"deeper inland" is correct and usable in written English.
It is used to refer to a location that is further away from a coastline, or border of some kind. For example: "We decided to explore deeper inland."
They bubbled up swollen and drowned, rag-dolled in the current, moving deeper and deeper inland, toward Issaquena".
The park is wild enough in its deeper inland reaches for there to be golden eagles, gray foxes, coyotes, mule deer, bobcats and even, occasionally, mountain lions.
The army marched deeper inland and pitched camp near a Galatian stronghold called Cuballum.
Prolonged dry seasons and sea-level rise, brought about by climate change, is pushing saltwater from the South China Sea deeper inland, compromising farmers' irrigation channels.
The harbor (which the two new icons overlook) is the best place to begin almost any tour of Oslo, and the one we took probes deeper inland it leads through streets lined by examples of architecture from various centuries, including a look at how distinguished older buildings have been repurposed to modern uses.
The real Bali can be found deep inland.
The Coston flare proved an immediate success, from rivers deep inland to the high seas.
There are deep inland extensions of this alluvial region, which in central Java cut through to the southern coast.
Until recently, glaciologists had assumed it would take decades for coastal changes to make their way deep inland.
British troops, officially in Sierra Leone to train a new army, have helped push the rebels deep inland.
These states later combined into the Sakalava empire, which controlled most of western Madagascar and several adjacent areas deep inland.
