Sentence:  Very frequently, however, to shorten the distance to the upper nurseries, where they[the ants] have to take the eggs, they project an arch of about ten inches in length, and half an inch in breadth, <WSD>groved</WSD> or worked into steps, on its upper surface, to allow of a more easy passage. 
Meaning of the word marked with <WSD> tags:  To plough or gouge with lines.