:
• Igneous - solidified from molten material

• Metamorphic - recrystallized by heat and/or pressure.

• Sedimentary - composed of remains of preexisting rocks.

Each rock type has many subtypes. The first two types are often referred to as "hard rocks" by geologists. Typically, when you hit them with a hammer, the hammer rings in your hand and bounces back. Sedimentary rocks are called "soft rocks" because typically, when you hit them with a hammer, the rock goes "clunk" and preaks.

Maya Mountains and hard rocks

A quick glance at a relief map of Belize shows where we will encounter "hard rocks" - the Maya Mountains in the south. These rocks range from 125-320 million years old (Paleozoic and Mesozoic). During the Mesozoic and much of the Cenozoic, the Maya Mountains were probably the only part of Central America that rose above sea level.

Limestone

In Belize, we will be able to dismiss the hard rocks pretty quickly. In fact, most of the rocks we see will be of a single type of sedimentary rock - limestone. This geologic map shows the distribution of limestones of various ages:

Fortunately, limestone is interesting.

Limestone - is sedimentary rock made of calcite (i.e. calcium carbonate - CaCO₃). An unambiguous way to identify it is that it effervesces vigorously when brought into contact with dilute hydrochloric acid. The material for limestone comes from two cources:

• Calcite that precipitates directly from groundwater, as in cave formations.
• The remains of marine organisms with calcite shells.

• Algae
• Sponges
• Corals
• Bryozoans
• Molluscs
• Echinoderms
• Microscopic critters including foraminiferans.

Note: most calcite secreting organisms must live in sunlit environments, so limestone forms in greatest abundance in shallow sunlit marine environments. Because limestone forms in places where life lives from the remains of living things, it represents an intimate cross-section of the geological and biological worlds. By looking at limestones we can determine:

• What critters lived in an environment
• What the environment was like in terms of salinity, illumination, and temperature
• Analyses of stable isotopes enable us to make statements about global ocean chemistry and, from that, ancient climate.

But there's more....

Limestone dominated landscapes are weird.

All rainwater is slightly acidic because dissolved CO₂ reacts with water to form carbonic acid. Limestone is highly soluble in even a slightly acidic environment. Thus, wherever limestone comes into contact with rainwater (either on the surface or under the ground) solution features appear. These include:

• Natural caverns.

• Dissolution may thin the roof of a cavern to where it collapses, causing a sinkhole ("cenote" in Spanish).

• Karst topography: Named for the Karst region of Slovenia. Regions rich in dissolution show an odd topography showing
  • Caves
  • Sinkholes
  • Springs
  • Vanishing streams.
  For extreme example, consider the limestone towers of southern China.

• It forms on shallow sunlit ocean floors where living things thrive.
• When it is exposed to rainwater (i.e. above sea level), it tends to dissolve.

By applying this knowledge, we can infer a good bit about Belize's geological history. Starting in the Mesozoic, most of Belize was covered by a biologically productive shallow sunlit sea. The presence of limestone tells us this. At some point in the more recent past, however, sea level dropped. How far down did it go? Consider the blue holes beloved of scuba divers.

This is nothing but a sink hole - a dissolution feature. What's it doing below sea level? Obviously in the past, this place was exposed to rainwater, so sea level must have been even lower than it is today.

In fact, the world is just emerging from the ice age of the Pleistocene, from 1.6 million to 10,000 years ago. This interval saw numerous alternating glaciations - periods in which water became locked up in continental ice sheets, and interglacials, warmer intervals in which glaciers melted.

• Many of the youngest limestones in Belize date from the last interglacial, around 125,000 years ago, when sea levels were roughly 10 m. higher than today. At that time, barrier reefs built up in the same places as today. Between the reefs and the mainland, lagoonal deposits also built up.
• During the last glacial, sea level dropped to 80 m. below its current level, exposing all of the continental shelf near Belize to erosion, rainwater, and dissolution. The Pleistocene barrier reef was eroded.
• Over the last 18,000 years, sea level has returned to its current level. New barrier reefs have built up on top of the remains of the Pleistocene reef. Modern sea levels still don't match those of 125,000 years ago, however, so the Pleistocene lagoonal deposits of that age still stand above water in the form of the Cays - Ambergris Cay, etc.

  

  One last thing

  The presence of limestone strongly effects the living and human worlds.

  When limestone dissolves, whatever other sediments (clay, sand, etc.) were mixed up in it are concentrated to form fertile soils. Thus, the best agricultural land in Belize is found on limestone bedrock. In contrast, the soils of the Maya Mountains are pretty poor. Indeed, the Maya Mountains are covered in pine forests that are specifically adapted to the poorer soils.

  Limestone landforms played a prominent role in the Mayan imagination and culture. The Mayan underworld - Xibalba - was not an abstraction to the Mayas. It was physically connected to the here and now by conspicuous caverns and sinkholes. Mayas routinely offered sacrifice by throwing precious objects and victims in to cenotes. Often, their temple facades were designed to resemble the mouths of caverns, or witz monsters whose mouths represented the cavernous entrance ot the underworld. Finally, in the absence of temples, Mayas - ancient and modern - perform ceremonies in the mouths of caverns.