As there was the big crack of the earth crust at the end of Mesozoic Period and the beginning of Cenozoic Period, some parts of the earth crust were sunk in the sea. Later, the accumulation of sediment and the changes of sea level turned the area to be the land, that is, the central region of Thailand to the Thai Bay. The central plain is the most accumulation of sediment of Thailand because it includes the areas of Ping, Yom and NanRiver including the upper areas of the central region in Tak, Uttaradit, Pitsanulok, Petcabun, Pichit, Sukhothai and Kamphaengpetch.
The other part is the area of the delta of Chao Phraya River occurred by the accumulationof sediment from Chao Phraya River, Mae Klong River, Tha Cine River and Bang Pakong River, that is, the areas of lower central region from Nakhonsawan to Thai Bay. The central plain is very abundant and large. So, it is appropriate for the settlement of households, especially in the lower central areas as many rivers are converged since the Buddhist century of 11 – 12 such as Uthong, Nakhon Chaisri, Kubua, Lawoh Ut Tapao, Sri Mahosop. Later, people migrated to the new river due to the convenience of communication. Cities like Suphannabhumi, Praek, Sriracha, Ayutthaya, Ratchaburi and Lopburi were established. In addition, the central plains had a lot of importance in terms of the administration. As a result, they became the economic and political center in Ayutthaya Kingdom.
Ayutthaya Kingdom, the center of administration in Buddhist Century 19, was located in the central region where Chao Phraya River, Noi River, Lopburi River and Pasak River were converged with Chao Phraya River and flew to Thai Bay. The location of Ayutthaya Kingdom was described in the historical book, which was probabiy written in the era of Rattanakosin, as follows: “When mentioning about Ayutthaya Kingdom, we usually thought of the city surrounded by the main rivers. Such rivers were the main communication lines. Thus, the city was appropriate for the domestic and international trading. lt was also the economic, political and cultural center.” At present, Phranakhonsriayutthaya Province still has the importance as it was the capital of Thailand, which is full of ancient sites, relics and cultural inheritance of ancestors.
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