The Pillars of Hercules: Mountains or Pillars?

Luke

The Rock of Gibraltar, the northern pillar. In the background can be seen the purported southern pillar, Jebel Musa, Morocco. (Source: Wikipedia)

The Pillars of Hercules: The ancient name of two promontories which stand either side of the Straits of Gibraltar; the northern pillar, the rock of Gibraltar, the southern pillar, either Monte Hacho in Ceuta, or Jebel Musa in Morocco. 

Since ancient times there has been some dispute as to which pillar the southern one is, either Monte Hacho, which is smaller and less distinct and, Jebel Musa, which is taller and more distinct.

The Pillars of Hercules separate the continents of Europe and Africa and the Mediterranean Sea from the Atlantic Ocean. 
The Tenth Labour of Hercules Beyond the Pillars

In Greek Mythology, as part of his tenth labour, Hercules is said to have traveled to an island near Gades, to capture the cattle of the three headed Giant Geryon. This marked the farthest west of his travels. 

Hercules fighting Geryon, ca. 540BC, Louvre (Source: Wikipedia)

Herodotus describes where this place is located.

Geryon’s home was far away, on an island which the Greeks call Erythea, near Gades, which lies on the Ocean beyond the Pillars of Heracles. Legend says that Ocean is a great river running from east all round the world; but there is nothing to prove this. (Herodotus, BkIIII, p. 242) 

This Gades, is modern day Cadiz, in Spain. This location does in fact lie on the other side of the Pillars of Hercules, facing the Atlantic Ocean.

The Ocean Herodotus describes sounds like the Atlantic Ocean, which technically does not go around the world, but names aside, does flow into other oceans such as the Indian Ocean without separation. Herodotus also seems to indicate that the pillars are located in or close to, the Straits of Gibraltar.

On the map Cadiz can be seen beyond the Pillars of Hercules/Gibraltar (Source: see below)

The Phoenician Hercules and the Real Pillars

The Phoenicians, a great seafaring people, sailed beyond the straits and founded the settlement known as Gades, modern Cadiz in Spain, in about 1100 BC. There was said to be a temple located there dedicated to Melqart, who is identified with the Greek Hercules, which consisted of two columns thought by some to be the original pillars erected by Hercules. 

The Geographer Strabo records this temple and pillars, when he recounts the dispute about the real pillars:

Others say that it is the bronze pillars of eight cubits in the temple of Heracles in Gades, whereon is inscribed the expense incurred in the construction of the temple, that are called the pillars (Strabo, Geography, BKIII, Ch 5, p.136/7)

He further indicates that although some others consider this to be the original Pillars of Hercules, amongst other locations near or beyond the straits, he himself considers the original pillars to be in the vicinity of what is generally called the Pillars of Hercules, located in the Straits of Gibraltar.

For he makes the argument, “it was a custom in early times to set up landmarks like that,” at the limit of the known world, and even though the original landmark may have disappeared with time, the area maintains the original appellation (quotation and paraphrase, Strabo, Geography, BKIII, Ch 5, p 137).

Wherever the original pillars are (if they even existed at all), the Rock of Gibraltar and either Jebel Musa or Monte Hacho, exist in the Straits of Gibraltar, and carry the legendary appellation.

Aerial Photo of the Straits of Gibraltar. In the middle left side can be seen the Rock of Gibraltar. Jebel Musa, can ben seen opposite across the Strait. Cadiz is not present on this picture, but would be just beyond the bottom of the photo. (Source: Wikipedia)

Sources:

https://www.ancientworldmagazine.com/articles/pillars-hercules/
The Geography of Strabo, Published in Vol. II of the Loeb Classical Library edition, 1923

http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Strabo/3E*.html

Herodotus, The Histories, Translated by Aubrey De Selincourt, Revised with an introduction and notes by John Marincola, Penguin, 1954
https://en.wikipedia.org/wiki/Pillars_of_Hercules
https://www.livius.org/articles/place/pillars-of-heracles/
Map showing Cadiz: http://www.worldeasyguides.com/europe/spain/cadiz/cadiz-on-map-of-spain/