After receiving an inquiry from T-Bone, I decided to take a closer look at the volcanics of St. Lucia. I put together a PDF file of some images I took while on the island, and you can download it here. I have attempted to speculate as to what created the stratigraphic sequences as best I can; however, I'm not a volcanologist and many of my interpretations are undoubtedly wrong. If anyone has a better interpretation, I'd love to hear it.
Some background information: St. Lucia is located in the Caribbean, at about 13 degrees north latitude. The island is part of the Lesser Antilles island arc, which is itself located on the edge of a subduction zone. The Lesser Antilles are created from volcanoes similar to those found on Japan or in the Cascades mountain range. These volcanoes are highly explosive, with great amounts of volcanic ejecta such as pyroclastic debris and ash (tephra). Known as composite volcanoes, they tend to erupt rarely, but violently. The images in the PDF were all taken on the western edge of St. Lucia, and are in close proximity to the several volcanoes on the island. You'll notice in the images that there seems to be a great deal of volcanic debris along the western edge, which could lead one to believe that the volcanoes of St. Lucia erupt frequently. I cannot attest to the frequency of eruptions on the island; however, I think the last major eruption was in the late 18th century. The large amount of volcanic debris on the western edge is most likely due to the prevailing wind patterns. St. Lucia is located within the Trade Winds, which blow from east to west. This would lead to a greater amount of debris deposition on the western side of the island.
Some of the images show what appears to be tephra layers which fine downward. In other words, thre are rocks and pebbles which are underlain by finer material, probably ash. I have two possible explanations for this. First, erosion may have carried away smaller grains (tephra) at the top of the tephra layer, leaving only the larger pebbles and rocks. Second, almost pure tephra may have been deposited, followed by larger unsorted colluvium from mass wasting. The second scenario may be more likely, as I don't see many cobbles intermixed in the ash layer.
Other images show what appears to be unsorted rocks and boulders mixed into tephra. I would speculate that these are mostly pyroclastic flows, but I may be wrong. They simply may be the result of mass wasting. I would guess pyroclastic flows, as I didn't see any organic material within the strata.
Many of the stratigraphic sequences appear to be loosely cemented, probably due to calcite percolation and precipitation. The loose cementation is a testatment to the young age of these sequences. You'll also note the high angle of repose on these cliffs. This shows that the cliffs are eroding rapidly, which I would guess would be the result of the loose cementation.
Other images show sequences which may have been deposited underwater, and later uplifted. This may be the case, but I am unsure as I didn't see any corals or other macrofossils. My ultimate conclusion is that most of these cliff faces are the result of volcano cone-building (not sure if that's the proper term). Many of these sites are immediately adjacent to old volcanoes, so it seems likely that repeated small eruptions would deposit layer upon layer of debris and ejecta.
There are a couple of images of lava fields in the PDF. A couple things to note here. First, I toured just about the entire western side of the island, but only saw this one lava field; the rest appears to be volcanic ejecta. This is typical of composite volcanoes, which don't produce large amounts of lava. Second, note the strange cleaveage pattern of the lava. These large blocks break apart and fall into the water quite frequently, where they can become hazards for boats. In fact, the locals call this area the "Cemetery", due to a large number of boat accidents here.