Topographically, the peninsula is a low-level plain that rises gradually inland to the south. The coast on the north and west is low, sandy, and semi-barren; openings through the outer coastal strand lead to narrow brackish lagoons surrounded by mangroves. The eastern coast consists of bluffs, indented with bays and bordered by several islands, the largest being Cozumel. A small range of hills traverses the peninsula from the city of Campeche to Chetumal Bay; although appearing sizeable, these almost nowhere exceed 100 meters in elevation. The extreme southerly portion of the peninsula is moderately hilly and is covered with tropical rain forest. The peninsula is composed almost wholly of a bed of coraline and porous limestone covered with a layer of thin, dry soil formed from the slow weathering of the bedrock. The humus is rarely more than a few centimeters deep, and consequently cornfields (_milpas_) must be moved every two or three years. With each move a new area of the land is cleared and burned. This agricultural practice has been routine with the Mayan people for centuries and apparently has had a profound effect on the vegetation, severely limiting the extent of virgin forest. In many places, especially in the northern part of the peninsula, the rocky surface of the terrain is perforated by natural wells or sink-holes, which are called _cenotes_ locally. Many _cenotes_ support small "islands" of unique vegetation, the composition of which somewhat resembles the rain forest of the southern part of the peninsula. Temporary water-holes and shallow lakes (_aguadas_) are sparsely scattered throughout the southern half of the peninsula. There are no rivers of importance; most that are present are in the southernmost region. The climate of northern Yucatán is hot and dry, and the absence of high mountainous ridges to intercept the moisture-bearing clouds from the Atlantic accounts for less rain than would be expected. The southernmost part of the peninsula receives a maximum of 200 centimeters of rain annually. The amount decreases toward the north to an average of about 45 centimeters. Almost all of the rain falls from May to October. Daily temperatures range from 78° to 98° F. in the shade; cool sea winds prevail day and night throughout most of the year. The hottest months are May and June. In winter, fierce northers, _temporales_ or _nortes_, occasionally sweep across this open region, bringing rain and cooler temperatures, lasting several days. The areas adjacent to British Honduras and Guatemala receive sufficient rainfall to support forests containing mahogany (_Swietenia_); sapodilly (_Achras_); several valuable cabinet woods; logwood (_Haematozylon_) and other dyewoods. As one proceeds northward the flora and fauna change gradually from that of the wet tropics to that of the dry tropics.