The terrain is marked by numerous small lakes and swamps, and a mixed forest zone covers over 33% of the country. The growing season lasts 169 days in the east and 202 days in the west, with most farmland consisting of sandy- or clay-loam soils. Limestone, clay, sand, and gravel are Lithuania's primary natural resources, but the coastal shelf offers perhaps 1,600,000 m3 (10 Mbbl) of oil deposits, and the southeast could provide high yields of iron ore and granite.