THE PHYSICAL SETTING OF THE RIO BRAVO CONSERVATION AREA

Introduction

_______The Rio Bravo Conservation Area is situated in northwestern Belize between the Guatemalan border and the New River (Figure 2). This zone is inland from the coast, where some of the earliest Maya occupation has been identified, and on river routs which likely were used to reach the core area around Tikal. The terrain of the area descends from west-northwest to east-southeast, with gradual dipping of the limestone bedrock accompanied by faulting. Three main faults running generally north-northeast and south-southwest form distinct escarpments and define the courses of the Rio Bravo, Booth's River, and the New River (Figure 2).

_______The Rio Bravo Conservation Area is included within the Gallon Jug Development Region, as defined originally by the British Honduras Land Use Survey Team (Wright et al. 1959:195-211). The survey team report (ibid.) divided the Gallon Jug Region into eastern and western subregions. The Booth's River Escarpment forms the boundary between the two subregions: the Hills Subregion to the west, and the Lowlands Subregion to the east (Figure 2). These two subregions differ markedly in terrain, hydrology, soils, and vegetation. As a result of these environmental differences, these two physiographic subregions present significantly different settings for the study of prehistoric Maya development.

The Hills Subregion

_______The limestone of the Hills Subregion is deeply but incompletely weathered. As a result, the Eocene limestone bedrock is covered by a deep mantle of limestone gravel and boulders. The soils formed on top of this mantle are primarily shallow gravelly clays with good drainage, workability, and fertility characteristics.

_______The hilly terrain provides for a great deal of small-scale variability in soil and vegetation patterns. The highest terrain, reaching over 200 m above sea level, is found between the Guatemalan border and the Rio Bravo Escarpment. The steeper hill slopes of the area have quite thin soils of only 10-15 cm depth, while the more level areas have soils up to 50 cm deep. The vegetation in this area is dominated by deciduous seasonal forest between 20 and 30 m in height. This forest is characterized by numerous species of large trees including mahogany (Swietenia macrophylla), sapodilla (Manilkara zapote), and Ramon (Brosimum alicastrum).

_______Small swamps and seasonally inundated areas are found in shallow depressions, supporting in turn low and high marsh forest vegetation associations. Low marsh forest, referred to locally as "akalche", is characterized by logwood (Haematoxylon campechianum) and chucum (Pithecolobium albicans), while high marsh, known as "bajo", is dominated by botan palm (Sabal mauritiiformis), chichem (Metopium brownei and Cameraria belizensis), and cohune palm (Orignya cohune).

_______The terrain between the Rio Bravo Escarpment and the Booth's River Escarpment is gently undulating, with a greater proportion of both low and high marsh forest than is found in the rest of the Hills Subregion. Also characteristic of this area are patches of flat land strongly dominated by cohune palm (Oribignya cohune).

The Lowlands Subregion

_______The 45-60 m Rio Bravo Escarpment marks the boundary between the Hills Subregion and the Lowlands Subregion. Minor scarps are also found within the Lowlands Subregion, such as one running along the northern and western sides of Irish Creek. Colluvial swamps have formed at the foot of these minor escarpments. Most of the bedrock in the subregion is hard limestone of the Paleocene or Lower Eocene epoch, capped by softer Miocene limestone with a few remnant patches of Pleistocene coastal deposits (Flores 1952; Wright et al. 1959:205). The terrain of the area is flat to very gently undulating, and soils are mostly clays with impeded drainage.

_______Vegetation of the Lowlands Subregion is primarily low to high marsh forest, with restricted patches of deciduous seasonal forest and cohune forest. Extensive areas of herbaceous swamp, known locally as "sibal", are associated with the colluvial deposits along Booth's River and Irish Creek. Savanna vegetation associations containing oak (Quercus oleoides), palmetto (Acoelorraphe wrightii), and pine (Pinus caribaea), occur within the Pleistocene coastal deposits found east of Booth's River in the northern part of the Lowland Subregion.