Abstract
According to the location of Sarakhs Plain in the northeast of Iran and its location on the Silk Road, the Sassanid settlements in the region were investigated in the field and fourteen Sassanid settlements were identified and introduced in this research. In addition to studying historical geography texts, GIS and Spss software were used. This research seeks to answer the question that what were the effective factors of the cultural, social and political interactions of the Great Silk Road and Sarakhs Plain in Northeast Iran in the Sassanid period and what components did it include? During the Sassanid era, due to its suitable climate and strategic location, Sarakhs Plain served as one of the important intermediate regions between Central Asia, Northeast Iran, and the Iranian Plateau, and included cultural, commercial, economic, and political components with Merv. By analyzing the collected data, it was found that the Silk Road is one of the most important factors in the formation of Sassanid settlements in the Sarakhs plain with an area of ​​several hectares, and on the other hand, the Sarakhs plain has been the cause of the prosperity of the Silk Road due to its location on the Marv Road. One of the most important results is that Sarakhs Plain, as a city in the northeast of Iran, by providing security in this region, attracted goods and capital from neighboring lands and added to the prosperity of the east-west and north-south commercial highways.
Keywords: Khorasan, Sarakhs Plain, Merv, Silk Road, Sasanian Archaeology.
 
 
 
Introduction
The Fern Plain in the northeast of Iran and adjacent to the country of Turkmenistan has been a favorable habitat for the formation of various human communities due to its natural potential and optimal environmental conditions such as rivers, height above sea level, geomorphology, slope, soil and favorable climate. is the highest point of the plain is located in Mount Bezegan and it is 1950 meters above sea level, and the lowest point is in the Hariroud river bed and it is 250 meters above sea level. Historical geography texts show that Great Khorasan and Sarkhes Plain were the site of important events of Sasanian rule, especially from the time of Yazdgerd II Sasanian period onwards. Sarkhs plain is almost in the center of Khorasan, and the famous silk road that started from "Siank" in ancient China and ended in Rome entered Iran through the current Sarkhs entrance. Ferkhs was located at the entrance of the eastern part of the Silk Road, between Merv and Neishabur. In addition to being considered a favorable bed for commercial exchanges throughout history, this highway has also been used occasionally as a route for invasions and raids by invading tribes into the land of Iran. The existence of ancient cities such as Sarkhs and Neyshabur along the Silk Road is clear evidence of the role of this highway in the formation of population, economic and government centers in the Sassanid period. According to the geographical location of Fern Plain and with the aim of analyzing the cultural, social and political interactions of the Silk Road and Fern Plain in the Sassanid period, the settlements of this period were investigated in the field of archeology and fourteen Sassanid settlements were identified and introduced in this research. This research seeks to answer the question that what were the effective factors of cultural, social and political interactions between the Silk Road and the Fern Plain in the Sassanid period and what components did it include? The necessity of the current research is the unknown geography of the region and understanding why and how interactions between the Fern Plain and the Silk Road were formed in the Sassanid period.
 
Materials & Methods
The current research is based on the purpose of historical research and based on the nature and method of descriptive and analytical research. The method and tools of data collection in this research have been done in two ways: documentation and field activities. In the documentary stage, all historical written sources and documents, pictures and maps were collected and analyzed. In the field investigation, while identifying the Sassanid settlements, the required movable and immovable archeological information and findings were recorded and analyzed. Finally, the interactions of the Silk Road and the Fern Plain in the Sassanid period were carried out in the framework of documentary and field data by using the output of maps and historical geography texts. In the method of data collection and analysis, in addition to using conventional methods in field surveys and settlement patterns, GIS and Spss software were used. Finally, the cultural, social and political interactions of the Silk Road and the Sarkhes Plain were analyzed based on the output of Sassanid settlement maps and data.
 
