turn back , after riding a little over 10 miles .
Question: Why did they have to walk the bikes ?
Answer:


The gravel became unwieldy .