Final answer:
A prehistoric monument consisting of an upright megalith is known as a menhir. These monuments were significant in Neolithic times and could stand alone or be part of a larger alignment.
Step-by-step explanation:
The prehistoric monument described in the question is known as a menhir. Menhirs are upright megaliths, which often stand alone or are sometimes aligned with others in a significant manner. Megalithic monuments played a crucial part in Neolithic societies, serving various ceremonial and astronomical purposes. The most famous example of a megalithic structure is Stonehenge, constructed as an earthwork enclosure and featuring a series of giant stones in a circular layout. Over several centuries, megalithic stones were quarried and meticulously placed to create these prehistoric sites.
Menhirs, in particular, are known for their solitary presence or as part of larger formations, sometimes contributing to the alignment with celestial events similar to the carvings at Newgrange and Knowth, which align with solstices and equinoxes. Their construction demonstrates the advanced organizational and architectural abilities of Neolithic people.