The Gulf Coastal Plain is broad and fairly level near the coast, but rolling in the interior. The plain, throughout most of its length in the isthmus, is at least 75 kilometers wide. The majority of the region in the isthmus is drained by the Rio Coatzacoalcos, which flows in a northerly course to the Gulf of Mexico. The western part is drained by the Rio San Juan, the principal tributary of the Rio Papaloapan. Behind the coastal dunes are frequent, and sometimes large, lagoons. Immediately inland from Coatzacoalcos and along the lower stretches of the Rio Papaloapan are extensive marshes. Essentially the entire coastal plain, with the exception of the coastal dunes, consists of rich alluvial deposits. [Illustration: FIG. 1. Map of the Isthmus of Tehuantepec based on the American Geographical Society's "Map of Hispanic America on the Scale of 1:1,000,000." The localities shown are numbered in the gazetteer; the numerical sequence of localities is an arrangement whereby north takes precedence over south and west over east. 1. Alvarado. 2. Lerdo de Tejada. 3. Tlacotalpan. 4. Tula. 5. Tecolapan. 6. Amatitlan. 7. Cosamaloapan. 8. Chacaltianguis. 9. Novillero. 10. Ciudad Aleman. 11. Papaloapan. 12. Tuxtepec. 13. Cuatotolapam. 14. Hueyapan. 15. Berta. 16. Coatzacoalcos. 17. Ayentes. 18. Rio de las Playas. 19. Cosoleacaque. 20. Minatitlan. 21. Acayucan. 22. Aquilera. 23. San Lorenzo. 24. Naranja. 25. Suchil. 26. Jesus Carranza. 27. La Oaxaquena. 28. Ubero. 29. Donaji. 30. Tolosita. 31. El Modelo. 32. Sarabia, 33. Guichicovi. 34. La Princesa. 35. Santa Maria Chimalapa. 36. Matias Romero. 37. Santo Domingo Petapa. 38. El Barrio. 39. Palmar. 40. Chivela. 41. Santiago Chivela. 42. Nizanda. 43. Agua Caliente. 44. Portillo Los Nanches. 45. Ixtepec. 46. La Ventosa. 47. Zanatepec. 48. Union Hidalgo. 49. Tres Cruces. 50. Juchitan. 51. Escurano. 52. Salazar. 53. Santa Efigenia. 54. Tequisistlan. 55. Cerro de Quiengola. 56. San Pablo. 57. Mixtequilla. 58. Tapanatepec. 59. Zarzamora. 60. Limon. 61. Tehuantepec. 62. Bisilana. 63. Santa Lucia. 64. Cerro de Arenal. 65. Cerro de San Pedro. 66. La Concepcion. 67. Tenango. 68. San Antonio. 69. Huilotepec. 70. Salina Cruz.] [Illustration: FIG. 2. Topographic profile of the Isthmus of Tehuantepec showing major localities along the Trans-isthmian Highway and major types of vegetation. Vertical exaggeration approximately 165 times.]