Discussion & Result
Examining the importance of roads is to such an extent that the cultural level of each region, its level of development, description and analysis of settlement patterns, extent and clustering of settlements, investigation of economic, political, cultural issues and population estimation can be done according to its distance or proximity to communication routes. Due to the location of Sarkhas Plain on the main and secondary communication routes, commercial routes and also the roads, one can realize the necessity of paying attention and general survey of archeology and communication routes of this area. Twelve settlements (86%) out of the total of fourteen Sassanid settlements in the Sarkhes plain are less than five hundred meters away, Chel Kaman settlement (7%) is between 500 and 1000 meters away, and Bezegan settlement (7%) is less than 1800 meters away from the road. The main and secondary communication routes and trade routes of the Fern Plain are located. This case shows the prominent role of communication routes in the formation of Sassanid settlements in Sarkhes Plain. It should be remembered that our current understanding of this issue is the current conditions of the region and perhaps many roads have changed over time or that some new roads have been created in the region. Most of the Sassanid settlements of Sarkhes Plain were formed less than 1000 meters from the main and secondary communication roads. The special geographical location of the Fern Plain, especially its location between the cultural areas of the Iranian Plateau and Central Asia, especially Turkmenistan, on the one hand, and having favorable environmental conditions for human life on the other hand, as well as conducting archaeological studies through scientific circles. The world in Turkmenistan, at the same time, the cultural dependence of the said region on the fern plain and vice versa, In the Sassanid period, the archaeological investigation and analysis of the Sassanid settlements in the Sarkhs Plain and the communication routes of this region of Khorasan have a key role in the studies of historical cultures in general, Northeast Iran and Turkmenistan in particular. Sarkhs plain was located between two important cities of the Sassanid period, Merv and Neishabur. This problem shows us that the entire Sarkhes plain was continuously inhabited and populated from north to south and from east to west during the Sassanid period. This prosperity is due to the rivers and being on the Silk Road. On the other hand, Sassanid settlements with a distinct architecture can be seen in the central part of the Fern Plain, which are located on the Silk Road.
It is very likely that government centers were concentrated in the central part of the Sarkhes Plain during this period, and on this basis, strong and durable buildings were left behind. During the Sassanid period, Sarkhes Plain was a populated land with many rivers and water resources, and innumerable cities and villages were established in all its parts. The proximity of the Fern Plain to Turkmenistan, which potentially indicates the cultural connection between these two regions, is clearly visible in the archaeological data of the settlements in the Fern Plain, including pottery.  This case, while showing the position and importance of Ferkhs Plain, considering the size and area of ​​the settlements, also shows the existence of important settlements of the Marv base in Ferkhs Plain. Based on this, we can mention Sarkhes Plain as one of the important Sasanian regions of northeastern Iran.
 
Conclusion
During the Sassanid era, due to its special strategic and geopolitical position and its location between Turkmenistan and North-Eastern Iran as a connecting bridge with the Central Plateau of Iran, the Fern Plain played a very important role in the cultural exchanges between these lands. In addition to the military importance of this region, the passage of communication routes between east-west and north-south through this region has doubled its economic impact, which shows the importance and prosperity of this region in this period of Iran's history. The results of archaeological studies and investigations show that in this plain, fourteen Sassanid settlements were formed on the banks of rivers and secondary and main communication routes compared to the Silk Road. The important point is the communication location of Sarkhs plain, so that this plain is one of the inter-mountain plains in the north-east of Iran. The continuation of economic production, the life of the political and social structure of Fern, was due to the connection and interaction of this city with other production centers and demographic and cultural areas. Sarkhas, in the middle of the silk road, was considered one of the centers of export and import of commercial goods and one of the main meeting centers of different ideas and cultures, as a residence of merchants. According to the archeological investigations of the Fern Plain, during the Sassanid period, it was a large city and was naturally influenced by the economic, commercial, and political processes of the Silk Road, and it played a role as the most influential demographic pole of Northeast Iran on the Silk Road. Sarkhas, which in its economic life was a creditor of the Khorasan and Silk Road, has been on the path of cultural development since the 4th millennium BC and reached the peak of urban and cultural development in the Sassanid period. Although the route of the Great Khorasan Road existed before history and the Silk Road of the Sassanid period, before the formation of the city of Sarkhes, the emergence of population centers such as Sarkhes increased the importance of this route and the Silk Road became more and more popular with merchants and tourists. By providing the elements of a city, which is the concentration of population, the creation of social institutions, economic development and political development, the Silk Road prepared the preparations for the development of Sarkhs as the largest city in the northeast of Iran, and by providing security in this region, Sarkhs attracted goods and capital. from neighboring lands and added to the prosperity of the east-west commercial highway. The Sassanid settlements in the Fern Plain show easy access to good pastures, abundant water, sedimentary soils and fertile agricultural lands, and express the dependence of the Sassanid period people on agriculture and animal husbandry.
Proximity to fertile lands, concentration in places near the slopes of pastures, main and secondary roads, living in lands with a low slope, most likely, a sign of relying on agriculture as the main way of living, as well as relying on animal husbandry and exploitation of natural resources on the foothills of the mountains. It is limited.