The north end of the inland passage according to Vingboons opens into endless ocean. It’s doubtful the ocean was endless, but apparently Vingboons’ sources included no information on what lay beyond. Other maps of California as an island provide various versions of what lay north, but Vingboons’ map is silent.
The exact location of the north end of the island has been elusive for several reasons. One reason is that the Pacific coast has suffered the most change and offers no distinct reference points all the way down to Point Conception. Another reason I suspect is the projection, although I am not a cartographer. Another is not knowing what the error in latitude really is.
The north end of the island lies just south of the southern end of the Willamette Valley, bounded on the west by the Southern Oregon Coast Range and on the east by the southern end of the Cascades. This formation is important because on many “California as an Island” maps from the 1600s and the 1700s the north end of the island is a distinct “M”. McLaughlin categorizes maps by whether they have a “flat” or “indented” top throughout his catalog of California as an island maps (“The Mapping of California as an Island, an Illustrated Checklist”, Glen McLaughlin). The obvious explanation for the existence of maps with two distinct northern coasts is that the ocean level was higher and/or the continent was lower, and the ocean had flooded the Willamette Valley when the “M” map was made. I believe a great deal of variations in other California as an island maps can be accounted for by changing ocean level during the ice age, as the ocean water was transferred to the land in the form of the great glaciers. Equally important would have been the rising elevation of the land as the whole Nevada, Utah, Colorado area was uplifted.
Vingboons puts the northeast corner of the island at 44 degrees latitude. Based on Point Conception being shown one degree too far north by Vingboons, the top of the island is probably at the 43rd latitude, not the 44th. Putting the northeast point of the island at 43 degrees, we find that the top of the island falls on an indistinct end to an equally indistinct plateau that covers the north end of the state of California. The salt lake at the north east corner is Summer Lake in Southern Oregon. Notice that the salt lakes in this area lie generally east of the Island California, as if there was a lot of salt water there.
The Pacific coast extends out onto the continental shelf, as it does for the rest of the California coast down to Point Conception.
Now let’s look at the eastern side of the north end of the inland passage.I have shown the entire east coast from Pheonix at the south end to Boise at the north in order to show that the map scale is consistent throughout the entire east coast of the passage. The islands line up correctly, as does Phoenix at the south end of this map. The background map is from Marble with Mercator projection, which gives very good alignment with Vingboons all the way up the east coast.The north end of Vingboons’ eastern coastline fits nicely into the valley that Boise, Idaho rests in. The little bit of coastline north of Boise follows the base of the mountains. The bump in the coastline just southwest of Boise wraps nicely around the Owyhee mountains in the southwest corner of Idaho. Vingboons’ unnamed river northeast of the north most island runs northeast up the valley toward Elko, Nevada. Today a river runs down this valley through Elko. To the north of Boise the Vingboons map trails off into nothingness. However, some other maps of the time, such as the Sanson map of 1657, show a rectangular peninsula pointing west from the end of Vingboons’ coastline. I have so far avoided branching off into other maps because this project was so large by itself, but here I will just point out this one feature. The name of the peninsula on the Sanson map is Agubela de Cato. It points generally northwest on Sanson’s and most all other maps of the era, but it points more southwest in the Louis Hennepin map of 1697 (or 1698 depending on your source). I think that it matches up well enough with the rectangular plateau northwest of Boise to be more than a coincidence. You can see the plateau in the very upper left corner of the above image.