the rivers Mureş, Olt and Someş. The so-called "Mediaş group" of cremation or mixed cemeteries emerged in this period near salt mines. The Hungarian and the Romanian vocabulary of salt mining was taken from Slavic, suggesting that Slavs were employed in the mines for centuries. Bistriţa ("swift"), Crasna ("nice" or "red"), Sibiu ("dogwood"), and many other rivers and settlements with names of Slavic origin also evidence the presence of Slavs in Transylvania. The Turkic-speaking Bulgars arrived in the territories west of the river Dniester around 670. At the Battle of Ongal they defeated the Eastern Roman (or Byzantine) Emperor Constantine IV in 680 or 681, occupied Dobruja and founded the First Bulgarian Empire. They soon imposed their authority over some of the neighboring tribes. The great variety in burial rites evidences the multi-ethnic character of the Bulgarian Empire. Even the Bulgars were divided in this respect; some of them practiced inhumation and others cremation. Initially, a sharp distinction existed between the Bulgars and their subjects, but the Slavicization of the Bulgars soon began. Opreanu writes that the "new cultural synthesis" known as the "Dridu culture" developed in the Lower Danube region around 680. New settlements and large cemeteries show that the region experienced a steady demographic rise in the 8th century. The large, unfortified "Dridu" settlements were characterized by traditional semi-sunken huts, but a few houses with ground-level floors have also been unearthed in Dodeşti, Spinoasa, and other places. "Dridu" communities produced and used gray or yellow fine pottery, but hand-made vessels were still predominant. Fine, gray vessels were also unearthed in the 9th-century "Blandiana A" cemeteries in the area of Alba-Iulia, which constitutes a "cultural enclave" in Transylvania. Near these cemeteries, necropolises of graves with west–east orientation form the distinct "Ciumbrud group". Female dress accessories from "Ciumbrud graves" are strikingly similar to those from Christian cemeteries in Bulgaria and Moravia. From an earlier date are the cremation cemeteries of the "Nuşfalau-Someşeni group" in northwestern Transylvania, with their 8th- and 9th-century tumuli, similar to the kurgans of East Slavic territories. Contemporaneous authors rarely dwelled on early medieval Southeastern Europe. For instance, the Royal Frankish Annals makes a passing reference to Abodrites living "in Dacia adjacent to the Danube near the Bulgarian border" on the occasion of their envoys' arrival in Aachen in 824. Bulgaria's territory increased under Krum (c. 803–814), who took Adrianople and forced at least 10,000 of the