A/N: In 'The Atlas of Tolkien's Middle-earth' by Karen Wynn Fonstad, no forest is shown in the lands of Thargelion or Ossiriand, but I needed a reason why the Teleri would stop for so long in these places (none is given in 'The Silmarillion'), and the easiest way to do this was to make them seem as homelike as possible. Besides, I can't imagine that these places were devoid of trees in any case, especially since the Green-elves dwelt in Ossiriand in later years.
The Chronicles of the Teleri
Gelion
Passing out of the Ered Luin we beheld yet another new land, green and pleasant like Eriador that we had just left, and also with great forests like Eryn Galen where we had lived for so long. But strangely we felt greater kinship with this new land than we had with either the rolling countryside of our travelling or the Greenwood that had been a home to us. We felt a connection to this place that we had not felt the like of since we had dwelt in our birthplace of Cuiviénen.
As we descended from the mountains we entered the first of the great forests we had espied from the heights. And behold! It was more like Cuiviénen than we had at first perceived. Our road led us alongside a river falling steeply from the foothills. And we named this river Ascar, for it seemed to us to be a rushing, impetuous entity, impatient to reach its destination. And that destination we discovered after several days of travelling to be another river, the Gelion, wider and slower moving than Ascar, reminding us of the Great River that had flowed on the western boundary of Eryn Galen. But more than anywhere this land of trees and rivers reminded us of our birthplace, and we felt a peace settle on us that we had not truly known since leaving the Waters of Awakening.
So drawn were we to this place that once again we chose to halt in our westward journey, settling in Thargelion. Our first years in our new home were devoted to exploring the country about us. To the north we discovered that an arm of the mountains jutted out from the main range of the Ered Luin, preventing further exploration. To the south we found the landscape to be much like the one the one we had settled in. Indeed, we named it Ossiriand, 'the land of Seven Rivers', for besides Gelion and Ascar, we discovered five more tributaries flowing down from the Blue Mountains: Thalos, Legolin, Brilthor, Duilwen, and Adurant.
However, although we moved freely throughout Thargelion and Ossiriand, for many years we were reluctant to cross the River Gelion and explore the lands that lay on its further banks. We were loath to give up our new home, uncertain whether we would find another like it further west.
Eventually though, certain of our people who volunteered were sent across the river, being instructed to discover all they could about the land to the west, and above all to try and gain tidings of our kin the Vanyar and the Noldor. These scouts were gone for a long time, so long indeed that we began to fear that some terrible fate had befallen them and the lands between us and the sea were truly perilous.
However, return they finally did, and they brought news of those we especially wished to hear of. Immediately to the west of Gelion, they said, lay a great plain of grassland, a place for travelling and not living. Beyond this was another massive forest, and it was here that they had gained tidings of our kindred, for many of the Noldor lived in the eastern reaches of this forest. The scouts had gone no further than this, but the Noldor had told them that the Vanyar inhabited the western part of the woods, beyond a river named Sirion, and in Nan-Tathren, 'the Land of Willows', further south on that same river. Elsewhere beyond their forest home the plains began again, their slope down to the sea interrupted only by ranges of rocky hills.
It seemed to us that there was no better home for us than where we currently resided, so we chose to remain in Thargelion. Our Lord Elwë, however, was well-pleased to find that Finwë remained in Middle-earth still, and he immediately set forth to a joyful reunion with his friend.
After this Elwë went often to visit Finwë, catching up on many years of separation. His sojourns in the forest home of the Noldor were lengthy, and thus we did not immediately worry when one extended for longer than those previously. But as the extra days turned to weeks, we began to wonder at our lord's absence. For although he spent much time with Finwë, he never forgot that he was Teleri, and did not neglect us.
Finally, our concern over Elwë's absence became fear for his safety, and Olwë his brother sent messengers to Finwë, asking if some injury had befallen him so that he could not return to us. But the messengers brought back far worse news, for Finwë had told them that the Lord Elwë had departed his company many weeks since, and therefore should have arrived home long before now. But he had not, and we were afraid for him.
And so we searched for him. Many who had been reluctant to cross the Gelion previously now did so without a second thought, scouring the plains west of the river, and the great forest where dwelled the Noldor and Vanyar. Some of our number even went south down the River Sirion to Nan-Tathren, to ask those of the Vanyar who desired to live closer to the sea if they had had any tidings of our lord. But all of our searching was in vain, for Lord Elwë was nowhere to be found. It was as if he had vanished from the face of Middle-earth.
The loss of our lord was a severe blow to us, for as well as being well-loved by all, he was the only one among us who had beheld the light of Valinor, and without him we were leaderless in our journey to the Blessed Realm.
But this was not the only blow dealt us. In seeking guidance from Finwë about the Lord Elwë's disappearance, we were dismayed to discover that both the Noldor and the Vanyar had suddenly vanished from their great forest home. Only one circumstance could have caused such a desertion – a summons to Valinor.
However, our knowledge of this summons came too late, for we knew we could never hope to catch up with our kindred before they travelled over the sea. Still, we perceived that the time had come to move on again.
But there were those among us who did not wish to leave off their search for the Lord Elwë, and so they remained in the wooded lands around the River Gelion. So, taking Olwë to be our lord, the rest of us left our home once more and moved west and south down the Sirion, knowing that we were too late, and yet hoping that the Valar would not forsake us.